Dead-Rail Range Improvement with a Wireless Repeater

A simple repeater using two ProMini Airs. A ProMini Air receiver picks up DCC transmissions on a channel in the 433MHz ISM band, and its 5V logic level DCC/GND output is directly connected to an 869/915MHz ProMini Air transmitter’s DCC/GND input. The 869/915MHz ProMini Air’s wireless DCC transmissions are picked up by DCC receivers onboard a locomotive.
A DCC repeater in action. The transmissions from the repeater’s ProMini Air 869/915MHz transmitter are picked up by the ProMini Air receiver located in the tender of the Cab Forward. The Base Station’s 433MHz ProMini Air transmitter sends wireless DCC to the repeater’s 433MHz ProMini Air receiver that is directly connected by wire to the repeater’s 869/915MHz ProMini Air transmitter.

The Range Performance Problem in Dead-Rail

An often-heard complaint in Dead-Rail is wireless range performance. The regulatory limits on transmitting power in the unlicensed “ISM” (Industrial, Scientific, and Medical) bands used for Dead-Rail applications force dead-rail transmitters to emit at low power, usually in the few milliwatts range. By contrast, licensed amateur radios can transmit at tens of watts!

Many radio-control applications work well with low-power transmitters because of either short transmission range or unobstructed line-of-sight between the transmitter and receiver. However, we often do not have these luxuries in our Dead-Rail applications, where we have huge layouts and line-of-sight obstructions.

OK, enough of the problem. Let’s get to a reasonably simple solution: a repeater.

Making a Simple Dead-Rail Repeater

There are many ways to make a repeater. I’ll discuss a very simple (simple-minded?) repeater design that is easy for us to implement in Dead-Rail using ProMini Air transmitters and receivers that I have described in a previous post.

The idea for my design of a Dead-Rail repeater is straightforward: receive transmissions from an often-weak signal at one RF frequency and retransmit this signal at full power at another RF frequency to prevent interference with the reception of the weak signal at the received RF frequency. So, right off the bat, you see that you need a wireless receiver operating at one RF frequency, a wireless transmitter operating at a different RF frequency, and a wired connection between the two to send 5V logic-level DCC from the receiver to the transmitter.

Repeater Base Station

Before we get to the actual repeater, let’s discuss a tiny variation in the transmitter “base station” that will give us a better transmission range than typical Dead-Rail transmitters that operate in the 869/915MHz ISM bands. The idea is to initially transmit in the 433MHz ISM band, which is legal in many parts of the world, especially in Europe. Contrary to popular perceptions, it is legal to transmit in North America in the 433MHz band if the transmitted power is low enough.

Why bother with a 433MHz base station? You certainly get better obstacle performance at 433MHz than you do at higher frequencies, and you may get better direct line-of-sight performance as well. The downside to using the 433MHz ISM band is longer antennas are needed, roughly twice as long as in the 869/915MHz ISM bands. The longer length makes it impractical to mount a 433MHz antenna for a mobile receiver onboard a locomotive. For fixed transmit and receiver installations, the longer antenna is far less inconvenient.

The photo below shows the “base station,” which converts the track DCC from a standard DCC throttle to wireless DCC transmitted in the 433MHz (433.05MHz to 434.79MHz) ISM band. The design is almost identical to the ProMini Air transmitter described in my previous post. The only differences are the Anaren radio module (with its approved antenna) designed to operate at 433MHz rather than 869/915MHz and a tiny bit of specialized transceiver initialization data in the software. That’s it for the base station!

Transmitter “Base Station.” A standard DCC throttle provides track DCC to a “DCC Converter” that converts the track DCC to “5V DCC” and 5V power/ground for the ProMini Air transmitter, which in turn, transmits wireless DCC on a channel in the 433MHz ISM band.
A base station operating the the US 916MHz band transmitting on Airwire channel 15

The Repeater

The photo at the very top of the page shows the repeater that you place at some distance from the “Base Station.” The repeater consists of a ProMini Air receiver that is identical in design to the ProMini Air receiver described in my previous post. The only difference is the Anaren 433MHz radio module instead of the 869/915MHz radio module (you cannot easily tell the difference between the two because they have the same pinouts and form factor), and a tiny bit of transceiver initialization data in the software.

You directly connect the receiver’s 5V DCC/GND to a ProMini Air transmitter’s 5V DCC/GND inputs. The transmitter outputs wireless DCC transmissions on channels in 869/915MHz ISM band that are picked up by mobile 869/915MHz receivers on-board the locomotives. As described in my previous post, compatibility with CVP Airwire, Tam Valley Depot, GWire, and ProMini Air receivers is assured.

Close-up of pin connections
Pin connections showing the wired connection between the 433MHz ProMini Air receiver and the 869/915MHz ProMini Air transmitter.
Alternative repeater power connections. A battery-powered voltage regulator set to +5V powers the 433MHz receiver. The 3-wire connection from the 433MHz receiver provides ground (Blk), +5V (Red), and 5V logic DCC (White) to the 869/915MHz transmitter.
A repeater operating in the US 916MHz band: the receiver operates on Airwire channel 15, and the transmitter operates on Airwire channel 0 (zero).
The “surrogate locomotive” with a receiver operating in the US 916MHz band on Airwire channel 0 (zero).

As a further option for the repeater, you can connect a second ProMini Air transmitter to the repeater’s ProMini Air receiver to wirelessly re-transmit DCC at a different frequency (channel) in the 433MHz band to other repeaters whose receiver is “listening” on the same 433MHz channel.

Some Possibly-Important Details

Below are possibly-important details.


My previous post discusses how to compile the ProMini Air software (found at this GitHub site) and download the resulting “firmware” to the ProMini Air’s Pro Mini MCU (micro-controller unit). The software the ProMini Air uses to operate at 433MHz is the same software that you use for the ProMini Air receivers and transmitters operating in the 869/915MHz ISM bands. All that changes is the selection of the 433MHz band and the correct crystal frequency (26MHz for the Anaren radio module) in the config.h file. See the relevant part of the config.h file below, and note the “#define EU_434MHz” (operate in the 433MHz band), “#undef TRANSMIT” (compile for a receiver), and “#undef TWENTY_SEVEN_MHZ” (the crystal frequency is NOT 27MHz).

// Set band of operation
/* Use ONLY ONE #define*/
/* For 896/915MHz EU/NA ISM bands*/
// #define NAEU_900MHz
/* For EU-only 434MHz ISM band*/
#define EU_434MHz
/* For World-Wide 2.4GHz ISM band*/
// #define NAEU_2p4GHz

// Set Transmitter or Receiver
/* Uncomment ONLY ONE #define*/
/* For receiver*/
#define RECEIVER
/* For transmitter*/
// #define TRANSMITTER

// Set the default channel for NA/EU 900MHz only!
#if defined(NAEU_900MHz)
/* Uncomment ONLY ONE #define*/
/* To set the default to NA channel  0 for 869/915MHz ISM bands only!*/
#define NA_DEFAULT
/* To set the default to EU channel 17 for 869/915MHz ISM bands only!*/
// #define EU_DEFAULT

// Set the transceiver's crystal frequency
/* Uncomment ONLY ONE #define*/
/* For 27MHz transceivers (e.g., Anaren 869/915MHz (CC110L) and Anaren 869MHz (CC1101) radios)*/
/* For 26MHz transceiver (almost all other radios, including Anaren 433MHz (CC1101), 915MHz (CC1101), and 2.4GHz (CC2500) radios)*/


We use a transceiver daughterboard with a surface-mounted Anaren “chip” designed to operate on multiple channels in the 433MHz ISM band instead of 896/915MHz ISM bands. The two chips have different discrete surface mount components optimized for the respective ISM band. Transceiver daughterboard offerings that claim operation at 433MHz and 869/915MHz are not credible – you cannot use the same discrete components for multiple ISM bands. Your range performance will be inferior if you use these offerings. And, these offerings are NOT usually FCC/IC/ETSI approved as “intentional transmitters.” The transceiver daughterboard with the Anaren radio module we recommend is available from Blueridge Engineering, or you can contact me directly.

Repeater connections

The best way to supply power to the two (or three) ProMini Air receiver/transmitter(s) is battery power or a voltage converter using a battery power source. The ProMini Air transmitter/receiver can accept direct B+/B- battery power connections, usually 14.8V LiPo batteries, or 5V/GND inputs from a voltage converter. Power connections are described in my previous post on the ProMini Air. I strongly recommend using the 5V/GND power inputs from a voltage converter (they are inexpensive) to prevent overtaxing a small 5V power converter onboard the ProMini Air.

All that remains to do is connect the 433MHz ProMini Air receiver’s GND/DCC output directly to the 869/915MHz ProMini Air transmitter’s GND/DCC input. The GND and DCC Input/Output connection are the same pins on both ProMini Airs. The 3 pin row for the connections from left to right is marked GND/+5V/DCC I/O (T/R). You can see the connecting wires in the photo at the top of the page. DO NOT connect the 5V pin in the 3-pin row between the two ProMini Airs UNLESS you are supplying a 5V/GND supply to one of the ProMini Airs via the two-pin row marked left to right as GND/5V.

Repeater power and data connections using battery connections
Repeater power and data connections using a +5V/GND voltage converter

Changing Configuration

The ProMini Air transmitter/receiver’s DCC address is by default 9000/9001, respectively. My previous post describes how to reconfigure the ProMini Air using the DCC throttle’s “OPS” mode by sending changes to the Configuration Variables’ values. Important CVs are CV255 to set transmission power level (0-10) and CV254 to set channel #. The 433MHz ProMini Air has eight channels (0-7) that can be used, and channel 0 (434.00MHz) is the default.

When you have multiple ProMini Air transmitters and receivers “listening,” beware that sending OPS mode commands to either 9000 or 9001 will change the CV values on all listening ProMini Airs that have one of these default addresses. Global changes are probably NOT what you had in mind and will disable any “two-step” repeaters if they retransmit to other repeaters since the repeater’s 433MHz transmitter must transmit on a different channel from the repeater’s 433MHz receiver.

You have two strategies for preventing inadvertent reconfiguration using OPS mode: change the ProMini Air’s DCC address as discussed here, or turn off all ProMini Airs you don’t want to reconfigure. Giving a unique DCC address to each ProMini Air is probably the safest strategy! Of course, you can “play” useful games by giving “groups” of ProMini Airs the same DCC address so that they are all reconfigured at the same time, but other “groups” at a different DCC address will ignore these commands.


With a simple repeater that requires no new hardware or software, I hope you will agree it is simple to extend the range of wireless DCC! Perhaps these ideas will inspire you to develop an even better range extension technique.


Assembling the ProMini Air Receiver/Transmitter


Typical application. In some cases, such as the Airwire transmitters, the throttle and transmitter are combined. Also, the receiver and amplifier may be combined, such as for Airwire and Tam Valley Depot receivers. The ProMini Air transmitter and receiver will require, respectively, an inexpensive, easily-obtained (or made) “DCC Converter” or “DCC Amplifier.”

I was inspired to fully develop a wireless DCC transmitter and receiver by two sources: Martin Sant, who runs the BlueRidge Engineering website, and an article by Mark and Vince Buccini titled “Build Your Own Wireless DCC System” that appeared in the April, June, and August 2014 editions of Garden Railways magazine. These back issues are still available.

The Buccini’s showed that it was possible to home-build a wireless DCC system at all. And Martin became a great collaborator who got me concretely started with the initial version of the “ProMini Air” wireless DCC transmitter/receiver hardware and the wireless DCC software for the Pro Mini microcontroller board. I am deeply indebted to these people.

Disclaimer: I have a close association with Blueridge Engineering, and I have worked with Blueridge to make the ProMini Air transmitter/receiver available as a kit. I am a hobbyist providing a hobbyist’s kit. I have meager experience designing PCB’s and providing kits, and it will have less “polish” than commercial “kit” offerings.

Note: Some photographs may show a prototype transceiver (a red PCB), but we recommend using the Anaren “daughterboard” that comprises the very small, FCC/IC-approved Anaren transceiver PCB that is surface-mounted to a PCB with a 10-pin interface for easy mounting to the ProMini Air PCB.

You can mount other, less expensive transceiver daughterboards to the ProMini Air PCB using the 10-pin interface. However, most of these transceiver daughterboards are not FCC/IC-approved as an “intentional transmitter.” In many cases, the manufacturers have NOT optimized these transceiver daughterboards to operate in the US/Canadian 915MHz ISM band (or the European 868-870MHz ISM band), resulting in reduced range performance.

Feature Comparisons

Our goal for offering the ProMini Air receiver/transmitter is to allow those interested in “dead-rail” (radio control, battery power of a model railroad locomotive) an inexpensive way to build wireless, DCC compatible transmitters and receivers for radio-control of model railroad locomotives in the US/Canadian 915MHz ISM band – the same band and protocol as the Tam Valley Depot and CVP Airwire transmitters and receivers (and the QSI Gwire receiver). Also, you can operate the ProMini Air transmitter and receiver in the European ISM band at 869.850MHz, and we have verified interoperability with Tam Valley Depot European DRS1 transmitters and receivers.

The “ProMini Air” receiver is compatible with the Tam Valley DRS1 transmitter, both the CVP AirWire T5000 and T1300 wireless throttles, and the no longer manufactured NCE GWire CAB. The ProMini Air transmitter is compatible with the Tam Valley Depot DRS1 receiver, the CVP Airwire CONVRTR receivers, and the QSI Gwire Receiver. Of course, the ProMini Air transmitters and receivers are compatible with each other!

The ProMini Air has some features that may be of interest when compared to commercial offerings. See the Comparison Tables below.

NameAirwire Receiver
Level Adj
Ch 16 (or 17(E))
Air Transmitter
Yes0-16, 17(E)YesYes
Comparison of wireless DCC transmitters

In fairness, the Airwire T5000 wireless throttle was never intended to interface to standard DCC throttles. But, as Tam Valley Depot recognized, it is advantageous to use any device that supplies DCC to the rails and transmit this DCC wirelessly to DCC-compatible receivers. A notable limitation of the Tam Valley Depot DRS1 transmitter is that it does not provide DCC “IDLE” packets that the Airwire receivers require unless the original DCC throttle does so (which most, if not all, do NOT). Also, the Tam Valley Depot DRS1 transmitter can only broadcast on one Channel (near Airwire Channel 16).

The comparisons for wireless DCC receivers are shown in the Comparison Table below.

Channel Auto
DRS10-16, 17(E)NoneYes
Yes (Limited)
0-16, 17(E)None or
Comparison of wireless DCC receivers

Perhaps the most important difference among the receivers is “DCC filtering,” i.e., how does the receiver behave when valid RF DCC signal is lost.

When the Tam Valley DRS1 or QSI Gwire lose valid RF signal, they output random pulses. I have discussed the pros and cons of this in another post.

On the other hand, the Airwire CONVRTR outputs constant-level DC when it loses valid RF signal, or more correctly, when it doesn’t receive enough DCC “IDLE” packets. Again, as discussed in another post, the DCC decoder may halt the locomotive dead in its tracks when it receives this constant-level DC, which may or may not be what the user wants.

The Airwire CONVRTR performs “DCC filtering” by evaluating whether it’s receiving DCC “IDLE” pulses periodically. So, even if a stream of completely-valid DCC packets are received, but there are few or no “IDLE” packets, then the Airwire CONVRTR will become inactive and output constant DC to the decoder.

This is why the Tam Valley DRS1 transmitter will usually NOT work with Airwire CONVRTR receivers because the DRS1 will not insert additional DCC “IDLE” packets! The Tam Valley Depot DRS1 transmitter is a passive participant: if the input DCC throttle doesn’t produce frequent DCC “IDLE” pulses, then the Tam Valley Depot DRS1 will not transmit frequent DCC “IDLE” pulses.

The ProMini Air receiver, via OPS mode reconfiguration (by default at DCC address 9001), can select either option – output random pulses or output constant-level DCC when valid RF signal is lost. This reconfigurability makes the ProMini Air receiver the most versatile of the wireless DCC receivers. The ProMini Air receiver’s valid RF detection is somewhat more sophisticated than Airwire’s. The ProMini Air receiver detects how long it’s been since it received ANY valid DCC packet. And, after a preset time interval (which is reconfigurable via OPS mode changing of a CV), the ProMini Air receiver will output either the random pulses it’s receiving (DCC filtering “off”) or output constant-level DC (DCC filtering “on”). The DC level that is output when DCC filtering is “on,” and there is no valid RF signal is also reconfigurable via an “OPS” mode setting of a CV at the ProMini Air’s DCC address.

Once valid RF signal is received again, the ProMini Air receiver detects this condition and outputs these valid DCC packets to the “DCC amplifier” that in turn sends “track-level” DCC to the decoder as discussed below.

Another important feature of wireless DCC receivers is Channel selection and searching.

The Tam Valley Depot DRS1 receiver will “listen” on a fixed Airwire Channel if you set some hardware jumpers. Otherwise, the DRS1 will automatically search all of the Airwire Channels for a valid RF signal if you do NOT set the jumpers. This behavior may or may NOT be a good idea if multiple wireless DCC transmitters are transmitting simultaneously on different Channels. And, changing the Channel selection behavior (fixed Channel or auto-scan) requires physical access to the receiver to connect or disconnect jumpers.

On startup, the Airwire CONVRTR “listens” for a valid RF signal on its “startup” Channel (which is reconfigurable by accessing a CV using the wireless throttle’s “OPS” mode). If the CONVRTR finds no valid RF signal after a given time, the CONVRTR will switch to Channel 0. This behavior is usually a good idea.

Like the Airwire CONVRTR, on startup, the ProMini Air receiver will “listen” for valid RF on its “startup” Channel (default, 0) stored in EEPROM memory. This startup Channel is changeable using the transmitting throttle’s “OPS” mode by setting CV255 to a value of 0 through 16 at the ProMini Air transmitter’s DCC Address (default, 9001). Like the Tam Valley Depot DRS1 receiver, if the ProMini Air does not find a valid RF signal on its startup Channel, the ProMini Air receiver will then auto-scan Channels 0, 16, 17(E), 1, 2, …, 15 (in that order) for valid RF signal. This scan sequence guarantees that a wireless DCC transmitter (if one is available) is selected, but only if the ProMini Air does NOT find a valid RF DCC signal on its startup Channel from another wireless DC transmitter.

If no valid RF DCC signal is found at all on any Channel on startup, the ProMini Air receiver will select Channel 0 and wait for a valid RF DCC signal. Also, upon reset, the ProMini Air’s Channel search process will be unchanged: try the “startup” Channel stored in EEPROM memory, then try auto-searching Channels, and if all else fails, just wait on Channel 0.

So, in summary, we are offering the ProMini Air DCC transmitter and receiver kit to provide a low-cost alternative with a set of features not completely found in commercial offerings.

The downside (or flexibility) to our offering is that you need a few additional items. In the case of the ProMini Air transmitter, you need a simple “DCC Converter” PCB (available very inexpensively from Blueridge and is discussed in detail below) that converts DCC output to the track into Ground, 5V power, and 5V logic DCC. These outputs provide the ProMini Air transmitter with power and DCC packets to transmit. For the ProMini Air receiver, you will need a low-cost “DCC amplifier” that converts the ProMini Air receiver’s 5V logic DCC back to DCC that the onboard DCC decoder would in its customary configuration pick up from the track (again, discussed in detail below). The ProMini Air receiver can be powered either directly from the battery or a small external 5V power supply.

ProMini Air transmitter connections
ProMini Air receiver connections

And, you will need an antenna of your choosing! I love antennas, but your antenna requirements are too diverse to offer a “one size fits all” antenna solution. We provide an FCC/IC-approved Anaren “whip” antenna with the Anaren transceiver that is surface-mounted to a 10-pin interface daughterboard. This Anaren antenna-transceiver combination should work well for most transmitter applications and is FCC/IC approved for “intentional radiators.” For the ProMini Air receiver, some can use the small whip antenna directly mounted to the transceiver onboard the ProMini Air receiver; others will need to run an antenna connecting cable to a small, externally-mounted antenna.

We discuss several nice antenna options below.

Hang with me…

Skill Requirements

The ProMini Air receiver/transmitter (Rx/Tx) kit is easy to assemble, requiring only the soldering in place of some pin rows, battery terminal (for the receiver option), and a Pro Mini PCB (not to be confused with the ProMini Air PCB) that are provided with the kit. The potential kit-builder should also be somewhat familiar with downloading firmware into micro-controller boards (an Arduino or clone Pro Mini in our case) using the Arduino IDE.

Kit Contents

ProMini Air kit contents

Note: We may include some of the “male” and “female” pin row connectors with the kit that may differ from the photos or descriptions here. Which pin rows go where is easy to determine since the transceiver daughtercard mounts with dual 5-position pin rows and the Pro Mini PCB mounts with two 12-position pin rows and a one two-position pin row, regardless of whether you use round or square pin rows.

The ProMini Air PCB, as you receive it, has the difficult-to-solder and easily-confused components already installed: the transceiver daughterboard, the surface-mount resistors and MOSFETs, and the 5V and 3.3V regulators. The transceiver has a U.FL antenna connector that interfaces to the provided Anaren whip antenna. If a remote antenna is required, U.FL to SMA or RP-SMA antenna cables are widely available.

Shown below is a close-up of the round pin headers. Compared to standard square-pin headers, round pin headers reduce the overall height of the assembled ProMini Air PCB when the Pro Mini PCB is mounted.

Round pin connectors

Kit Assembly

Step 1: Solder the 6-position DIP pins into the Pro Mini PCB as shown below. You can carefully bend the pins inward over the Pro Mini PCB to prevent clearance interference with other components on the assembled ProMini Air PCB. The angle of the bent pins should allow for the insertion of the USB connector that we discuss later.

Installation of 6-position pin-header on the Pro Mini

Step 2: Install the supplied the two 5-position male pin rows into the transceiver daughterboard. The Anaren PCB is surface-mount, so we mount it on a dual 5-position pin row interface PCB for ease of soldering and interface. The kit provides two female rows of 5-position pin rows that you can solder to the ProMini Air PCB that allows easy insertion/removal of the transceiver daughterboard with its corresponding dual rows of male pins. Or, you can directly solder the male pins to the ProMini Air PCB for a strong, but permanent mount.

The Anaren daughterboard that comprises an Anaren transceiver PCB surface-mounted on a PCB with a 10-pin interface for convenient mounting to the ProMini Air PCB. Note: the appearance of this board may be slightly different from the kit.
Anaren PCB daughterboard with two 5-position male pin rows soldered in place.

Step 3: Install the standard DIP pins and battery terminal on the ProMini Air PCB, as shown below. If you are using the ProMini Air as a transmitter, you can omit the battery terminal installation because the “DCC Converter” PCB will supply GND and +5V power as well as 5V logic DCC on the column of 3 pins adjacent to the 4-pin I2C display interface at the lower right of the ProMini Air PCB shown below.

DIP pin and battery terminal installation on the ProMini Air PCB. Note this photo also shows the round pins that connect to the Pro Mini PCB.

Step 4: Solder the round DIP pins to the Pro Mini PCB and the ProMini Air PCB. The male rows will be mounted underneath and soldered on the top of the Pro Mini PCB, and the female rows will be mounted to the top and soldered underneath the ProMini Air PCB.

The technique I use is to insert, but do not solder, the round DIP pins onto the Pro Mini PCB, as shown below (12-position left, 12-position right, and two-position P4/P5). I then overlay the ProMini Air PCB on top of this temporary assembly, aligning the round pins with the corresponding mount holes in the ProMini Air PCB. Be sure that the extra, unused holes in the ProMini Air PCB align with the Pro Mini PCB’s USB pins!!!

Holding all of this temporary assembly together, solder the pins to the bottom of the ProMini Air PCB and then carefully turn the assembly over and solder the round pins to the top of the Pro Mini PCB. I use this technique to provide accurate alignment when the Pro Mini PCB removed and then re-inserted into the ProMini Air PCB.

Of course, you may have a better or more comfortable technique for soldering pin rows into place!

Temporary insertion of round DIP pin rows into Pro Mini PCB for accurate alignment

Below is the final mating of the Pro Mini PCB to the ProMini Air PCB with the Pro Mini PCB soldered to the male round pin rows. Once you solder the round pin rows to the Pro Mini PCB “in situ,” the Pro Mini PCB can be carefully lifted off the ProMini Air PCB, along with its male rows of round pins, for firmware loading.

Final installation of Pro Mini into the ProMini Air PCB. Note that the USB connection pins overlay the unused holes in the ProMini Air PCB.

That’s it for hardware assembly!

Firmware Installation

The ProMini Air can be configured as either a receiver (Rx) or transmitter (Tx) by loading firmware into the Pro Mini using a “USB Breakout Module” (available here or here) that inserts into the USB DIP pins as shown below. Since the Pro Mini PCB operates at +5V, please ensure that you use a 5V USB Breakout Module, NOT one that supplies 3.3V! Note the alignment of GND and DTR on the Pro Mini and the USB Breakout Module. Some clones of the Pro Mini have the USB pin-out reversed, so you will need to turn over the USB Breakout Module before insertion!

Pin-out alignment between Pro Mini and USB Breakout Module
Insertion of the USB Breakout Module into the Pro Mini USB pins

Once you connect the USB Breakout Module to a PC via a USB cable, the Arduino IDE can be used to download the ProMini Air’s compiled firmware. In the Arduino IDE, to set the correct microcontroller board, select: Tools > Board > Arduino Pro or Pro Mini. To set the correct chip, select: Tools > Processor > ATmega328P (5V, 16 MHz). To set the correct port, select: Tools > Port > the_correct_USB_serial_port, which you will need to figure out. I use the “AVRISP mkII” Programmer (Tools > Programmer > AVRISP mkII) because it was the default selected by the IDE.

Important Note: Since the ProMini Air’s firmware makes extensive use of the EEPROM, before installing the ProMini Air transmitter or receiver firmware, we strongly advise that you completely clear the Pro Mini’s EEPROM. Use the Arduino IDE and selecting File > Examples > Examples for Arduino Pro or Pro Mini > EEPROM > eeprom_clear program. Once eeprom_clear has been compiled and downloaded to the Pro Mini, there will be a short delay, and then one of the Pro Mini’s diagnostic LED’s will turn on continuously. Then, compile and install the ProMini Air firmware as described below.

The source code is available from this GitHub site. The source code should be placed in a directory where the Arduino IDE can find it. The subdirectory structure should be maintained so that the Arduino IDE can properly access the “project.”

How to download the GitHub zip file that will maintain the directory structure

Depending on whether you want a transmitter or receiver, edit libraries/config/config.h to select the “define” for the transmitter or receiver.

For a receiver (Rx), config.h should look like:

//    vvv User Entry Area Below vvv

// Set band of operation
/* Use ONLY ONE #define*/
/* For 896/915MHz EU/NA ISM bands*/
#define NAEU_900MHz
/* For EU-only 434MHz ISM band*/
// #define EU_434MHz
/* For World-Wide 2.4GHz ISM band*/
// #define NAEU_2p4GHz

// Set Transmitter or Receiver
/* Uncomment ONLY ONE #define*/
/* For receiver*/
#define RECEIVER
/* For transmitter*/
// #define TRANSMITTER

// Set the default channel for NA/EU 900MHz only!
#if defined(NAEU_900MHz)
/* Uncomment ONLY ONE #define*/
/* To set the default to NA channel  0 for 869/915MHz ISM bands only!*/
#define NA_DEFAULT
/* To set the default to EU channel 17 for 869/915MHz ISM bands only!*/
// #define EU_DEFAULT

// Set the transceiver's crystal frequency
/* Uncomment ONLY ONE #define*/
/* For 27MHz transceivers (e.g., Anaren 869/915MHz (CC110L) and Anaren 869MHz (CC1101) radios)*/
/* For 26MHz transceiver (almost all other radios, including Anaren 433MHz (CC1101), 915MHz (CC1101), and 2.4GHz (CC2500) radios)*/
// #define TWENTY_SIX_MHZ

//    ^^^ User Entry Area Above ^^^


If you want a transmitter (Tx), then config.h should be

//    vvv User Entry Area Below vvv

// Set band of operation
/* Use ONLY ONE #define*/
/* For 896/915MHz EU/NA ISM bands*/
#define NAEU_900MHz
/* For EU-only 434MHz ISM band*/
// #define EU_434MHz
/* For World-Wide 2.4GHz ISM band*/
// #define NAEU_2p4GHz

// Set Transmitter or Receiver
/* Uncomment ONLY ONE #define*/
/* For receiver*/
// #define RECEIVER
/* For transmitter*/

// Set the default channel for NA/EU 900MHz only!
#if defined(NAEU_900MHz)
/* Uncomment ONLY ONE #define*/
/* To set the default to NA channel  0 for 869/915MHz ISM bands only!*/
#define NA_DEFAULT
/* To set the default to EU channel 17 for 869/915MHz ISM bands only!*/
// #define EU_DEFAULT

// Set the transceiver's crystal frequency
/* Uncomment ONLY ONE #define*/
/* For 27MHz transceivers (e.g., Anaren 869/915MHz (CC110L) and Anaren 869MHz (CC1101) radios)*/
/* For 26MHz transceiver (almost all other radios, including Anaren 433MHz (CC1101), 915MHz (CC1101), and 2.4GHz (CC2500) radios)*/
// #define TWENTY_SIX_MHZ


//    ^^^ User Entry Area Above ^^^


Note that two additional options are available for using transceivers that operate with 27MHz (such as the FCC/IC/CE-approved Anaren transceivers) or 26MHz transceivers and whether the operation is for North American or European operation (the latter of which only operates at 869.850MHz).

After downloading the firmware into the Pro Mini completes, please do not remove the USB connection from the computer until the “secondary” LED, which indicates attempted communication over the SPI (serial peripheral interface), flashes on (it will not be bright). This will ensure that the initial round of writes to the blank EEPROM was completed!

Alternatively, you can load the firmware into the Pro Mini MCU using an “AVR ISP,” such as the Sparkfun Pocket AVR Programmer or a less-expensive clone. This “ISP” downloading mode will bypass and erase the bootloader to directly load the firmware into the Pro Mini MCU. On boot-up with the bootloader now erased, the Pro Mini MCU will almost instantly supply “5V logic DCC” to the DCC amplifier, which in turn, provides the DCC decoder with standard DCC waveforms. There is no “boot-up DC” to speak of and no need to set CV29, bit2=0. (I set it anyway.) With this solution, all DCC decoders I’ve tried (ESU, Zimo, MTH) start-up without the “boot-up jerk.”

This “ISP” form of loading firmware is not as extensively used by folks using the Arduino IDE, but ISP loading is easily accessible within the Arduino IDE. The overly-brief method of ISP programming steps are the following:

1. Connect the USBtinyISP Programmer to the following six Pro Mini pins: GND, RST, VCC, SCK (pin 13), MISO (pin 12), and MOSI (pin 11). Be sure the “power the target” switch is set so that the USBtinyISP will supply power to the Pro Mini MCU. I have developed a very simple Pro Mini ISP Programming Board that I will make available at a nominal cost (<$3.00+shipping). This board has a pair of pins for optional GND/VCC power input because some ISP programming boards, unlike the USBtinyISP, do NOT provide +5V DC power.

Pro Mini MCU connections to a USBtinyISP for loading firmware without a bootloader.
Pinout for USBtinyISP programmer.
USBtinyISP connections to the Pro Mini MCU using a programming board available from the author.

2. From the Arduino IDE, Select Tools → Programmer → “USBtinyISP”

3. Select the AirMiniSketchTransmitter sketch.

4. Select Sketch → Upload using a Programmer.
The Arduino IDE will then compile the sketch and download the resulting firmware to the Pro Mini via the USBtinyISP, bypassing (and erasing) the bootloader. 

Once the ProMini Air receiver or transmitter firmware is installed in the Pro Mini and inserted into the ProMini Air PCB, the ProMini Air is ready for integration!


To complete the integration of the ProMini Air receiver (Rx) or transmitter (Tx), you must establish several connections.

Overview of Connections

See the picture below for an overview of the connections to and from the ProMini Air. Which connections used depends on whether the ProMini Air will act as a receiver (Rx) or a transmitter (Tx). THERE IS NO PROTECTION AGAINST INCORRECT BATTERY OR EXTERNAL POWER CONNECTIONS!!! The ProMini Air will be destroyed immediately if the GROUND and POSITIVE POWER SUPPLY are reversed!

Data and power connections

The U.FL connector on the Anaran transceiver may be directly connected to the provided Anaren whip antenna, or you can connect a U.FL to SMA or RP-SMA cable the ProMini Air to a remotely-mounted antenna. Also, a two-pin output provides Ground and the DCC input to (Tx) or output from (Rx) the RF transceiver board as signals to an oscilloscope for waveform review.

ProMini Air antenna connector (female RP SMA) and transceiver DCC input/output

The ProMini Air has several connections that provide power, I2C display outputs, and 5V logic DCC inputs or outputs.

ProMini Air connections for power, I2C display output, and 5V logic DCC input or output

We will break down these connections for the ProMini Air receiver and transmitter in the next two sections.

Receiver Connections

Starting with the ProMini Air configured as a receiver (Rx), there are several options for providing power. The first option is to use external battery power and jumper the +5V and +5V (Battery) pins together to use the on-board 5V regulator to provide board +5V supply.

ProMini Air receiver battery-powered option

Since you may not like the heat generated by the onboard 5V regulator when you supply power with external battery power and install the jumper, as an alternative, you may use an external +5V power supply, as shown below where the external power supply provides Ground and +5V. Of course, you do NOT install the jumper.

ProMini Air receiver powered by an external +5V power supply
Close-up of ProMini Air receiver power connections to an external +5V power supply

The ProMini Air receiver must connect to an external DCC amplifier that converts the 5V logic DCC from the ProMini Air receiver to DCC A/B that a DCC decoder requires. This DCC amplifier uses battery power and the inputs from the ProMini Air receiver to provide the power and DCC messages, coded as a bipolar DCC waveform, to the decoder for both power and DCC messages. These “DCC amplifiers” are usually medium to large amperage amplifiers that accept pulse width modulation (PWM) input to provide precision output control for electric motors. The maximum PWM frequency of these amplifiers is usually high enough (> 20kHz) to reproduce DCC packets accurately.

ProMini Air receiver connections to a DCC amplifier. Note: the ProMini Air receiver does not have its power connections in the photo above.
Close-up of ProMini Air receiver output to the DCC amplifier
Close-up of the inputs to the DCC amplifier from the ProMini Air receiver

Some DCC amplifiers have their specialized connector configurations, as shown below, for a GROVE-compliant amplifier.

Example of another DCC amplifier’s connections to the ProMini Air receiver

Integration of the ProMini Air Receiver into a Locomotive

Of course, the real purpose of the ProMini Air receiver is to integrate it into a locomotive for wireless DCC control using an onboard battery as power. An excellent, reasonably-priced (about $14 US as of 2020), high-power (13A continuous) DCC amplifier may be purchased here as shown below. Considerably smaller (0.8″ x 1.3″), but more expensive high-amperage amplifiers (about $30 US as of 2020) that have been used with success can be found at Pololu here or here.

ProMini Air receiver integration with battery power, DCC amplifier, and antenna
Example Installation

Transmitter Connections

Now, let’s turn the ProMini Air used as a transmitter (Tx) of DCC messages from any DCC compatible throttle.

The photo below shows the connections between an interface board that takes throttle DCC A/B inputs (“track” DCC) and rectifies these inputs to provide Ground and +5V power supply output. This “DCC Converter” PCB also “taps off” the DCC A input and converts it to a 5V logic DCC output that is suitable for the ProMini Air transmitter. These outputs provide to the ProMini Air transmitter as shown below with Ground, +5V power, and 5V logic DCC input. The “DCC Converter” PCB is available from Blueridge Engineering, or the user can make this uncomplicated board based on the schematic shown below. Note: the ProMini Air transmitter IS NOT connected to a battery since it receives its power from the “DCC Converter” PCB.

ProMini Air receiver connections to a “DCC Converter” PCB that supplies the ProMini Air transmitter with Ground, +5V power, and 5V logic DCC. The ProMini Air transmitter is NOT connected to a battery! The jumper connecting +5V to +5V (Battery) is NOT used!
Close-up of ProMini Air transmitter connections to the “DCC Converter” PCB. The jumper connecting +5V to +5V (Battery) is NOT used!

Below is a schematic of the straightforward circuit that takes throttle DCC input and converts it to Ground, +5V power, and 5V logic DCC for the ProMini Air receiver.

Schematic for the “DCC Converter” PCB that converts throttle DCC A/B into Ground, +5V, and 5V logic DCC for the ProMini Air transmitter. This circuit uses a 2W10 rectifier (from here), an L7805 5V regulator (from here), and two high-speed switching 1N4148 diodes (from here).

The user can change the ProMini Air transmitter’s Channel (Airwire channels 0-16) and Power Level (0-10) by setting the DCC throttle’s address to that of the ProMini Air transmitter’s (9000 by default). Then, using the throttle’s OPS mode, change the value of a configuration variable (CV255 for Channel: 0-16, and CV254 for Power Level: 0-10), exiting OPS mode, and changing the throttle back to the locomotive’s DCC address.

Receiver/Transmitter Antenna Connections

For the ProMini Air transmitter, we strongly urge you to use the FCC/IC-approved Anaren “whip” antenna supplied with the Anaren transceiver that is surface-mounted to a 10-pin interface daughterboard. This Anaren whip antenna/transceiver combination is the FCC/IC-approved as an “intentional radiator.” For experimentation purposes, antennas for the ProMini Air transmitter can be purchased on-line from many sites. For fixed installations of the ProMini Air transmitter, we suggest the reputable products from Linx such as their SMA one-half wave antennas with an internal counterpoise. You can find these antennas at Digi-Key, e.g., ANT-916-OC-LG-SMA ($10.55) and ANT-916-CW-HWR-SMA ($12.85). The former antenna has a slightly better gain (2.2dBi versus 1.2dBi) but is somewhat longer (6.76” versus 4.75”).

Linx half-wave antennas. The ANT-916-OC-LG-SMA has better gain than the ANT-916-CW-HWR-SMA at the expense of being 42% longer.

For the ProMini Air receiver or the ProMini Air transmitter where a small, remotely-mounted antenna is needed, we again recommend Linx antennas such as the ANT-916-CW-RCS or ANT-916-CW-RAH.

The ANT-916-CW-RCS is an excellent choice for a small antenna with 3.3 dBi gain. It is available from Digi-Key or Mouser. Note the male RP SMA connector.
The ANT-916-CW-RAH is another excellent choice for a small antenna (2.2 dBi) available from Digi-Key or Mouser. The connector shown here is a male RP SMA, but male SMA connectors are available from Digi-Key and Mouser as well.


Diagnostic Outputs

The ProMini Air receiver or transmitter provides diagnostic outputs that are not required for operation, but are useful for troubleshooting or just for fun:

  • The transceiver’s output (in Rx mode) or input (in Tx mode) can be monitored on the output DIP pins that are described above.
  • “I2C” outputs that can drive an inexpensive 2 row, 16 column I2C LCD display.
The 2-pin connector provides Ground and the RF transceiver’s transmitted or received DCC signals. These signals can be monitored by an oscilloscope.
ProMini Air receiver/transmitter connections to an I2C LCD display
Close-up of ProMini Air receiver/transmitter connections to an I2C LCD display

The ProMini Air software now automatically searches for a valid LCD I2C address. Please make sure only ONE display is connected to the ProMini Air.

You can also change this address by going into the “OPS” mode to reset either the ProMini Air transmitter or receiver. For the transmitter, you use the DCC throttle that connects to the ProMini Air transmitter (by default at DCC address 9000). For the ProMini Air receiver, you use the wireless DCC throttle transmitting to the ProMini Air receiver (by default at DCC address 9001). In both cases, you set CV243 to the appropriate I2C LCD address. The EEPROM permanently stores the changed address, but this new address is not operative until you power cycle the ProMini Air.

Configuration and Testing

By default, the ProMini Air receiver and transmitter are configured to operate on Airwire Channel 0. This default can be changed by setting the DCC address to 9001(Rx)/9000(Tx) (the default, which can be changed as described in the Users Manual) to access the ProMini Air transmitter, and in OPS or Programming-on-the-Main (POM) mode setting CV255 to the desired channel. Valid channels are 0-16 for North Americal Operation or Channel 17 (869.85MHz) for European operation.

Should the ProMini Air receiver fail to detect valid DCC packets on its default Channel during startup, it will cycle through all Airwire Channels to find a Channel producing valid DCC packets. If this cycling fails to find a valid Channel, the ProMini Air receiver will change to Channel 0 and wait for a valid RF DCC signal. This change of Channel is not permanent, and on a restart, the ProMini Air will revert to its default channel.

There are several other configuration options available through “OPS” mode programming, as described in the ProMini Air Users Manual.

We strongly urge the user to test the ProMini Air before the final deployment. At the least, an inexpensive I2C LCD display can be purchased here or here (and numerous other locations) to gain some insight into the ProMini Air’s state. This display is especially valuable when using the ProMini Air as a transmitter.

Examples of Testing

In the examples below, the Yellow waveform is the signal from/to the RF transceiver for Rx/Tx, respectively, and the blue waveform is one channel of the resulting DCC (Rx) sent to the decoder or DCC received from the throttle via wireless transmission (Tx).

Receiver Testing

The photo below shows the ProMini Air operating as a receiver. Of course, an RF transmitter wirelessly sends DCC packets. This transmitter may be a dedicated wireless DCC throttle, such as the Airwire Tx5000. Or, it may be a transmitter that converts standard “track DCC” to wireless DCC such as the Tam Valley Depot DRS1 transmitter or the ProMini Air used as a transmitter (as discussed in the next section)!

On the LCD, “My Ad: #” is the DCC address of the ProMini Air itself. The “(L)” means “long” address. Displayed on the second line is the Channel number and whether DCC “filtering” is “off” (Filter: 0, as shown) or “on” (Filter: 1).

Example of output from a ProMini Air receiver. The yellow signal on the oscilloscope is from the T/R DCC output pin on the ProMini Air receiver (the green PCB on the left with the red RF transceiver PCB mounted on the left end). The blue trace is the DCC signal produced by the DCC amplifier (the PCB on the right with the blue power/DCC out terminal) from inputs from the ProMini Air.

The photo below shows the oscilloscope waveforms when there is no valid RF DCC signal. With filtering off (Filter: 0), the DCC sent to the decoder reproduces the random pulses generated by the receiver.

The ProMini Air receiver’s outputs when no valid RF DCC is found. The yellow signal is the RF receiver’s DCC, and the blue signal is one of the DCC outputs from the DCC amplifier that provides input to the onboard DCC decoder.

These two photos show the ProMini Air’s transceiver and DCC amplifier output when valid RF DCC is received and when no valid RF DCC is received. DCC filtering is off, so the DCC amplifier’s output reproduces the transceiver’s DCC in both cases.

Valid RF DCC received. The decoder DCC mirrors (blue) the receiver’s DCC (yellow).
No valid RF DCC. The random pulses produced by the RF receiver are reproduced by the decoder DCC when DCC filtering is off (Filter: 0).

The user can reconfigure the ProMini Air receiver using the throttle’s “OPS” mode. Setting the wireless throttle DCC address to 9001 now shows that the Msg address (“Msg Ad: #”) matches the ProMini Air receiver’s address (“My Add: #”).

Set DCC filtering “on” by selecting the ProMini Air’s address (9001 in this case). Note that the MSG address now matches the ProMini Air’s address.

In OPS mode, change CV246 to “1”, which will turn “on” the ProMini Air receiver’s DCC filtering.

In “OPS mode” setting CV246 to “1.” The display will indicate that CV246 has been changed.

The display now shows that DCC filtering is “on.”

In “OPS mode” setting CV246 to “1.” The display will indicate that CV246 has been changed.

Exiting OPS mode and changing the throttle to the locomotive’s address, now shows an updated “Msg Ad: #” with DCC filtering “on.”

Then change the address back to the locomotive’s. The display now shows DCC filtering is “on.”

Here is the transceiver’s and DCC amplifier’s DCC output since we are transmitting valid RF DCC.

Again, the receiver and decoder DCC when a valid RF DCC signal is received.

If we turn off the wireless transmitter/throttle sending RF DCC, now the transceiver outputs random pulses (yellow). Since filtering is “on,” the ProMini Air receiver firmware detects “bad” waveforms that do not appear to represent a valid DCC packet. The ProMini Air receiver then outputs a constant-level signal that causes the DCC amplifier to output a high level on DCC A (blue), and zero on DCC B (not shown). This behavior is similar the that of the Airwire receivers. However, the detection mechanism for Airwire receivers is simply the lack of a sufficient frequency of DCC “IDLE” packets, not an analysis of the transceiver’s pulse train.

The waveforms when there is no valid RF DCC signal. With filtering on (Filter: 1), DCC A sent to the decoder is positive and DCC B is zero. This is assuming that CV248 is set to “1”. If CV248 is set to zero, then DCC A is zero and DCC B is positive.

Repeating the process of changing the wireless throttle’s DCC address to 9001, going into “OPS” mode, changing CV246 to “0”, exiting “OPS” mode, and changing back to the locomotive’s DCC address will now set DCC filtering to “off.”

You can repeat the process of selecting the ProMini Air’s address and in OPS mode set CV246=0 to turn the filtering back off, and then set the address back to the locomotive’s.
Changing the address back to the locomotive’s now indicates that the DCC filtering is off (Filter: 0).

So, now when we turn off the wireless DCC throttle/transmitter, the DCC amplifier’s output (blue) will reproduce the random pulses output by the RF transceiver (yellow).

Now, when no valid RF DCC is received, the decoder DCC reproduces the random pulses generated by the RF receiver.

Transmitter Testing

We now turn our attention to testing when the ProMini Air is used as a transmitter.

With exactly the same ProMini Air, the Pro Mini was re-flashed with the transmitter firmware. The “DCC Converter” PCB (the PCB on the right) converts any throttle’s DCC to Ground, +5V power, and 5V logic DCC for input to the ProMini Air transmitter (the PCB on the left).

The display will alternate between showing the ProMini Air transmitter’s DCC address (“My Ad: #”) and the transmitted DCC packet’s DCC address (“Msg Ad: #”). The transmitting Channel (“Ch: #”) and Power Level (“PL: #”) display on the second line.

Note the ProMini Air transmitter’s ID.
The throttle’s address is alternately displayed with the ProMini Air’s address. The Channel number and Power Level are indicated.

Below is an oscilloscope trace of the input DCC from the throttle (blue) and the DCC transmitted by the RF transceiver on the ProMini Air transmitter. Since the wireless DCC must keep Airwire RF receiver’s “happy” with numerous DCC “IDLE” packets, the ProMini Air transmitter evaluates the incoming DCC from the throttle. When the throttle outputs frequent, redundant DCC packets, the ProMIni Air transmitter occasionally inserts DCC “IDLE” packets instead of one of the redundant packets. So, the input DCC and the transmitted DCC will not precisely match. Since DCC throttles send many redundant DCC packets, the locomotive will receive sufficient DCC packets to operate correctly.

The DCC sent out (yellow) will not exactly match the throttle DCC because of slight timing delays and the occasional insertion of DCC “IDLE” messages that are required to keep Airwire receivers “happy.”
A shorter time scale of the previous photo

The ProMini Air transmitter can be reconfigured by setting the throttle’s DCC address to 9000 (which can be changed) and then going into the “OPS” mode to set configuration variables (CV) to new values.

Setting the throttle’s address to 9000 allows the throttle to reconfigure the ProMini Air in OPS mode.

Once we have changed the throttle’s DCC address to 9000, note that the message address (“Msg Ad: #”) now matches the ProMini Air’s address (“My Ad: #”).

The display now indicates that the message address matches the ProMini Air’s.

For example, while in OPS mode, changing CV246 to “6”, will reset the ProMini Air transmitter’s Power Level to 6, as indicated by the display shown below.

In OPS mode, setting CV254 to 0-10 changes the output power level, as indicated here.

After exiting the “OPS” mode, we see that the display reflects the new Power Level (“PL: #”).

The Power Level is now 6.
Note that Msg and My Address are the same.

Changing the throttle’s DCC address back to the locomotive’s address will sometimes show “Msg Ad: 255(S)”, which means that the ProMini Air transmitter sent out a DCC “IDLE” packet to make Airwire receiver’s “happy.”

Changing the throttle’s address back to the locomotive’s allows the ProMini Air to insert occasional DCC “Idle” messages, indicated by a message address of 255. The IDLE message keeps Airwire receivers “happy.”

A display refresh (every 4 seconds) will most likely display the locomotive’s DCC address, which is 1654. The “(L)” means “long” address.

The display will alternately show the locomotive address and the ProMini Air’s address.

Conclusion and Further Information

The ProMini Air is an inexpensive and hopefully fun introduction to wireless DCC control of your model railroad locomotive!

Please see this Blueridge Engineering site for further information on the purchase and assembly of ProMini Air kits. The prices will range from $40 to $50 that includes a DCC amplifier for the receiver and a DCC converter for the transmitter.


Dead-Rail Transmitter/Receiver Options and Installation

Numerous wireless RF transmitter/receiver (Tx/Rx) options for locomotive control are available both in the US and abroad. My discussion is confined to wireless RF transmitter/receiver options that are DCC compatible, which means that the transmitter sends “logic-level” DCC packets, and the receiver converts the “logic-level” DCC packets back to “bipolar” DCC packets, as would be transmitted on tracks, that an onboard DCC decoder can “understand.”

Schematic of representative application

Why am I limiting my discussion? Because DCC is a standard, and if you don’t go with solutions that have standards behind them, then you are likely to suffer “vendor lock” where a single vendor holds you “hostage” with “their” solution. Perhaps that attitude is a bit overblown, but vendors with proprietary solutions tend to lag in innovation for lack of competition, and what happens if the vendor goes out of business?

I know that the NMRA DCC standards have some problems including the following issues: pending issues under consideration for years; vendors ignoring some parts of the standards; some vagueness in places; and lack of standards for wireless. The DCC standard is imperfect, but it’s miles better than no standard at all. Plus, the DCC decoder market is competitive and feature-rich – you can almost assuredly find a DCC decoder that will satisfy your needs.

As a further limitation of this post, I will mostly confine my discussion on DCC-compatible wireless Tx/Rx options to the 902-928 MHz ISM (Industrial, Scientific, and Medical) band because this is where I have direct experience. There is significant and exciting activity in the DCC-compatible 2.4 GHz ISM band (using Bluetooth technology) band as well (see BlueRailDCC), but I have no personal experience with this band. Another advantage of the 902-928 MHz ISM band is that there is some interoperability between transmitters and receivers, although there is currently no firm standard behind this interoperability.

DCC-compatible Tx/Rx options are a very large topic that I cannot fully cover in this blog. These options are well-covered in the following links:

  • Dead Rail Society: This should always be your first stop when looking at topics related to dead-rail. This site is the epicenter of dead-rail. In particular, this page discusses vendors for dead-rail Tx/Rx.
  • Facebook Dead Rail page: This social media page is a valuable source for the latest announcements and discussions for dead-rail, including Tx/Rx options.


Below is my personal experience with 902-928 MHz ISM DCC-compatible receivers.

General Comments

How each of these DCC compatible wireless receivers handles loss of valid RF signal from the transmitter is discussed here.

CVP Airwire

A CVP Airwire CONVRTR-60X wireless DCC-compatible RF receiver mounted to the side of the tender hull using Velcro. The U.FL antenna cable was later connected. The DCC “A/B” output of the CONVTR-60X connects to the “Track Right/Left” inputs of a wiring harness for a LokSound L V4.0 DCC decoder (not yet inserted) on the opposite side of the tender hull.

The company CVP manufactures and supports its Airwire series of products that include hand-held wireless DCC-compliant throttles (such as the T5000 and T1300) and receivers, such as the CONVRTR series that seamlessly connects to DCC decoders onboard the locomotive. As a general comment, CVP provides excellent, detailed installation and operation documentation, and that’s in part why they are dominant in some segments of wireless model railroad control. The CONVRTR receiver has some sophisticated features, such as setting its Airwire RF channel purely in software, that are described in its User Guide.

However, the CONVRTR interacts with the Airwire wireless throttles in ways that make it difficult to impossible​ to transmit just “garden variety” DCC wirelessly to the CONVRTR for proper operation. The Airwire throttles transmit numerous DCC “Idle” packets as a “keep-alive” message for the CONVRTR. A red LED on the CONVRTR board indicates received signal quality and flickers least when receiving a large number of DCC Idle packets. The brightness of the LED indicates the received RF power. Typical DCC throttles are not designed with these “keep-alive” concerns in mind, and do not output DCC Idle packets often enough to keep the CONVRTR “happy.”

Other than the CVP Airwire transmitters (the T5000 and T1300), the only currently-available (the no longer manufactured NCE GWire Cab was also Airwire-compatible) RF transmitter that I am aware of that is capable of communicating with the Airwire CONVRTR is the ProMini Air, whose open-source software (at GitHub AirMiniTransmitter) intercepts “garden variety” DCC from the throttle and interleaves a sufficient number of DCC Idle packets to communicate correctly with the CONVRTR. This “keep-alive” requirement for the Airwire CONVRTR is challenging to produce, so sometimes a reset of the DCC throttle ​or the ProMini Air is required to initially send enough DCC Idle packets to initiate communication with the CONVRTR.

Like the Gwire receiver below, the Airwire CONVRTR “X” versions have a ​U.FL connector for connecting a shielded antenna cable from the receiver to an externally-mounted antenna. An internal antenna option is available as well for CONVRTR mountings that are not surrounded by metal.

QSI Solutions Gwire

Gwire U.FL connector. If using the U.FL connector, detach the wire antenna.

The Gwire receiver operates on Airwire RF channels 0-7 ​that the user must select from a dial on the device itself. A nice feature of this receiver is an onboard U.FL connector (see Figure  above) that allows the user to connect a shielded antenna cable from the receiver to an externally-mounted antenna – useful when the antenna needs to be on the exterior of a metal locomotive or tender shell. See Blueridge Engineering’s website for details on how to interface the Gwire to any onboard DCC decoder. The Gwire presents no difficulties for wireless 902-914 MHz ISM band DCC-compatible transmitters, and you can find it on eBay at relatively low ($20 US or less) prices.

Tam Valley Depot DRS1, MkIII

Tam Valley Depot DRS1, MKIII in an open-cavity install. Note the built-in long wire antenna.

The Tam Valley Depot DRS1, MkIII receiver operates only on Airwire RF channel 16 (actually 916.49 MHz, which is close enough to Airwire channel 16 at 916.37 MHz) and makes a suitable wireless DCC receiver. This receiver has a long, single-wire antenna that provides efficient RF reception (see the figure above). However, you must place this wire outside any metal shell, which may be inconvenient in some mounting applications. The DRS1, MkIII, presents no difficulties for the 902-914 MHz ISM DCC-compatible transmitters as long as they transmit near 916.49 MHz. The DRS1, MkIV described in the next section supersedes this receiver.

Tam Valley Depot DRS1, MkIV

The recently-released Tam Valley Depot DRS1, MkIV receiver. Note the internal antenna on the right side of the board.

The Tam Valley Depot DRS1, MkIV receiver is a significant upgrade from the DRS1, MkIII, and operates at the original Tam Valley 916.49 MHz frequency, Airwire Channels 0-16, and at 869.85 MHz (for European operation). The DRS1, MkIV presents no difficulties for the 902-928 MHz ISM DCC-compatible transmitters and is an interesting choice because it changes channels automatically until it finds a sufficient RF signal carrying DCC packets. See the figure above for the version that employs an internal antenna that is useful when the receiver is not mounted inside a metal shell.

The DRS1, MkIV with a U.FL antenna connector (and a heatsink update) is now available (see picture below), making it very useful for connecting to external antennas outside of metal shells. This version of the DRS1 makes it highly competitive in capability and quality with the Airwire CONVTR. Perhaps a future version will provide DC output to the onboard DCC decoder when no valid RF signals carrying DCC packets are available, making it possible to program the DCC decoder’s behavior when there is no DCC signal available.

Tam Valley Depot DRS1, MkIV receiver with U.FL connector.

Blueridge Engineering ProMini Air Receiver

ProMini Air receiver/transmitter

The inexpensive ProMini Air receiver kit presents no issues when used with 902-928 MHz ISM DCC-compatible transmitters. It operates on Airwire RF channels 0–16 and requires a separate, low-cost amplifier (e.g., the Cytron MD13S) to convert the ProMini Air’s unipolar 5V DCC to bipolar DCC that provides sufficient power to the decoder. See the Blueridge Engineering web page for details on how to build the kit and properly connect the ProMini Air to the amplifier that is in turn connected to the onboard DCC decoder.

The ProMini Air’s open-source software is available for download at the GitHub site AirMiniTransmitter.


So far as I’m aware, there are four 902-928 MHz ISM DCC-compatible transmitters: the CVP Airwire T5000 and T1300, the Tam Valley Depot DRS1 transmitter, and the Blueridge Engineering ProMini Air transmitter.

CVP Airwire Transmitters

The CVP Airwire T5000 and T1300 transmitters are excellent choices for operating with 902-928 MHz ISM DCC-compatible receivers, all of which will properly-communicate with these two transmitters. When I am testing wireless receivers, the T5000 is my “go-to” because, in addition to serving as a DCC-compatible throttle, it can program onboard DCC decoders, via the wireless receiver, in either “OPS” (or Programing-on-the-Main, PoM) or “Service” mode. While the T1300 cannot program the onboard DCC decoders, it serves as a typical DCC throttle.

Of course, the Airwire transmitters send sufficient DCC “Idle” packets to keep the Airwire CONVRTR receivers “happy.”

Tam Valley Depot DRS1 Transmitter

The Tam Valley Depot DRS1 transmitter uses DCC packets produced by any DCC throttle or command station that outputs “bipolar” DCC to tracks. The DRS1 transmitter converts the “bipolar” DCC to “logic-level” DCC and transmits it at only 916.49 MHz, which is close enough to Airwire Channel 16 at 916.36 MHz to be received. This frequency limitation means that only the Tam Valley Depot DRS1, MkIII, and MKIV, and the Blueridge ProMini Air receivers can operate with this transmitter if they are receiving on 916.48 MHz or Airwire Channel 16.

While the Airwire CONVRTR can operate on Airwire Channel 16, the DRS1 transmitter is not designed to transmit sufficient “Idle” DCC packets to keep the CONVRTR “happy” since it passively sends along only the DCC packets it receives from the DCC throttle or command station.

Blueridge Engineering ProMini Air Transmitter

The ProMini Air transmitter with optional LCD. The antenna in the picture was replaced with a high-quality 1/2-wave antenna Linx ANT-916-OC-LG-SMA antenna from either Mouser or Digi-Key for improved transmission.

Blueridge Engineering provides the ProMiniAir transmitter/receiver kit that uses open-source software at the Github AirMiniTransmitter site. Like the DRS1 transmitter, it is designed to take inputs from any DCC throttle or command station’s “bipolar” DCC output to tracks (via a simple, low-cost optocoupler provided by Blueridge Engineering) and transmit the “logic-level” DCC on Airwire channels 0-16.

The ProMini Air transmitter inserts a sufficient number of DCC “Idle” packets into the original throttle-produced DCC to keep the Airwire CONVRTR “happy.” This keep-alive capability coupled with transmission on Airwire channels 0-16 ensures that the ProMini Air transmitter is capable of communicating with any of the 902-928 MHz ISM DCC-compatible receivers discussed in this blog.

This transmitter’s settings, like channel number and output power, can be controlled by the DCC throttle or command station in the “OPS” mode by setting the throttle address to that of the ProMini Air, which is 9000 by default. An optional LDC display can be attached to the ProMini Air transmitter for status display. More configuration information is available at the GitHub AirMiniTransmitter site.

Full disclosure here: I am one of the contributors to the AirMiniTransmitter open-source software, and I am heavily-involved with Blueridge Engineering with the design of the ProMini Air transmitter/receiver board.

Dead-Rail Conversion of an MTH 2-8-8-8-2 Virginian Triplex


This post describes my most difficult dead-rail conversion to date: an MTH O scale 2-8-8-8-2 Virginian Triplex (MTH product number 20-3101-1) that I purchased on eBay circa September 2019. Previously, I converted a Sunset 3rd Rail Allegheny with an MTH Proto-Sound 3.0 board to dead-rail, but the Triplex was my first complete dead-rail conversion of an MTH locomotive to 2-rail operation, which included lathe turning high-profile wheels to approximate an NMRA RP-25 flange profile (also see NMRA standard S-4.2) so that the locomotive would operate reliably on track meeting NMRA standard S-3.2.

Box details
Locomotive side view
Tender side view

The inside view of the tender below demonstrates a significant challenge: space is very tight with the Proto-Sound board and the large speaker consuming a large part of the tender’s internal volume where we need to install additional dead-rail components: DCC-compatible RF receiver; 14.8V LiPo battery; and switch, charging, and antenna wiring.

Inside view of the tender: Note the battery cradle, the space-consuming speaker, and the end-of-tender smoke unit.

The strategy starts to emerge:

  1. Replace the Proto-Sound 2.0 (PS2.0) board with a PS3.0 board that can operate in DCC mode.
  2. Remove the original rechargeable battery and its cradle and locate the 14.8V LiPo battery pack there.
  3. Remove the large speaker and replace it with a smaller 4-ohm speaker so that we can make room for the 14.8V LiPo battery pack and the Airwire CONVRTR-60X DCC-compatible RF receiver that operates in the 902-928 MHz ISM band on Airwire channels 0-16.
  4. Lathe down the high-rail wheel flanges to approximate an NMRA “RP-25” profile for 2-rail, dead-rail operation.

An advantage of this strategy is retaining almost all of the control the PS2.0/PS3.0 provides, including directional head/tail lamp, marker lights, cabin lights, flickering firebox, sound, and fan-driven smoke units.

Proto-Sound 3.0 Conversion

The first step of the dead-rail conversion was easy: replacing the Proto-Sound 2.0 board with a Proto-Sound 3.0 (PS3.0) board from Ray’s Electric Trainworks. As I have mentioned in other posts, working with Ray Manley is a great pleasure. I sent my PS2.0 board as a trade-in to Ray, and he took care of the rest, providing me with a fully-functional PS3.0 board, complete with DCC capability.

The heatsink for the PS3.0 board necessitated drilling and tapping a new mount hole with spacer, as shown in the figure below.

New Proto-Sound 3.0 heatsink mount with a spacer.

The following photos show the original electrical power inputs to the PS2.0 board and their modified connections for the replacement PS3.0 board.

Original power connections to the Proto-Sound 2.0 board.

As you can see below, the AC power from the center rail pick-up (hot) and the outside rails (ground) were disconnected – we will be getting our power from a 14.8V LiPo battery pack in the tender. In this case, there is no Constant Voltage Unit, so no Battery +(14.8V)/Battery -(Ground) connections are required.

Locomotive power connection modifications. There is no Constant Voltage Unit in this locomotive, so the B+/B- wires indicated above are NOT used.

The AC power connections in the tender are also disconnected, and the power inputs to the PS3.0 connect to the switched battery power. The Battery +(14.8V)/Battery -(Ground) connection on the wiring harness was NOT required.

Tender power connection modifications. There is no need for B+/B- power supply to the locomotive.
Power input modifications for the PS3.0 board. The DCC A/B input power comes from the connection to the Airwire CONVRTR-60X DCC-compatible RF receiver.
Final wiring connections

Locomotive Electrical Modifications

There were two aspects to the electrical modifications in the locomotive:

  1. Headlamp replacement
  2. Electrical power supply

The original headlamp was a power-hungry incandescent bulb. An LED with a polarity-independent plug from Evan Designs was used to eliminate the need to determine the polarity of the original headlamp wiring.

Headlamp LED replacement details

The power-related modifications consisted of removing the center-rail pick-ups, which is very easy on MTH locomotives and disconnecting any wiring to the center-rail pick-up (hot) and the outside rails (ground).

The original center-rail pick-ups. The “Center-Rail Pick-up” is disconnected.
The center-rail connection underneath the motor is disconnected

Battery Installation

Battery installation was very challenging since the only practical placement location was the original rechargeable battery and its cradle mounting beside the PS2.0 board. A special-order 2x2x1 14.8V, 2600mAh (38.48 Wh, 5A rate,  LxWxT: 133 mm x 40 mm x 25 mm) LiPo battery purchased from provides the one cell-diameter thickness required to fit the battery pack between the PS3.0 board and the tender hull.

Original rechargeable battery and cradle location. The speaker volume control potentiometer was moved to accommodate the more extended 14.8 V LiPo replacement battery pack.
Volume control potentiometer. It was moved from its original location, and UV glue provides stress relief to prevent breakage of the very fine wires.
Final location of the volume control potentiometer. UV glue holds the potentiomenter in place.
Final battery location. Velcro attaches the battery to the side of the PS3.0 board.

Mechanical Modifications

In MTH steam locomotives, the wheel axles insert into a solid cast chassis frame, so the driver wheels must be pulled off the axle before machining the high profile wheels to approximate an RP-25 profile that is compatible with two-rail, dead-rail operation.

The driver wheels must be pulled off the axle for machining after removal of the side-rods. The axle and wheel are scribed to maintain proper “quartering.”
A wheel puller to separate the driver wheel from the axle.
Comparison of lathe-cut and unmodified high-profile wheels. The high-profile flanges were lathe-cut to approximate an RP-25 profile for 2-rail, dead-rail operation.

Tender Mechanical Modifications

The tender’s mechanical modifications involve adding a Kadee 740 coupler and accommodating additional dead-rail electronics.

Coupler Modifications

The original coupler pivot, rather than using a frame-fixed mounting, was used to mount a Kadee 740 coupler. This strategy ensured that tight curves would not bind the coupler.

The original coupler assembly

A Kadee 740 coupler was mounted on the original coupler pivot, as shown in the Figure below. The brass screw heads were ground down to provide clearance with the tender frame.

Modified coupler assembly for a Kadee 740 coupler

Additional Dead-Rail Electronics

The added dead-rail electronics include the charging plug, the ON/OFF/Charging plug, a smaller speaker, and the antenna mount.

Charging plug (left) and ON/OFF/Charge switch (right) mounting
Antenna mounting location. The antenna is a  Linx ANT-916-CW-RCS discussed in this blog.

The original speaker was far too large to provide clearance for the additional battery, DCC-compatible RF receiver, and other electrical components needed for the dead-rail installation. So a 16mm x 35mm speaker was placed in the bottom of the original speaker’s cavity, and UV glue holds the speaker in place.

The original speaker. It is far too large to accommodate the additional dead-rail components: battery, DCC-compatible RF receiver, ON/OFF/Charging switch and charging plug.
Candidate speaker. The actual 16mm x 35mm speaker was even smaller and mounted into the bottom of the original speaker cavity.

Final Demonstration

With the locomotive reassembled, it’s time to test it out! If your locomotive has a smoke unit(s), always ensure sufficient smoke fluid is loaded. Even if you don’t intentionally turn on the smoke unit – sometimes it’s unexpectedly activated.

Initial demonstration

Dealing with Loss of RF Signal in Dead-Rail for Onboard DCC Decoders

Note: This post deals with details of various brands of DCC-compatible, wireless RF receivers operating in the 902-928 MHz “ISM” band that connect to onboard DCC decoders. Some aspects of the discussion may apply to other RF bands as well.

Typical application. In some cases, such as the Airwire transmitters, the throttle and transmitter are combined. Also, the receiver and amplifier may combined, such as for Airwire and Tam Valley Depot receivers.

The designers of various DCC-compatible RF receivers have a couple of strategies for what output to provide to the onboard DCC decoders when a valid RF signal is lost:

  1. Output the random pulses that the RF receiver naturally outputs when a valid RF signal is lost. This option will cause most DCC decoders to maintain direction and speed while the DCC decoder “sifts” the random pulses searching for valid DCC packets.
  2. Output a fixed, positive Direct Current (DC) voltage to one of the DCC decoder’s “Track” inputs and a zero voltage DC the other “Track” input when either a) RF signal is lost, or b) when the RF transmitter does not send sufficiently-frequent “keep-alive” DCC packets. The latter is true for the Airwire CONVRTR. How the DCC decoder responds to these DC “Track” inputs depends upon DCC decoder configuration and, unfortunately, DCC decoder manufacturer discretion.

There are several NMRA-specified Configuration Variables (CV’s) that affect how DCC decoders handle the loss of valid DCC packets and are important to understand when the DCC decoder is connected to the DCC output of DCC-compatible RF transmitters because the RF receivers may lose or receive corrupted RF signal from the dead-rail RF transmitter.

The NMRA standard S-9.2.4, section C “Occurrence of Error Conditions” states “Multi Function Digital Decoder shall have a Packet Update time-out value.” Further down on line 60 the standard states “A value of 0 disables the time-out (i.e., the user has chosen not to have a time-out)”. This part of the NMRA standard is not universally-implemented by manufacturers, and it affects how decoders will respond to the loss of RF transmission of DCC packets. To implement this requirement, the NMRA standard S-9.2.2 has defined the recommended (R), but not mandatory (M), CV11, Packet Time-Out Value. A value of CV11=0 is defined to turn off the time-out, but CV11 is frequently not implemented.

However, another CV that is often implemented addresses some aspects of loss of DCC. The optional (O) CV27, Decoder Automatic Stopping Configuration, is under re-evaluation by NMRA, but the NMRA has taken no definite action some time. Here is what the NMRA standard S-9.2.2 currently (as of 2019) states about CV27: 

Configuration Variable 27 Decoder Automatic Stopping Configuration
Used to configure which actions will cause the decoder to automatically stop.

Bit 0 = Enable/Disable Auto Stop in the presence of an asymmetrical DCC signal which is more positive on the right rail.
“0” = Disabled “1” = Enabled

Bit 1 = Enable/Disable Auto Stop in the presence of an asymmetrical DCC signal which is more positive on the left rail.
“0” = Disabled “1” = Enabled

Bit 2 = Enable/Disable Auto Stop in the presence of an Signal Controlled Influence cutout signal.
“0” = Disabled “1” = Enabled

Bit 3 = Reserved for Future Use.

Bit 4 = Enable/Disable Auto Stop in the presence of reverse polarity DC.
“0” = Disabled “1” = Enabled

Bit 5 = Enable/Disable Auto Stop in the presence forward polarity DC.
“0” = Disabled “1” = Enabled

Bits 6-7 = Reserved for future use.

Since DCC decoder manufacturers frequently do implement CV27, what electrical output the DCC-compatible RF receiver provides to the DCC decoder upon loss of a valid RF signal will influence how the DCC decoder responds. We will break this down for various brands of DCC-compatible RF receivers in the 902-928 MHz ISM band in the following subsections.

Note that some DCC decoders will not honor CV27=0; i.e., all auto-stopping features disabled. For example, with CV27 set to 0, the Zimo MX-696, and probably other Zimo DCC decoders as well, will continue speed and forward direction if positive DC level is input to the “Right Track” DCC input, and a zero DC level is input to the “Left Track” DCC input. Under these “track voltage” conditions, the locomotive will stop if originally moving backward. Some (but not all) DCC-compatible RF receivers, such as the Airwire CONVRTR, provide these DC inputs, if a valid RF signal is lost, but only if connected correctly.

The “correct” connection relates to how the user connects the DCC output from the RF receiver to the “Track Right” and “Track Left” inputs of the DCC decoder. Under normal circumstances, when there is a valid RF signal, which way the DCC decoder connects to the RF receiver does not matter. Under the exceptional case of DC-only output by the RF receiver, if it loses a valid RF signal, which way the DCC decoder connects to the RF transmitter does matter. The user will likely want the locomotive to continue forward with the loss of a valid RF signal, so some experimentation is required to determine which of the RF transmitter DCC outputs should connect to which of the DCC decoder’s “Track” inputs to achieve the desired behavior.

Example DCC waveform output from a DCC-compatible RF receiver when there is a valid RF signal
Example random pulse output from a DCC-compatible RF receiver when there is no valid RF signal. Note the waveform’s superficial similarity to valid DCC output.

As a further complication, the user should probably turn off the decoder’s “analog” mode of operation by setting Bit 2 of CV29 to 0 to force the decoder to use “NMRA Digital Only” control of ”Power Source Conversion” (see the NMRA standard here). If Bit 2 of CV29 is set to 1, and again we emphasize the user should probably not activate this feature, then “Power Source Conversion Enabled” and then CV12 determines the power source; the most common of which is CV12=1, “Analog Power Conversion.”

Airwire CONVRTR Series

CVP Airwire CONVRTR-60X tender installation. The CONVRTR operates on Airwire channels 0-16. Note that the U.FL antenna lead was later connected to the CONVRTR. The LokSound L V4.0 DCC decoder mounting harness can be seen mounted on the tender wall opposite the CONVRTR, and its Track Left/Right inputs are connected to the CONVRTR-60X’s DCC A/B outputs.

When the CVP Airwire CONVRTR loses a valid RF signal or receives insufficiently-frequent DCC Idle packets, it detects these conditions and outputs a fixed DC voltage to the decoder. Consequently, the user should set CV27 according to the description above.

While it may seem that the user would want the locomotive to stop if its RF receiver loses a valid RF signal, consider what might happen in tunnels or locations remote to the DCC RF transmitter. Getting stuck under these circumstances if a valid RF signal is lost is probably not what the user wants, so we strongly suggest that the user set CV27=0.

The user is cautioned, however, that some DCC decoders, such as the new ESU LokSound 5 L DCC, do not honor the CV27=0 setting unless the “polarity” of the “Track Right/Left” is connected “correctly” to the CONVRTR’s “A/B” output. Experimentation may be required to determine the correct connection, but my experience is: CONVRTR A <–> Decoder Track Right & CONVRTR B <–> Decoder Track Left

QSI Solutions Gwire and Tam Valley Depot DRS1 Series

The QSI Solutions GWire operates on Airwire Channels 0-7. If the U.FL plug (at the upper-left corner of the Linx Transceiver chip) connects to an externally-mounted antenna, the antenna wire at the upper-left corner of the GWire board should be cut off at board level, or better yet, unsoldered.
The Tam Valley Depot DRS1, MKIII, operates on Airwire Channel 16
The Tam Valley Depot DRS1, MkIV, operates on Airwire Channels 0-16 (as well as other frequencies). Note the internal antenna on the right-hand side of the board.

The QSI Solutions Gwire and Tam Valley Depot DRS1, MkIII and MkIV DCC-compatible RF receivers will output random pulses to the onboard DCC decoder when a valid RF signal is lost, so setting CV27 is probably of no use. On the “plus” side, most DCC decoders will maintain locomotive direction and speed in the presence of these random pulses since the DCC decoder is actively sorting through these pulses for valid DCC packets, which is usually the behavior the user wants.

A Blueridge Engineering webpage describes how to easily modify the GWire for use as an RF receiver for any onboard DCC decoder.

Blueridge Engineering ProMini Air Receiver

Blueridge Engineering ProMini Air receiver operates on Airwire channels 0-16. The ProMini Air can also be configured to operate as a DCC-compatible transmitter that wirelessly transmits DCC from any DCC source on Airwire channels 0-16.

The Blueridge Engineering ProMini Air receiver has a default long address of 9001. Like the ProMini Air transmitter, the ProMini Air receiver’s channel can be reset in “OPS Mode” by setting CV255 to a value in the range of 0–16. The ProMini Air receiver has the following options when a valid RF signal is lost:

  • Output random pulses to the onboard DCC decoder: The user can set the ProMini Air receiver to output the random pulses when it loses a valid RF signal by setting CV246 to 0 in “OPS mode” at the ProMini Air’s address. In this case, setting CV27 for the onboard DCC decoder is not relevant, because the random pulses from the ProMini Air receiver will cause the onboard DCC decoder to maintain speed and direction of the locomotive while it is “sifting” through the random pulses for valid DCC packets.
  • Output either fixed positive or negative voltage DC to the onboard DCC decoder: In this case, setting CV27 for the onboard DCC decoder at its address is relevant. The user can set the ProMini Air receiver to output fixed DC voltage when it loses a valid RF signal by setting CV246 to 1 in “OPS mode” at the ProMini Air’s address. A positive DC voltage is output by setting the ProMini Air receiver’s CV248 to 1 in “OPS mode” at the ProMini Air’s address, or a negative DC voltage is output by setting CV248 to 0. If the user does not want the locomotive to stop with the loss of a valid RF signal, then set CV27=0 for the onboard DCC decoder at its address. Of course, setting CV27 to other values (see above) in the DCC decoder will determine how the DCC decoder responds to the fixed DC voltage that the ProMini Air outputs to the onboard DCC decoder upon loss of a valid RF signal.


It’s an unfortunate fact of life that we can lose a valid RF signal from our DCC-compatible transmitter. However, with a little study of DCC decoder documentation, and possibly a bit of experimentation, gracefully coping is definitely possible.

Retro-Fitting Smoke Units with Thermistors and Set-up for LokSound Decoders

This is a slight modification of a post I made titled: O Gauge Forum Post on Smoke Units

It is possible to modify a non-ESU smoke unit so that it connects to the LokSound L or XL decoders just as an ESU smoke unit does by connecting the smoke unit to the specialized ESU smoke unit terminals: HTR+/-, MOT+/-, and TMP+/-. This capability allows you to take direct advantage of all the LokSound capabilities provided for ESU smoke units. The missing component in some smoke units is a Negative Temperature Coefficient (NTC) thermistor.

What started me down this road was a “deadrail” conversion of a Sunset 3rd Rail Big Boy (3-rail, “Late Version”) originally outfitted with TMCC and a nice Lionel smoke unit with dual output (photo below).

Dual stack, fan-driven smoke unit for retro-fit.

I wanted to retain this beauty and use a LokSound L V4.0 decoder that is controlled by an Airwire CONVRTR-60. THOR73’s posts on the O Gauge Form inspired me to work through using this smoke unit with the LokSound L V4.0 decoder. I thought that if I could figure out how the ESU smoke units created their “temperature” inputs to the LokSound decoder, then I could retrofit the Lionel smoke unit so that it would be “input compatible” with an ESU smoke unit. This retrofit turned out to be simple.

I reverse-engineered an ESU 54678 smoke unit by measuring the resistance between the heater resistor leads (HTR+/-): ~23 ohms; motor leads (MOT+/-): ~16 ohms; and thermistor leads (TMP+/-): ~100K ohm at room temperature. Each of these components is electrically-isolated from the others. When powered by a 14.8V LiPo battery, the LokSound L V4.0 decoder I had on hand produced the following results on the ESU Profi board using the LokProgrammer (with ground measured at the Profi board’s ground terminal):

Terminal Smoke off Smoke on (Throttle=10)
HTR+ (not connected to heater resistor*) 13.4V 13.2V
HTR- Open Switched open/ground @500Hz ~30% duty-cycle PWM
Fan+ 0V Pulsed <= 5V (difficult to determine with low Frequency chuffs)
Fan- 0V 0V
TMP+ 5.1V 5.1V
TMP- 1.3V 3.7V
* Battery+ (14.8V) connected to heater resistor + input

Cut-away of a 54678 ESU smoke unit showing the thermistor

The difference in TMP- between unheated and heated conditions suggests, but does not prove, that the thermistor’s decrease in resistance with increased temperature is manifested by a voltage increase at TMP- as part of a voltage divider where the thermistor is in series with a fixed resistor resident in the decoder, possibly with a low-side voltage offset:

Probably thermistor circuit

Guesses: RFIXED~1.5K based on probe measurements and derived @ 25C Voffset~1.24

So right off the bat, the ESU smoke unit’s heater resistance (23 ohms) is similar to Lionel’s (27 ohms), and both smoke units use 5V fan motors. The Lionel was missing only the thermistor. Lower resistance smoke units (around 8 ohms) might be problematic to convert unless retrofitted with a heater resistor in the 20-ohm neighborhood or use an externally-supplied, lower HTR+ voltage. The heater and fan motor similarity between the ESU and Lionel smoke units seemed to make this particular Lionel smoke unit an excellent surrogate candidate.

Thermistors with 100K ohm resistance at 25 Celsius are commonly-available, usually with a “B” parameter of around 3900 Kelvin. You can Google what this parameter means (simplified Steinhart-Hart Equation: R(T in Kelvin)=R@TRef*(exp(B/T-B/TRef)) ). While I was not able to verify that the ESU smoke unit used precisely this type of thermistor, testing described later supports this selection.

The photo below is the Lionel 27 ohm smoke unit PCB, part #610-PCB1-045, Rev C (Lionel replacement part #691PCB1045), that was retrofitted with an “axial,” glass-coated 100K NTC thermistor with a B of 3892 Kelvin. (Well, it’s actually a Lionel replacement PCB since I cut some traces retrofitting on the original PCB that I regret doing. Interestingly, the original PCB did not have the mangled lettering of the replacement PCB that some like some have noted.)

The 3-pin power plug on the PCB can be used to power the heater resistor since the outputs from the rectifier/5V converter on the PCB do not connect to anything after removing the fan motor plug. The ground on the PCB MUST be isolated from the heater unit metal case since the PCB’s “ground” wire will be connected to the LokSound L’s HTR- terminal that regulates the heating resistor’s current path to the electrical ground! Electrical measurements revealed good electrical isolation of the metal case from the heating element.

Lionel fan-driven smoke unit with a 100K thermistor added.

I drilled two holes in the smoke unit’s PCB board, and the thermistor was inserted and soldered to two-wire leads that connect to the LokSound L’s TMP+/- terminals. I used high melting-point solder because conventional solder might melt at the high operating temperatures of the heater resistor and thermistor (max around 250 Celsius according to documentation for the ESU smoke unit).

The two heater wires from the three-pin PCB plug connect to the LokSound L’s HTR+/- terminals. (Pins 1 and 3 are shorted together on the PCB and connect to one side of the heater resistor. Pin 2 is ground and connects to the other side of the heater resistor.)

The motor wires directly connect to the MOT+/- terminals. Out of sheer luck, when the red motor lead from the smoke unit is connected to MOT+, and it’s black lead to MOT-, the fan motor spins in the “correct” direction.

As others have suggested, I also replaced the original 27 ohm ceramic resistor with a Lionel 27 ohm replacement #6008141055.

Once you connect the smoke unit’s six outputs to the LokSound L’s ESU smoke unit terminals, some modifications are needed in the ESU sound files and decoder set-up, since they did not originally activate the ESU smoke unit. First, follow THOR73’s directions regarding the connection between sound and smoke chuffing under the “Smoke unit” menu. Note especially that the smoke unit’s automatic power-off time should be reset since the default is 0 seconds. I don’t know if 0 means never turn off, but a non-zero setting seemed like a good idea to me.

LokSound change decoder settings menu

What differs from THOR73’s discussion is the sound-file set-up for an ESU smoke unit. Editing the sound files reveals that most “nodes” have an option to set the “ESU Smoke Unit” parameters. Frequently these settings are turned off, but there are some useful “presets” you can select and experiment with.  An especially interesting preset is the “preheating” preset that is available in the stopped state.

Mute State:

LokSound Stop state settings.

Here are the other states I modified, but I am by no means expert or knowledgeable about these settings. Usually, I chose a “Preset” and then selected the “Steam Chuff” checkbox, which preserves the parameters of the preset (unless you change them), but turns off the Preset name.

Stop State:

LokSound Mute state settings

DCX State:

LokSound DCX state settings

Coast State:

After editing these sound nodes, the next step is to set an “F#” to turn the smoke unit on/off on the “Function mappings” menu. The “logical” outputs column provides an “ESU Smoke Unit” selection, so I selected F23 as the ESU Smoke Unit on/off toggle.

TESTING WARNING: The ESU 53900 Profi Decoder Tester does not appear able to provide adequate power to either an actual ESU Smoke Unit or surrogates described here! In deployed operation, the LokSound L is perfectly capable of delivering sufficient power, but the Profi board is, in my experience (or inexperience), NOT able to do so. I initially thought the culprit was the puny AC to DC converter provided to power the Profi board. But, the power connection to a very hefty 14.8V LiPo battery did not solve the problem. The workaround uses either THOR73’s high-side MOSFET switch mentioned in this thread or the low-side MOSFET switch described in the same topic thread. Either way, you will need to take power (about +14V DC) from the source providing power to the Profi board and use the Profi board’s HTR- output to control the MOSFET switch. In turn, this switch controls the smoke unit’s heater. If using THOR73’s high-side FET switch, then you connect the smoke heater as he describes. If you use the low-side FET switch I presented, the smoke unit’s HTR- output connects to the switch control input, and the switch’s ground connects to the power ground. 

Reiterating, YOU ONLY NEED THIS SPECIALIZED MOSFET SWITCH FOR TESTING WITH THE Profi BOARD! In actual operation, the LokSound L adequately powers an ESU smoke unit by direct connection to the decoder’s ESU smoke unit terminals, as is the modified smoke unit described here. 

Here’s the “proof in the pudding” video:

Surrogate smoke unit in action

Please forgive the disassembled state. I haven’t finished the dead rail conversion, but this video does demonstrate battery power with the LokSound L V4.0 controlled by an Airwire CONVRTR-60 wireless receiver.

Follow up

To be pretty linear, my guesses on RFIXED and Voffset are 1.5K and 1.24V. The 1.5K came from an “off” measurement of resistance between decoder GROUND and TMP-, which is fraught with potential for error.

These values will give you the following approximate curves where the left axis is the voltage at TMP-, and the right axis is the estimated thermistor resistance.


Example thermistor T versus V and T versus R curves

Dead-Rail Conversion of a Sunset 3rd Rail Allegheny 2-6-6-6 Locomotive with MTH Proto-Sound 3


Don’t jump to conclusions; MTH did NOT manufacture the C&O 2-6-6-6 Allegheny with MTH Proto-Sound 2.0 (PS2.0) I found on eBay. Instead, it’s a brass locomotive produced by Sunset 3rd Rail (Figure 1) and converted to PS2.0 (see MTH PS2.0 Upgrade Manual). See Figure 2. The retrofit replaced the original tender QSI-OEM Digital Soundboards, the wiring harnesses for the tender and locomotive, and the Suethe smoke units and their voltage regulator board in the locomotive.

Figure 1: Sunset 3rd Rail C&O Allegheny box info

Figure 2: Original tender Proto-Sound 2.0 electronics and harness wiring

It was a well-done conversion, so I was very reluctant to tear out the tender PS2.0 control board, and the wiring harnesses in the tender and locomotive. The PS2.0 conversion used an MTH smoke unit that has both fan speed and smoke intensity controls.

Figure 3: MTH Smoke Unit with funnel

The CVP Airwire receiver boards I typically use for dead-rail conversion don’t have this level of smoke unit control. And, the PS2.0 board used a speed encoder on the locomotive motor’s flywheel to synchronize the PS2.0 board’s sound. See Figure 5 for the speed encoder reader and flywheel strip and 5b for additional electrical connections.

Figure 4: Original Constant Voltage Unit wiring with dead-rail modifications indicated

Figure 5: Speed encoder, flywheel strip, and electrical connections for the Constant Voltage Unit.

Figure 5b: Locomotive harness wiring after modifications connecting the Constant Voltage Source to Battery +/- and replacing incandescent bulbs with LED’s

All of these built-in features were pretty nice, but I still wanted a dead-rail conversion.

Hmm… Looking around, I found out that Proto-Sound 3.0 (PS3.0) had DCC/DCS control options and the wiring harnesses are the same for PS2.0 and PS3.0 boards (see for instance MTH PS3.0 Upgrade Manual). Things are looking up. Then I found a great site, Ray’s Electric Trainworks, that provides PS3.0 replacement boards and great support.

My thought was this: if I can upgrade the locomotive to PS3.0, then I can take the following steps. Jumper the PS3.0 board to DCC operation, disconnect the original rail power/communication wiring, and re-connect the rail power/communication wiring to the DCC outputs of a CVP Airwire CONVRTR-60X receiver (CONVRTR Users Guide). Easy, right? Not so fast.

Ray at Ray’s Electric Trainworks was a great help: he steered me to the right PS3.0 card that I needed for the tender, and he loaded the Allegheny sound file for me. Otherwise, I would need a bunch of DCS infrastructure to load the sound file. And, he gave me a rebate for the old PS2.0 card! Great guy.

OK, I have the PS3.0 card from Ray. The PS3.0 card came mounted in its plastic carriage that is screw-mounts on the tender chassis through pre-existing holes. The heatsink orients a bit differently between the PS2.0 and PS3.0 – no big deal – I just needed to drill a hole in the tender chassis in a slightly different place. The PS3.0 doesn’t use a Ni-MH battery, so out it went. That was a good thing, too, since I needed the real-estate for the replacement LiPO battery that would supply power to the control boards, lights, smoke unit, and the locomotive.

Locomotive Modifications

Here is a diagram of the locomotive wiring harness from the MTH PS3.0 Upgrade Manual with my annotations.

Figure 6: Locomotive PS3.0 wiring harness with modifications indicated

Since I wouldn’t be sending track power to the tender, I cut the Ground Lead and Pickup Roller Leads wires. I re-purposed them by connecting these harness wires to the Constant Voltage Unit, which is a heat-shrink blob whose input leads were cut from their original chassis connections (see Figure 5 again).

While I was at it, to reduce power consumption, I removed the incandescent cabin and headlight bulbs, and I replaced them with Yeloglo LED’s (see Yeloglo description) whose + input was in series with the Yeloglo’s 470-ohm resistor for 10-16 Volt operation. Yeloglo LED’s have an excellent yellowish output reminiscent of incandescent light.

Tender Modifications

The diagram below shows the tender’s wiring harness with my modifications.

Figure 7: Tender PS3.0 wiring harness with modifications indicated

Continuity testing revealed that the locomotive Roller Pickup Leads and the Ground Lead connected to Pin 1 and 3 of the 7-pin connector, respectively.

Note that BOTH locomotive and tender Ground Leads (both are black) that are input to the PS3.0 on pins 3 and 4 of the 7-pin connector, respectively, MUST BE DISCONNECTED FROM THE LOCO/TENDER FRAMES AND CONNECTED to the “B” DCC output of the CONVRTR. This pair of connections was the trick. I initially connected ONLY the Ground Lead coming from the locomotive (pin 3 of the 7-pin connector) to the CONVRTR, and it did NOT work! I don’t know if only connecting pin 4 of the 7-pin connector would work – I didn’t try it.

Similarly, I disconnected the red wire that is input to the PS3.0 on pin 1 of the 7-pin connector (which originally connected to the locomotive Roller Pickup Leads) from the plug bundle that connects the tender to the locomotive, and I connected it to the “A” output of the CONVRTR.

The “harness side” of the two cut wires originally going to the Pickup Roller Leads (red) and Ground Lead (black) on the locomotive, were connected to switched Battery + and Battery – (ground), respectively, to provide power to the locomotive’s Constant Power Unit.

Note: “Switched B+” means battery power coming from the Battery’s + terminal that is turned on or off with a switch (you want to be able to turn off the power!). “Battery – (ground)” means the connection to the Battery’s negative terminal that is usually grounded to the tender chassis by a battery charging plug.

A picture is worth a thousand words, so studying Figures 6 and 7 will give you the story of what wiring cuts and re-connections that are needed to convert a Proto-Sound 3.0 steam locomotive to dead-rail.

Other Dead-Rail Conversion Details

Of course, there are other aspects to the dead-rail conversion that are required. These aspects include the addition of battery power and CONVRTR connections and removal of center-rail pick-ups and electrical connections that are part of a typical 3-rail to dead-rail conversion for an O scale steam locomotive. These conversion aspects are discussed in another blog.


In summary, if you have a locomotive with the PS3.0 installed, conversion to battery-powered DCC operation and radio control (dead-rail) is straightforward once you know the few wiring cuts and re-connections you need to make. The DCC operations for this particular locomotive can be found in the MTH document “Premier 2-6-6-6 Allegheny Steam Engine .” What you preserve with the PS3.0 is good DCC functionality, the original sound, and coordinated smoke – and that’s a pretty nice combination.