Integrating the ProMiniAir Receiver with QSI Decoders using Gwire Connections

Users of the QSI Revolution or Titan series of DCC decoders may want to take advantage of these decoders’ built-in Gwire connections for radio control but cannot find the discontinued Gwire receiver. This post describes how you can use the ProMiniAir Receiver as a “drop-in” replacement for the Gwire receiver so that you can control the locomotive with the following radio-control throttles:

  • CVP Airwire throttles such as T5000 and T6000
  • NCE’s discontinued Gwire throttle
  • Our ProMiniAir Transmitter
  • Stanton Cab
  • Tam Valley Depot DRS1 transmitter

Integrating the ProMiniAir Receiver with the QSI decoder’s Gwire capability is easy.


I strongly recommend you review Greg Elmassian’s excellent post, “QSI AirWire & GWire cab.” It deals with using the Gwire receiver with various QSI decoders, which have a built-in flat flex cable (FFC) plug initially designed to connect to the discontinued and now hard-to-find Gwire receiver.

The QSI Titan FX-DO decoder is an example of a QSI decoder equipped with a Gwire flat flex cable (FFC) plug.

The ProMiniAir Receiver is highly compatible with the Gwire receiver and is somewhat more flexible with more channels and DCC-based reconfigurability. The ProMiniAir Receiver also has a smaller footprint (1.1″ x 0.76″). The modular design approach of the ProMiniAir makes it feasible to take full advantage of the QSI’s Gwire capability to use the ProMiniAir without an additional amplifier.

Integration of the ProMiniAir Receiver using Gwire Connections

The only additional item required is a Parlex HF05U-03-ND 5-position flat flex cable (FFC, 5 Position, 1.00mm conductor spacing, 3″ length, one side connector on both ends), found at Digi-Key here, that plugs into the QSI Gwire plug. Other brands of 5-position flat flex cables are available, notably from Molex.

The Parlex Flat Flex Cable (FFC) 5Pos 1.00mm, 3″

The ProMiniAir Receiver is easily connected to one end of this cable by soldering 30 gauge wires that match up well with the dimensions of the flat flex cable, as shown below.

The soldered connections join 30 gauge wires to one end of the flat flex cable. Only three connections are required: GND, +5V, and DCC.

The other ends of the 30 gauge wires are soldered to the ProMiniAir Receiver, as shown below.

The three 30 gauge wire connections to the ProMiniAir Receiver. Optional connections to an LCD were soldered in place for diagnostic purposes, and these LCD connections are NOT required for final operation.

No DCC amplifier is required because the QSI decoder creates DC voltage from its onboard rectifier. The QSI Gwire connections provide the +5V/GND needed to power the ProMiniAir Receiver, which sends 3.3V Logic DCC back to the QSI decoder.

The QSI decoder must be configured to accept DCC from the Gwire connection by setting “indexed” CV56.1 to 1. This means first setting CV49 to 1 and then CV56 to 1. See Greg Elmassian’s excellent post for more details. Once the decoder accepts this setting, it will only respond to DCC from the ProMiniAir receiver until the decoder is reset, as Elmassian’s post described.

My only addition to Elmassian’s information is that a firmware update may be needed before the decoder will “accept” this setting. I could not successfully update CV56.1 on a QSI Tital FX-DO I used for the testing shown in this post until I updated the decoder’s sound file.


The photo below shows the demonstration setup.

Demonstration setup

The Digitrax DCS52 DCC throttle supplies Track Right/Left DCC, which only powers the QSI Titan FX-DO and the ProMiniAir receiver via the Gwire flat flex cable. The Airwire T5000 throttle will then transmit DCC commands received by the ProMiniAir Receiver, sending 3.3V Logic DCC back to the QSI Titan FX-DO that controls sound and the motor. DCC commands from the DCS52 DCC throttle are ignored.

Below is the video demonstration

Demonstration of the ProMiniAir Receiver using the QSI’s Gwire connections. No additional amplifier is required. Note that the decoder is responsive only to the Airwire throttle. The motor displays an observable “jerk” when changing directions.


I have just recently started using QSI Titan decoders. They are challenging to find but produce fantastic sounds and are very flexible. The modular design of the ProMiniAir Receiver makes it very simple to take full advantage of the QSI decoder’s Gwire capability. I can provide a ProMiniAir Receiver integrated with the modified flat flex cable (FFC) for $34.99. Plug the flat flex cable into the QSI decoder’s Gwire connector, program CV56.1=1, and you are ready to go with radio control!

My thanks to Greg Elmassian for his very useful posts.

A Magnetic Power Switch for Large Scale Locomotives

Recently, I was modifying an O-Scale Sunset 3rd Rail L-105 locomotive for combined 2-rail track power or battery-powered radio control (bprc or dead-rail), where the switches were inconspicuously located at the front of the tender but were challenging to reach once the tender and locomotive were coupled.

Inconspicuous, but inconveniently-located 2-rail/dead-rail switches on a L-105 locomotive

To make matters worse, power to the QSI Titan FX-DO decoder mounted in the tender must be cycled OFF and quickly back ON to activate the smoke unit. Given the location of the switches, this operation just wasn’t possible. What to do?

Well, how about using a “Reed Switch” to turn the power on or off using a magnet? In my research, I found numerous model railroad applications of Reed Switches inserted “inline” with power. However, Reed Switches cannot typically handle the large currents (3A or more) we encounter in O-Scale and larger. So, direct inline use of a Reed Switch was not feasible.

This site mentioned using a Pololu Big Pushbutton Power Switch that can handle large currents (up to 8A), so I thought I might be able to connect a “normally-open” Reed Switch to the convenient “push button” inputs on the Pololu Switch shown below.

Close-up of the Pololu Big Pushbutton Power Switch, MP, showing the intermittent pushbutton input.

This short post shows how to connect the Pololu Big Pushbutton Power Switch, MP, which can handle up to 8A, with a normally-open Reed Switch for convenient magnetically-controlled power ON/OFF.


As shown in the following three photos, assembly is straightforward. The only caveat is the Reed Switch is somewhat delicate, so bending the leads to the easily-broken glass capsule requires gripping the lead between the capsule and the bend, as shown in the photo below.

Technique for bending the leads to a Reed Switch to prevent breakage

Below is the finished device. Only eight solder joins are needed, and heat shrink is used to cover the Reed Switch’s solder joints. The long wiring lead to the Reed Switch provides mounting in a convenient location, including INSIDE non-magnetic metal shells!

Power Switch/Reed Switch Connections

The close-up below shows the straightforward connections to the Pololu Big Pushbutton Power Switch, which handles up to 8 A. Larger-capacity switches (up to 16A) are available from Pololu.

Close-up of Power Switch/Reed Switch Connections

Let’s demonstrate using this device.


The video below is the “proof-in-the-pudding” showing that the Reed Switch controls the ON/OFF of the Pololu Big Pushbutton Power Switch.

Demonstration of the Pololu Big Pushbutton Power Switch controlled by a Reed Switch to turn the power ON and OFF with a magnet. A battery supplies power, and the DC output is provided to a ProMiniAir receiver/amp that generates the DCC output shown on the oscilloscope.


So there you have it: a simple magnetically-controlled switch that handles large currents.

An added benefit is the Reed Switch can be mounted INSIDE a non-magnetic metal shell, such as brass! Magnetic fields pass through these metals.

A Simple AC-to-DC Converter

Sometimes folks want radio control of their locomotives but prefer to use track AC (including DCC) instead of a battery to provide DC power for the radio receiver and amplifier. This post shows how to repurpose the DCC converter PCB customarily provided with the ProMiniAir transmitter to convert track AC to DC power for the ProMiniAir receiver.

DCC Converter Modifications

The original purpose of the “DCC Converter” is to use a DCC throttle’s Track Right/Track Left output and convert it to 5V DCC and 5V power for the ProMiniAir transmitter. See the Figure below.

The original purpose of the “DCC Converter” is to provide 5V power and 5V DCC signals to the ProMiniAir transmitter.

One of the strengths of the modular approach used for the ProMiniAir transmitter and receiver is that you can “repurpose” components. The DCC Converter can be modified to use the AC track input to provide filtered, higher-voltage DC power.

Below is the repurposing idea: add a large capacitor (in series with a 100 Ohm resistor and a 1N4001 diode) across the “+” and “-” terminals of the rectifier and route out the rectifier’s DC output. Smaller onboard capacitors (10uf and 100nf) also filter out higher-frequency noise that large capacitors sometimes do not effectively filter.

One end of a 100 Ohm resistor and the + terminal of a 1N4001 diode are connected in series (and in parallel to each other) to the capacitor’s + terminal. The other end of the 100 Ohm resistor and the – terminal of the diode are connected to the rectifier’s + terminal. The capacitor’s – terminal is directly connected to the rectifier’s – terminal to form the DC ground. My thanks to for the idea of adding a resistor and diode in series with the capacitor.

When the throttle is turned on, the 100 Ohm resistor prevents an “in-rush” short circuit that might cause the throttle to cut off. When charging, the 1N4001 diode is reverse-biased with a large resistance. If AC power is interrupted, current flows out of the capacitor through the low resistance path of a forward-biased 1N4001 diode to maintain DC power output.

How keep-alive works. The resistor regulates charging, and the diode regulates discharging.
A large capacitor (along with a 100 Ohm resistor and a 1N4001 diode) can be added to the DCC Converter to output heavily-filtered DC power.
Connections between the AC-to-DC Converter with a large keep-alive capacitor and the ProMiniAir receiver/amp. The switch is NOT required if a large keep-alive capacitor is not used.

In fact, it is possible to forgo the large capacitor since these onboard capacitors do a pretty good job of “cleaning up” the DC output of the rectifier. This is a good option if space is at a premium.

The filtered DC output can now provide DC power to a ProMiniAir receiver/amp, just as a battery would. If the added capacitor is large enough, it will function as a “keep-alive” capacitor many DCC decoders use to prevent track power interruptions.

An Example

The photo below shows a real-world example of the conversion using a 10000uf “keep-alive” capacitor originally used with a Zimo decoder. The size of the capacitor dominates that of the DCC converter!

A modified DCC Converter using a very large 10000uf “keep-alive” capacitor.
Close-up of the modified DCC Converter

The oscilloscope trace below demonstrates the ability of the modified DCC Converter to produce clean DC power for your ProMiniAir (or other) receiver.

Track AC input and filtered DC power output from a modified DCC Converter with an added large (10000uf) capacitor

Note how “clean” the DC power output is (13.8VDC). Square wave track inputs at 16.8V are a severe test because they produce frequencies at odd multiples of the square wave’s frequency, e.g., at 6KHz, 18KHz, 30KHz, etc. for the example above, but very little of these frequencies “bleed through” to the DC power output.

Simplifying further, we can use the DCC Converter without an added capacitor, relying on the onboard capacitors to filter the rectifier’s “+” and “-” output. This option might be useful if space is at a premium.

Modified DCC Converter with NO capacitor

The DC power output is still clean, but you lose the “keep-alive” that a large capacitor provides.

Track AC input and filtered DC power output from a modified DCC Converter with NO added capacitor


So there you have it – the DCC Converter can be slightly modified to provide filtered DC power and “keep-alive” capability. The modified DCC amplifier with a sizeable keep-alive capacitor costs $10 + shipping. Without the capacitor, the modified DCC Converter is $7 + shipping.