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.