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.
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.
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.
Assembly
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.
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!
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.
Let’s demonstrate using this device.
Testing
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.
Wrap-Up
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.