16mm Projects

Guards Van Electronics upgrade : Rob Holmes

We’ve had a small guards van for a while at the SLR which has needed some TLC. This month I’ve finally got around to doing it.

The guards van had a twin AA battery pack currently which powered a small red LED rear light via the switch. This looked good, but as the van was open you could see the battery pack and wiring. So I wanted to replace the electronics, all bar the LED. The first step was removing the old battery pack and wiring, remembering which LED leg was positive and negative of course!

The new electronics will be based on a Li-ion battery cell (The same size as AAA but 3.7V) and a USB Type-C (As found on phones) to allow easy charging and to protect the Li-ion cell. Protecting Li-ion cells is important and using a board like this makes using them very simple indeed.

As a hole was left from the old switch location, I widened it out so the board could fit up inside the van itself, with the charging plug underneath. The hole was larger than the board as I’d need wiring to go in and out as well, plus it made fitting it in easier later once wired up.

I then started to wire up the board to the now two LEDs. I decided to add another warm white LED on the inside of the van to provide some light for the guard. These USB Type-C charging and protection boards look complex at first but are in truth easy to connect up. The battery + and – connect to B+ and B-, then your load (In this case the LEDs via a switch) connects to the Out+ and Out-. The board should always be powered so it can monitor the battery, but uses very little power at all to run, so it won’t drain the battery. I took the negative side of the Out- connection on the board to my slide switch, then up to the two LEDs. The Out+ side of the board goes to a 30ohm resistor which protects the LEDs. Remember that these Li-ion batteries are 3.7V and not 1.5v or 1.2v despite the fact they are AAA in size. I planned to secure the USB board using two brass posts which go into the wagon frames. The board has two input pads with holes either side of the USB. I’m not using these for power (As charging will be by the USB port) so I’ll use them to hold the board in place, by soldering the bars to the pads once I’m ready to fit it.

The USB board sits up into the guard’s van body with the cables up to the two LEDs heat shrunk to keep them tidy in the corner. I’d come up with a plan to cover this board over later on, but already you can see it’s a big improvement.

Under the van, the battery holder (One made for AAA batteries) was installed and connected to the two spare wires shown above. The switch was glued to the side of the battery holder but the holder itself isn’t glued in. I had planned to, but I was happy with the friction fit, making changing the battery easier if needed. The battery used was one I had spare and holds 350mah, plenty for two LEDs.

Once the electronics were finished, I turned my attention to some details and hiding the USB board in the van body. To do this I used a luggage box from the SLR Models scrap bin (It’s handy having it just the other side of your workbench) which was hollowed out, so the board fitted up inside. Once painted and weathered it joined a few more items inside the van, now I had plenty of room without all the wires.

The end result is a lovely little guard’s van which will run for hours without needing new batteries. When the light does finally go out, simply plug it in to your phone charger and when the light turns blue on the charging board, it’s ready again!

I would ask what Stan the guard thinks of it, but he’s sleeping on the job again.

Rob Holmes – Southgate Light Railway

August 2021

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