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Caravan Lights Test Box

Category: Caravanning
12 Jul 2018
Written by Andy Hits: 2365

With a motor mover to get the caravan on and off the drive, I got concerned about stopping on the main road through our village and having the caravan moving, but disconnected from the car electrics, so no lights showing.
I know it is a big white box, and should be obvious, but I also know that there are a number of drivers that seem to interpret 30 mph differently to me!

I found online some trailer light test boxes, but they seemed from the descriptions I could find to be more like the network cable testers I've got, and will light up an individual circuit, or perhaps try and measure that there is resistance less than infinity, and more than a short circuit, and light up a status light.  Useful, but not to try and provide some indication of what the caravan in the road is trying to do

After starting this project, I did find some videos on Youtube of others endeavours, like these: Trailer Lights Test Box and DIY Trailer Plug Light Tester Tutorial.  I took some of the ideas on board, but made it my own.

First off I read up on the 12 pins used by the standard caravan cable.  I thought that an easy way to power the box would be to use the caravan battery via pin 9 (+ve) and its associated ground on pin 13 (-ve).  My multimeter probing quickly proved that there was nothing live there.  I guess diodes and such like prevent the car from drawing power from the caravans leisure battery when starting the car.  I ended up hardwiring an SAE connector, which I can then connect to a plug I've run from inside the caravan that is on the 12v sockets circuit as a power source.

For switches, I got an 8 pack of rocker switches with built in LED.  This saved having to wire in a light per switch so it is easy to see what is on.  I also bought a 13 pin trailer socket, a length of 12 core trailer wire, lots of crimp spade connectors, and finally a motorcycle style variable speed flasher unit so that I could get the indicators to flash, rather than be constantly on.

I used some nice big block connectors to act as a common contact for positive and negative parts of the circuits, as the switches needed a negative connection for the integrated LED, and as the rocker switches needed a 20mm hole (along with a small key to stop them rotating) I got a set of stepped drills to make the right sized hole, rather than buying a single 20mm spade bit

This is my circuit diagram, with a master switch, and the integrated LEDs on the switches included.  The wires which appear to go nowhere go to the trailer socket, and then to the lights on the caravan, and their ground is shown too

I did some testing before I started trying to fit it all in the box, and then started drilling the holes.  My next problem was trying to fit it all in the box, as it was quite a few wires in a smallish box, as I still wanted it to be comfortable to hold in the hand.  A bit of chopping up of block connectors and routing the wires round the switches and it did fit in, and this was the finished result

Working clockwise from the top, the switches are:

Reverse, Right indicator, Right side/tail light, Fog light, brake lights, left side/tail light, left indicator
To the rear is the trailer socket, and the master power switch is on the lower edge.  The wire coming from the back is the SAE connector

My shopping list of parts is was follows:

Item Quantity Cost
Trailer Socket 1 £5.99
Trailer Wire 1 £4.94
Rocker Switches 1 £5.99
Electronics Box 1 £7.85
Flasher Relay 1 £5.25
Block Connector 1 £2.59
SAE connector Pack 1 £10.99

If I'd been happy to wait for shipping of parts from China rather than UK based parts I could have saved a little

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