The Berkshire Driving School  
Trailer, Minibus & Taxi tuition
  105 Sutherland Chase, Ascot. SL5 8TE
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Berkshire Driving School - Trailer Socket Project

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This website has been published formost open communication between towing vehicles and trailers and an improvement on the multi-pin system currently used to control and power lights and equipment on trailers and semi-trailers and to replace the break-away cable on smaller trailers

Edited by John Silvester
Project leader

First draft 30 November 2003
Latest update 29th June 2014

Project funded by:

105 Sutherland Chase
Berkshire SL5 8TE

Version 01.06.14


At the beginning of the century proposals were discussed to improve radio communications by re-allocating frequencies and free up parts of the spectrum for short range communication. One obvious application of this is the control of trailer lighting. This website is published to prevent companies patenting this simple but useful application and inhibiting further development. It is hoped that an open standard could be developed.

This project was initiated by John Silvester, owner of The Berkshire Driving School. This business trains clients to draw a trailer and pass a driving test with a trailer. It has been found that the conventional multi-pin trailer plug connecting the electrics has frequently been unreliable and an improvement was needed. John Silvester has had success in the past with electro-mechanical projects. He aims to assemble a team capable of developing a replacement for the trailer socket which will become the new standard for powering and controlling trailer lighting and equipment. The present standards do not allow significant information to be passed back to the towing vehicle and this project is designed to address this shortcoming.

Contributors to this project

John Silvester – project leader
Neil Manuel – logic designer

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Early Developments

Project Update - June 2014

An early prototype has been developed to act as 'proof of concept' using a pair of Arduino Pro Mini boards for control and Ciseco XRF 868MHz modules for the radio link plus a 'no-name' chinese 5v power modules to provide logic power from 12v battery.

The Arduinos were chosen for the huge worldwide programming support they have and the radio modules were chosen because they were functionally identical (apart from frequency) to, but half the price of, Xbee modules (and are made and supported in the UK).

Car Module
The 'car' module

The 'car' unit was fitted into an old headlamp package box and included some buffer resistors to reduce the lamp outputs from the car's trailer socket to safe logic levels. For this demo the 'car' unit was plugged into the car's trailer socket, picking up its 12v supply from the permanent battery output on pin 9.

Trailer Module
The 'trailer' module

The 'trailer' unit was fitted into a die-cast box and included a suitable short-whip aerial, a set of power MosFET transistors to work the trailer's lights and a trailer coupling socket so the trailer could be 'wirelessed' without modifying anything. The 'trailer' unit was powered from a small 12v 'burglar-alarm' type sealed lead-acid battery plugged into the side of the unit.

Individual parts were tested but this demo was the first outing for the complete kit and, as you can see (much to my delight) it worked first time. As the video shows there is no visible delay between the car's rear lights and the trailer - the communication link is that fast!

The video shows the 'car' unit placed on the boot lid followed by a monitor setup on breadboard on the path - if you look closely later on you can see LEDs on the monitor flashing the same as the car and trailer lights. At the front of the trailer you can see the die-cast box with the standard trailer lead plugged into it, the black aerial pointing up and the small battery alongside.

Update: One thing we noted was that the car no longer recognised a trailer was attached - we need to determine how the trailer electrics in the car test for a trailer plugged-in so when a radio link is established the car will be told 'there is a trailer connected'.

Update: June 29

A dummy load circuit board was constructed to place a switchable 27 ohm resistor (representing a 5 watt bulb load) for each of the trailer's seven circuits. Upon connecting the board to the car's trailer socket the car identified a trailer was present when the load was switched on and absent when the switch was turned off. This was good progress. Each load resistor was connected through a removable jumper to try to determine which which 'lamps' are used to detect the trailer.

Eventually it was determined that there only needed to be a resistor representing both of the indicators for a trailer to be considered as 'connected' however now the remaining (disconnected) lamps now show up as 'trailer bulb failed' alarms. So, to tell the car a trailer is fitted and to clear bulb failure alarms, all 7 lamp circuits need a dummy load. Even better progress - but annoying.

Next tests will need to determine (a) if we can use a higher value resistor bank so there's less power (and heat) generated when all lamps are 'lit' but still be identified as trailer present and all bulbs ok and (b) see if the load resistors can individually be turned on and off with a set of MOSFETs as 7 relays will be big and power hungry (for an Arduino to operate). This would then allow the trailer to monitor its own lamps e.g. failure of a trailer fog lamp would be reported back to the car which could disconnect the fog lamp dummy load causing the car dash to show 'failed trailer fog lamp' correctly.

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The present standards

The present standard for connecting a trailer to a vehicle in order to power electrical devices on the trailer is via a multiple pin connector. This connector typically has seven conductors arranged within a round shell. This shell protects the pins from the ingress of dust and water and prevents the conductors from shorting out should they inadvertently become in contact with the vehicle or trailer chassis. One conducting pin is used for one or a few vehicle lights which illuminate together when voltage is applied to that conductor. The many trailer lamps require many conductors. One conductor is used as an earth and carries the conventional current back to the vehicle. This conductor is typically no larger than the other conductors and this restricts the current carrying capability of the connector.

The National Marine Electronics Association (NMEA) has introduced standards which allows electronic equipment from various manufacturers to be easily integrated into a yacht and have compatability with each other. There is no move at present for car manufacturers to follow suit and adhere to one controlled area network (e.g. CANBUS) standard. A pan-national standard would allow trailer manufacturers to design more sophisticated, reliable and safer products.

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Current Wiring Diagrams

British system

Looking at Rear of Socket, Front of Plug
Pin Colour Function
1 Yellow Left Indicator
2 Blue Fog Light
3 White Earth
4 Green Right Indicator
5 Brown Right Side Light
6 Red Brake Lights
7 Black Left Side Light
British Plug

Continental system

Looking at Rear of Socket, Front of Plug
Pin Colour Function
1   Left Indicator
2   Fog Light
3   Earth (35 amp)
4   Right Indicator
5   Right Side Light
6   Brake Lights
7   Left Side Light
8   Reversing Light
9   Permanent Live
10   Ignition Live
11   Earth (35 amp)
12   Coupled Trailers*
13   Earth (35 amp)
European plug

Edited: Our original 13-pin listing repeated the first seven colours from pin 8 but standards do not specify wire colours for the trailer cable. (Each trailer installer seems to use their own colour scheme.)

* Hardly anyone implements this 'coupled trailer' pin and, most often, there is not even a pin 12 fitted to the plug or socket.

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Shortcomings of the present system

Please note all information on this site is as correct as best endeavours can make it (Errors and Omissions excepted).
Please report website problems to webmaster Neil Manuel
Website revised on Saturday 24th March 2012. Last edited on Wednesday 28th June 2017.