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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 12th January 2020

Project funded by:

105 Sutherland Chase
Berkshire SL5 8TE


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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As work progresses with the development of the trailer link system we will try to document our progress and research...

Project Update - 12th January 2020

Again it's been a while since the last update (we all still have day jobs and too many distractions) but we're getting somewhere...

John has some medical problems he needs to deal with so the project is on hold or slow-go for a while.

Having already tried "traditional" i.e. cheap Bluetooth serial modules I have been researching newer, more up-to-date Bluetooth Low Energy and mesh systems - still a bit too new on the market so not too much of the "this is really how to simply do it" help guides around. These new units seem to offer greater range and more functionality in the modules used but seem to be unduly complex in their setting up. All too often including Internet connectivity - which we totally do not want here.

We still haven't resolved the thorny problem of reliably "fooling" the tow car that a trailer is attached - we keep coming back to the tow car's trailer interface needing to use filament lamps to detect a trailer is connected. The most common "Ebay" solution is to used some ridiculously low ohmic dummy load resistor(s) - which get too hot - and will still show some trailer lights as "failed" unless ALL the light lamps have been fitted with resistors. So, still thinking our way round that.

Project Update - 17th April 2019

Again it's been a while since the last update (we all have day jobs too) but we're getting closer to something interesting...

John has fashioned a proper looking light board, albeit from wood, using commercial LED trailer/caravan approved lamp assemblies with additional number plate lighting and mounting bolts with wing nuts to hold the number plate. I (Neil) have been working on replacement electronics and ruggedised fitting within the light board body. It looks really good from the road side of view...

trailer light board outside
In the top corners of the board we have some aftermarket car reversing distance sensors to help with reversing - electronics not fitted yet but the one we have uses its own wireless link to a car dashboard display so we only need to fit the electronics and power it from the reversing lamp circuit. In the bottom corners we have some sockets. There is an 8-pin on the left intended to allow wired trailer lamp operation if needed and a 2-pin on the right for DC power and, ultimately, charging.

On the inside we have designed some 3D-printed plastic "rafts", taking advantage of the lamp and number plate mounting bolts, that hold the wiring blocks and electronic circuit boards securely...

trailer light board inside

We have a plan to put a high capacity LiPoly battery inside the board using a 2-pin waterproof plug to provide charging input - however this is still in design and development so will be a while before it is implemented. Until then the 2-pin plug can supply 12v DC power from an external small lead-acid battery box to power it.

trailer lamp right

The lamp wiring "rafts" use standard 6-way terminal blocks merging the 6-cores from the main lamp with the 2-cores from the number plate lamp and connecting the 8-core internal wire linking all the lamps. A bridging circuit board (not shown) next to the left lamp uses diodes to protect the wired car trailer feed input from the trailer board lamp driver and there are protection diodes on the lamp drivers for when the car is driving the lamps with a wired connection.

trailer electronics

This picture shows the redesigned main control board incorporating a Bluetooth module, Arduino Nano processor module, MOSFET lamp drivers and output protection diodes. The white temporary power feed wire coil with banana plugs were used for testing.

Still very much a work in progress but the unit is working and in a state to demonstrate a functional radio-linked trailer lighting board though more work is required to solve the car-side of things with reliable "trailer presence detection" and lamp testing/failure indications ... watch this space!

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Project Update - 3rd December 2018

At the time of writing, cars are not specifically designed to work with trailers fitted with LED's. This can cause problems because Incandescent bulb filaments are inefficient - hot filaments break with excessive vibration that typically occur with trailers needing a bulb failure monitoring system.
A car can determine that a trailer is attached and the bulbs are working by one or more of these four methods

  1. Sending a fast pulse through each bulb detectable by the car but such that the energy in the pulse is insufficient to light the bulb
  2. Putting a low voltage down the wires sufficient that it is detectable by the car but insufficient to light a filament
  3. Comparing the current flowing through the bulbs on either side of the trailer.
  4. Passing a high current down the indicator wires. If a high current can be detected then the car computer is aware that a trailer is present and modifies the car for operation in trailer mode.

Now LED's differ from bulbs because -

  1. They are very efficient and light up when the pulse (1) above passes through the LED's - they flash
  2. LED's need a minimum low voltage before they conduct a significant current and light up. Checking continuity as per (2) above might not cause sufficient current to be detected by the car
  3. Measuring current differences as in (3) above is difficult if the currents are so low
  4. Although LED's have a high resistance when a low voltage is applied, when they do 'strike' they could take too much current and overheat unless a current limiting device is in series
  5. LED's are diodes (the D in LED) so usually conduct in one direction only.

Many companies produce a range of devices which can make a car work with LED's but since car manufacturers have not standardised on their trailer light bulb detection circuitry, a number of devices must be manufactured to suit individual manufacturers and models. Problems with trailers fitted with LED's are not uncommon as a Google search on 'canbus resistors' would show.
Typical 'canbus loads' are high wattage 6 ohm power resistors each drawing 2 amps at 12 volts and dissipating 24 watts of heat.

Incandescent bulbs and their spring holders tend to be unreliable when fitted on the rear of trailers, particularly on unladen bouncy trailers.
Note that retrofitting LED bulbs into incandescent fittings may reduce heat and power but will not address the potential intermittency aspect of the bulb socket.
Complete sealed LED designed trailer lights are the way forward since they have -

  1. greater reliability
  2. resistance to vibration
  3. a long service life
  4. greater efficiency
  5. less weight
  6. more compact
  7. the possibility of a high immersion protection rating.

Clearly there is a need for a unit which is easily deployed and which can eliminate trailer LED problems but yet not get too hot.
We are working hard to devise such a unit and allow others to share our findings.

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Project Update - November 2018

Due to other commitments the project has been on 'the back burner' for far too long. It was time to revive it and move forward.
In the intervening time the 'Ciseco' company (provider of the 868MHz radio modules) changed to 'Wireless Things' and then the company folded so those radio modules have been abandoned. In their place we're now using some generic HC05 Bluetooth radio modules. We are considering using the new standard Bluetooth Mesh modules when they become more readily available.

We would also like to consider providing other sensors on the trailer to provide warnings like tyre deflation or brake binding in addition to proximity detection (parking distance sensors) and of course rear view video. All still in the theoretical pipeline but, for now, just working with lamp control eliminating the fat unwieldy umbilical cable to the car.

The original die-cast box has been changed to a waterproof plastic box (to prevent shielding the Bluetooth signal) and the original logic board has been rehoused along with the new radio module. The unit requires a 12v DC supply for the trailer lamps themselves. This is currently provided, for simplicity, by a small sealed-lead-acid battery, typically used for burglar alarms.

trailer radio unit

Using smaller circuit boards but still using an Arduino and the new Bluetooth radio module a smaller housing has been devised that becomes part of the trailer plug itself. The new unit incorporates CanBus resistors that are only connected in-circuit when a Bluetooth link is established. The CanBus resistors will be detected by the car's electronics as 'trailer present'.

car radio unit

The unit need not be physically removed from the car when uncoupling the trailer - all that is required is to disconnect the trailer battery to break the Bluetooth link and the car's CanBus resistors will be disabled and the car's electronics will no longer recognise a 'trailer present'.

We are woking on a video demonstration of the latest build. Watch this space...

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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'.

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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).
Copyright John Silvester ©2024. Please report website problems to webmaster Neil Manuel
Website revised on Saturday 24th March 2012. Last edited on Wednesday 3rd January 2024.