Berkshire Driving School - Trailer Socket Project
Project feedback to : email@example.com
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
First draft 30 November 2003
Latest update 29th June 2014
Project funded by:
THE BERKSHIRE DRIVING SCHOOL
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
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).
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.
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.
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.
Current Wiring Diagrams
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.
Shortcomings of the present system
- Large, often exceeding 800mm diameter
- The multiple pins and sockets are difficult to clean
- Limited current carrying ability for all the pins
- The small pins deform when used regularly and contact is lost
- The plug is difficult to connect in the dark since it needs to be correctly orientated. Being round, it is often difficult for the operator to find the correct orientation
- The pins of the British system are located mainly by friction and may be easily pulled out
- The socket is usually located beside the tow-ball and the cable must be slack enough to accommodate a bend between trailer and drawing vehicle. Physical movement renders copper cabling brittle and subject to fatigue.
- The slack cable may touch the ground when the trailer bends the opposite way
- Many contacts have to be connected and these are in the plug and socket which is subject to road dirt and spray thrown up by turbulence at the back of the towing vehicle. Corrosion of the contacts may affect reliability
- There is no provision for monitoring of equipment on the trailer
- A separate break-away cable has to be attached
- The current standard has a shutter which has to be lifted. This means that two hands are needed to effect a connection.
- Current standards do not convey sufficient information to an engine management system which modifies engine, suspension, braking and driver information screen which displays relevant data for the driver
- Retrofitting cables through body cavities and behind trim is a very time consuming and expensive process
- Wiring is aesthetically displeasing
- Many designs of rear trailer light clusters employ precarious mounting methods for delicate filament bulbs. This is frequently a source of failure, especially when using a trailer unloaded when the suspension is particularly harsh. Solid state (LED) lighting is much more robust. When annual testing is introduced, fail rates will be high with current designs.
- Modern vehicle design reduces mass by using the minimum quantity of cabling. The present standard does not comply with this design philosophy.
- Smaller trailers (under 750kg) need not have brakes. In this case a short chain or wire rope must join the trailer to the towing vehicle such that, in the event of a failed coupling, the trailer tow cup will be prevented from dropping enough to touch the road.
- A pair of robust connectors carry current to the trailer.
- The robust connector should be stout enough to act as a break-away cable
- The power cable must be reinforced to act as a break-away cable.
- The connector should be placed directly underneath the tow-ball, above or close to the towbar
- The connector should locate positively and be unlikely to be disconnected should the tow-ball coupling fail. The connector will then act as the break-away cable attachment.
- The standard should allow for future safety devices to be included in the specification.
- Bolt on tow balls
- Swan neck tow balls
- Pin and jaw couplings
- Light emitting diodes
- Antilock trailer brakes
- Acoustic parking sensors
- Antilock brakes
- Electronic stability control
- Overheating bearings
- Rear view cameras
- Tyre pressure monitors
- Charging of trailer batteries
- Monitoring of refrigeration compartments
- Unauthorised access alarm
- Alarm warning of faulty coupling
- Traction motors on the trailer
- Any conceivable electronic additions
- Extending of information to the engine management system so that the parameter of the towing vehicle will be optimised for towing that particular trailer.
- Tolerances are specified
- The neck dimensions are specified
- The metal used is restricted
- The origin of the metal must be traceable
- Corrosion resistant finishes are restricted
- Clearance round the ball is specified
- Markings and labels are mandatory
- The device must be tested independently and gain approval Generally this is a million pushes and pulls at its rated load
- The break-away cable has to exceed standards
- Mounting hole centres 190mm apart
- 15mm diameter
- Bolt head socket diameter 24mm
- End radius 250mm centred around bolt holes
- Allow 16mm diameter for socket clearance. This clearance may be 38mm deep before it would interfere with the torque wrench.
- A mild steel mounting plate for electrical connections is usually sandwiched between the bar and ball.
- Mounting the connector below or close beside the ball will reduce the amount of slack needed to accommodate the vehicle turning.
- This position will also offer some protection should the driver not completely reverse accurately when re-coupling.
- The downside of this arrangement is that the operator will not be able to see the position of the connector below the tow-ball.
- This need not be a problem if the connectors lock together easily and a guiding device is part of the design.
- Exposed contacts should not become completely live until hidden during coupling.
- Only two pairs of contacts are needed, live and earth.
- They will be at a low voltage and low current capability until the trailer transmits a signal back to the towing vehicle.
- The socket should be able to resist the maximum trailer weight, say 1.75 tonnes.
- Connecting and removal should be a one-handed process.
- Type 631 stainless steel has high strength
- Super dolce steel by the Sumitomo (SEI) Steel Wire corporation is as strong and more corrosion resistant
- 1x19 stainless steel wire 4mm diameter will have a breaking strength of about 1.5 tonnes
- Sta-Lok structural cable systems can be used to connect to the wire.
Units are mounted on the towing vehicle and trailer and produces coded radio signals. This is used to control the lights on the trailer. Signals also pass in the opposite direction and can be used for confirming the code used between the trailer and towing vehicle.
The standard should be compatible with:
The design should allow for the introduction of:
Tow-ball mechanical design standards
The tow ball is 50mm diameter
Design notes (mechanical)
Trailers towed by cars and leisure 4WD leisure vehicles in Europe use 50 mm
The absolute maximum towing weight for such vehicles is 1.75 tonnes.
Non-European countries may have higher permitted loads.
Car manufacturers designed towing equipment are often equipped with swan-neck attachments.
After market and leisure 4X4 vehicles usually use tow-balls mounted on a tow-bar or chassis plate.
The standard dimensions are:
Ideally the proposed connector would be cast as part of the tow-ball.
When the contacts touch, current flowing to the trailer starts the "hand-shaking" process. The towing vehicle allows the full battery voltage and current capability to be provided to the trailer.
Disconnecting the socket shuts of the main power to the connector.
The rear of a motor vehicle is an inhospitable environment. The two contacts needed for this project will be less susceptible than the current multi-pin arrangement but some environmental protection should still be incorporated into the design.
Wire and fitting standards manufactured to the forma B.S. 1290: 1983
now to the new standards: bs. 302.pt2: en 12385. bs en13414-1
B.S. 302: Part 1 1987
B.S. 302: Part 2 1987
All corresponding Test certificates supplied
Trailer and towing vehicle Number Plates
The ability for a numberplate to be read is covered by Statutory. Instrument 1971 No 450 - The Road Vehicles (Registration & Licensing) Regulations 1971. Part 1ll - Exhibition of Licences and Registration Marks states the following:
- The provisions of this regulation shall apply to vehicles, other than works trucks and agricultural machines, first registered on or after 1 October 1938.
- The registration mark of the vehicle shall be fixed and displayed on both the front and the back of the vehicle, so that in normal daylight the letters and figures are easily legible from every part of the 'relevant area', the diagonal of the square governing the 'relevant area' being 75 feet.
- For the purposes of this regulation, the expression 'relevant area' in relation to a registration mark on a vehicle means the area contained in a square described on the ground, either in front of or behind the vehicle, where one corner of the square is below the middle of the registration mark and the diagonal of the square from that corner is parallel to the longitudinal axis of the vehicle, but excluding any part of the square within 10 feet of the vehicle - see diagram.
It is an offence to allow the numberplate to be obscured. This is now a fixed penalty offence, and therefore more likely to be policed. Obviously, this is of some concern to towbar fitters and to drivers of vehicles with towbar attachments.
Electrical design standards (draft)
- The lead content of solder is reduced
- The device must comply with standards concerning the transmission of electrical signals
- Factors include transmission frequency, Baud rate and Transmission power
Design notes (Electrical)
- Transmission is by serial means
- Nominal bit rate 9,600 bps
- ASCII, divide bps by 10 to get cps (characters per second) because 1 start and 8 data and 1 stop bits ASCII will be 960 bytes (or characters) per second
- Half duplex more than halved the data rate since each unit must wait before receiving before transmitting so signals don´t collide and Tx must stabilise
- Therefore 200 bytes per second in reality (20ms)
- Full duplex speeds this up
Possible use of XBee 2.4 GHz modules
- ZigBee is the name of a wireless communication standard.
- It is a development of the virtually defunct Home RF (replaced by Wireless fidelity or Wi-Fi)
- The standard is supported by major players and they have formed a consortium called the Zigbee Alliance.
- XBee is the trade name of MaxStream. These units were new to the market at the start of this project.
- The units are half duplex ie they alternately transmit and receive.
- The modules are Bi-directional (most modules only transmit in one direction) It has 16 separate channels so 16 networks can be addressed.
- Current consumption is very low so heat build up will not be a problem.
- They have transmitting power which can be set within the legal European standards (10mW)
- The units also are low cost.
- Transmission speed is quite low but this is not a problem in this application.
- Each unit has a unique addressing system whereby the units have unique factory configured serial number.
- The modules have in-built data-packet and error-checking to ensure reliable data transmission.
- They can be easily be interfaced with computers and microcontrollers.
- A PICAXE-XBee module is also available
Specification of the XBee module as defined on their website
- Range: in the order of 30 m
- Transmit power: 1mW
- Data rate: 250kbps
- Receiver sensitivity: -92dBm
- Supply voltage: 2.8-3.4V
- Typical transmit current: 45mA
- Typical receive current: 50mA
- Power down current: less than 10 micro amps
- Frequency: 2.4gHz
- Dimensions: 244mm by 276mm
- Operating temperature: -40 to 85C
- Number of pins: 20
- Number of channels: 16 direct sequence channels, software selectable
- Agency approvals: FCC Part 15.247 Industry Canada (IC) Europe
This is a simplified view of the actual XBee hardware shown in the photo.