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STT trailer converter

News  Data  Photos

When I installed a trailer hitch on my 2008 VW GTI I found out they do not use any of the normal brake light systems. Instead they have moved to a PWM based system. The two types of PWM systems are called STT (stop tail turn) or ST (stop turn).  Looking around the web the pre-made converters run around $100 to $120. I decided to build my own. This page is the result of the build. Total cost (including a custom made pcb) was under $40. If you just use point to point wiring it can be done for less than $30)

 10OCT09  Project finished and web page created.

Circuit schermatic

Right click and chose "view image" to get a larger version.
Circuit explanation:
- W1 - W5 are the inputs from the GTI. Note the ST terminal is not used in this case (added it in case I ever buy a car with ST lights)
- D1-D3, R4, C2, Q1 Is a battery saver circuit. Running the micro draws about 5 to 10mA. If there are no tail signals for apx 1 minute C1 discharges and Q1 shuts off taking the current draw to 0. Startup on the micro is almost instant so there is no noticeable lag between the car and trailer lights on startup.
- U1, C1, C3 are the 5V reg for the micro
- R1-R3, R5-R6 make up the voltage shifter (~14V -> 5V)
- U2 is the micro (where the magic happens)
- R8-R10, Q2-Q4, R11-R13,Q5-Q7 Power driver output stage. Note Q5-Q7 will need heat sinks.
- R14-R16, D4-D6 Status indicators (Left, Tail, Right)
- W6-W9 Connections to trailer harness.

As for mounting just about anything will do as long as Q5-Q7 are properly heat sinked. I embedded mine in a block of epoxy but a project box would have worked just as well.

Download the full package (micro code, bill of material, logics, and PCB data) here GTI_STT_CONVERTER.zip.  

Also see the photo album for more details on how the circuit works and construction notes.