Trailer wiring using "current sensor"

Hey guys. Another project arrived on my plate... This time dealing with a trailer wiring for 2015 Audi Q5.

Here is the background story to get you set in the right direction. In the US trailers use some weird standard system of combined stop and turn signal. It employs so called 4-pole flat wire. Since I hate this, I added amber turn signals and installed a signal separator from 4-flat into the light. It works OK for most of the time if 4-flat is used.

All my cars so far had separate turn signals and stop lights. Hence, I thought I could actually use that. So, instead of relying on 4-flat, I added 5-flat ribbon and connector to both - the car and the trailer. It bypasses the separator and works flawlessly.

4-flat is: ground, left turn, right turn, tail. Stop operates as both turns at the same time. This is the moment the separator gets confused - if used hazard turn signals it flashes stop lights... 5- flat: ground, left turn, right turn, tail, and stop. All separate just like my cars have.

Now we are getting into the main question. We got a new car. All would be fine if the trailer wiring modules for this car were as in my older ones - simply plug and play using T-style connector, take current from the cars harness. I would add the 5-flat between the T-connectors (this way I connect to the trailer wiring, not car's), and all would be great. Both 4-flat and 5-flat are working. Unfortunately they started adding "power supply" and no T-connector anymore. Instead either splicing (I hate this one the most) or "sensor".

I know the car has separate stop and turns. What I want to do is the following.

I can see it go two ways. One: I get a hold of connectors to build my harness myself. Problem is to find the connectors... so may not be as easy.


Two: I get to build a circuit that would "sense/detect" current in the car harness and then send the signal through my custom 5-flat to the trailer. I could use the trailer wiring, sure, but it only sends the signal to the module and combines the 5 signals into the 4-flat style.

Any ideas are welcome. Could be as the wiring above - looks to me like some kind of Hall effect sensor, I suppose CT will not work on DC, right? Or maybe tapping into the harness (I may use 24 AWG wire that would be directly connected to the harness' connector pin) and sense the voltage/power and transform using a circuit and external power supply (car battery) to not draw current through the car harness.

I am not sure... I know you guys will come up with a great idea as all the other times. Like the very last one - I was looking how to use capacitor, charge, discharge rates... when I was advised to use DC-DC converter step up and down. Ideal small solution.

Sorry for long story, but I wanted to make it clear. I hope it did.

by PLP
July 01, 2019

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