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Cct to differentiate between 5v and 12v?

johns11

New Member
I decided to do towbar wiring on my wife's 7 yr old VW Tiguan so I can use it to tow my small boat.

It turns out that the tail light and brake light are actually the same lamp on the same circuit (seriously!?!), sometimes being supplied with 5v (for when tail lights are on) and sometimes with 12v (for brighter lamp when brake is pressed).

I need to split this single input supply of 5v/12v into separate outputs, depending on if the input is 5v or 12v, to supply my trailer's separated 12v LED lights. In other words:
1./ when input = 5v, output 1 is livened (at 12v – I have a separate 12v supply available)
2./when input = 12v, output 2 is livened (and output one can either be on or off)

Does anyone have any ideas on how I can achieve this? Can I use something like a 5v and a 12v solid state relay for example?

Thanks a lot
John
 

Pommie

Well-Known Member
Most Helpful Member
If you use two 6V relays with a 6V zener in series with one of them then they should switch at the different voltages. You'll need a series resistor for the relay without the zener.

Mike.
Edit, you may need a transistor for the 5V signal relay.
 

Diver300

Well-Known Member
Most Helpful Member
VWs often have PWM signals on the tail lights, so that they are on part of the time to appear dimmer. Your car may do that, so it may not work to distinguish between 5 and 12 V. You would have to smooth the signal first.

If you have LED tail lights on the trailer, and it is PWM, you could just wire up to the trailer brake lights and it would work, as the trailer lights would also be dimmed with PWM.

Some people, me included, find the 100 Hz PWM that VW use intensely annoying. Whoever decided that it was OK didn't ask enough people or investigate in enough conditions. It's worse at night, when there is movement involved, which I don't think is a particularly odd set of conditions for tail lights on a motor vehicle. I noticed the flashing / modulation first about 20 years ago, so it could well be that VW have changed things, and they are using 5 / 12 V without any PWM.

Combining brake and tail light wiring done to reduce the cost and weight of wiring.

You would probably be better off to just get the VW towing module. That will detect the trailer light current to shut off the car's reversing sensors and rear fog lights when there is a trailer connected. It will also detect blown trailer bulbs. I'm not sure about the rules in New Zealand, but I think that you have to have an indication that the trailer direction indicators are working or not, and the VW towing module will do that.
 

johns11

New Member
Thanks all for your replies, this has been awesome help.

Pommie, a simple idea that might just work. Will need to investigate.

augustinetez, Of course... the simplest solution of all but the slowest. Luckily I'm in no hurry since it's winter here. Thanks

Diver300 I didn't think of it as PWM, and there's a good chance you're right. I think this would mean the other ideas above won't work, and I don't have an oscilloscope. I really want the car wiring isolated from the trailer wiring so prefer something like a relay in between. I do have the VW towing module but no idea how to wire it in, as the specific sub-model of this Tiguan shows its a non-towbar version (even surprised the dealer when they looked that up). What that means is no towbar specific wiring/connections for that module, so I'd need to figure all that out wire by wire (tried VW forums and will relook), but decided that adding a few relays myself was going to be easier.

You guys are brilliant!
Thanks again
John
 

Ian Rogers

User Extraordinaire
Forum Supporter
Most Helpful Member
It could be even simpler if the tail is indeed PWM ( my thought as well ) A low pass filter will switch a relay on when braking but will switch off the relay when tail is used...

I would imagine the PWM will be @ 50% when on tail...

One pitfall.... Your circuit MUST NOT drain the existing circuit or the Module reports light failures
 

Diver300

Well-Known Member
Most Helpful Member
I would imagine the PWM will be @ 50% when on tail...
The one that I measured was about 10%, running at 100 Hz. That was with LED lights.

For me, those flash in a very distracting way. That was why I was looking at it with an oscilloscope.

I don't know if VW have increased the frequency, or gone back to incandescent lights, or smoothed the current flowing to the LEDs, or left it the same.
 

Ian Rogers

User Extraordinaire
Forum Supporter
Most Helpful Member
The one that I measured was about 10%, running at 100 Hz. That was with LED lights.

For me, those flash in a very distracting way. That was why I was looking at it with an oscilloscope.

I don't know if VW have increased the frequency, or gone back to incandescent lights, or smoothed the current flowing to the LEDs, or left it the same.
You are most probably correct... I have never tested one... BUT VW do use LED clusters...
 

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