Using a Pulse Width Modulation input as a relay trigger: doable?

[UWSkier]

Looking for some electrical knowledge here.

Problem:  My new truck provides a pulse width signal on the trailer running light circuit instead of a steady 12v from the battery.  This is fine for illuminating the LED running lights, but it will not allow the cameras (which draw their power from this circuit) on my RV trailer to power up properly.  I have a 7 way extension pigtail that bridges the 12v charge voltage to the running light circuit, but this makes the truck not "see" anything on the running light circuit and throw "check rear lamp" messages on the DIC.  I'd like to create a more permanent fix for this.

Proposed solution:  Disconnect the running light input where the 7 way wiring comes into the trailer and use the green running light input to a relay.  The relay power source will be the steady 12v circuitry instead of the pulse width.  The relay output will connect to the running lights.

The question:  Can a pulse width circuit be used to operate a relay?  Since it's mechanical, I'd imagine so.  It's basically just an electromagnet.  I'm not creating a fire hazard or anything by doing this, right?

Edited August 22, 2023 by UWSkier

[oldjeep]

Rewire the trailer to power the camera off the charge wire?  Seems goofy to have anything powered off a running light circuit.

[UWSkier]

4 hours ago, oldjeep said:

Rewire the trailer to power the camera off the charge wire?  Seems goofy to have anything powered off a running light circuit.

running new wires is not in the cards.  I could bridge the running lights into the 12v charge wire, but that's basically what my pigtail already does and gives me the messages on the DIC.

[shawndoggy]

the running lights are "always on"?

You can't be the only owner with a modern truck... what does the manufacturer say (assuming that the cameras-wired-to-running-light-circuit was an OEM choice, not yours)?

I'm OK with 12v and relays, but with a little googling I realize that pwm is way beyond my skillset.  

https://www.etrailer.com/Accessories-and-Parts/Voyager/VGR64VR.html

https://www.etrailer.com/Accessories-and-Parts/Furrion/FR87PR.html

 

[shawndoggy]

Seems like there is a homemade solution in this thread, but the inline plugin thing sure seems easier?

https://www.mygrandrv.com/forum/showthread.php/37200-21-GMC-Denali-2500-problems-connecting-to-Furrion-back-up-camera?p=457480&viewfull=1#post457480

[UWSkier]

I think I'm going to do this a different way.  Will leave the PWM wired to the two rear combo brake/turn/running lights since those don't have cameras piggy-backing on them.  For the others, I'll wire directly to the charge circuit on the truck-side of the diode I plan to install into that line.  That should keep my truck happy because it'll see the running light load, and will only turn on the cameras and the rest of the running lights when plugged into the truck with it's always-on 12v.

[RebesMalibu]

7 minutes ago, UWSkier said:

I think I'm going to do this a different way.  Will leave the PWM wired to the two rear combo brake/turn/running lights since those don't have cameras piggy-backing on them.  For the others, I'll wire directly to the charge circuit on the truck-side of the diode I plan to install into that line.  That should keep my truck happy because it'll see the running light load, and will only turn on the cameras and the rest of the running lights when plugged into the truck with it's always-on 12v.

If the pwm is of a high duty cycle and frequency, then it will appear as a DC voltage to most consumers including relays. 
 

Something else to keep in mind is when the 12 volt charge circuit is on. On my expedition, it is only on when the engine is running, the drivers door is closed and  it is in gear. I’m not sure if it is on while I’m in reverse.  

[UWSkier]

Just now, RebesMalibu said:

If the pwm is of a high duty cycle and frequency, then it will appear as a DC voltage to most consumers including relays. 
 

Something else to keep in mind is when the 12 volt charge circuit is on. On my expedition, it is only on when the engine is running, the drivers door is closed and  it is in gear. I’m not sure if it is on while I’m in reverse.  

It's always on on my Silverado.  Even when parked with the engine off and doors locked.

[justgary]

Sorry I'm late to the party.

Is the truck using PWM to dim the running lights on the trailer?  Does it also do it if you have the headlights on (not just DRL)?

[justgary]

And by the way, driving a relay with PWM will generally result in an early death for the relay.  They tend to have a limited cycle life.

You should be able to spoof the lamp circuit with a resistor.  I would assume about 100 mA of current should do, so maybe try somewhere around 130 ohms or so.

[justgary]

On second thought, that would use over one watt of power.  Maybe try 50 mA instead.  A 270 ohm, one watt resistor is a good starting place.

[justgary]

On 8/22/2023 at 10:48 AM, UWSkier said:

running new wires is not in the cards.  I could bridge the running lights into the 12v charge wire, but that's basically what my pigtail already does and gives me the messages on the DIC.

On third thought, they might be using PWM during full headlamps also to make them brighter than normal.

You could also try a capacitor on the leads of your camera.  That would smooth the PWM for you.  A 1uf 25v electrolytic is a good place to start.

[UWSkier]

On 8/27/2023 at 6:53 AM, justgary said:

On third thought, they might be using PWM during full headlamps also to make them brighter than normal.

You could also try a capacitor on the leads of your camera.  That would smooth the PWM for you.  A 1uf 25v electrolytic is a good place to start.

think I'll go with the resistor to spoof the running lights and wire a dedicated switch to the running lights off the 12v supply.  This way I can turn on my running lights and cameras at night for a little added security.  My camera monitor is also a DVR.