I've been trying to solve an automotive electronics problem for several weeks now, but everyone I've spoke to can't seem to come up with a solution.
In brief, I'm trying to add a relay in-line with the horn switch in my car, such that I can close my own circuit when the horn is pressed, without affecting the existing horn circuit in the car.
I had some JD1912 12V relays left over from a previous install, so I tried to use those. (Relevant image: Diagram)
First, I placed connected the trigger wire (pin 86) to the the wire coming into the horn switch, and the ground (pin 85). The relay triggered when the horn button was pressed as expected, but this also caused the actual car horn to sound continuously. Presumably doing this was enough to give the factory horn relay enough current to close.
Next, I tried placing the relay in series with the horn switch by splicing the wiring heading into the horn switch, and connecting the relay (pin 86 and 85) in line. Once again, the relay triggered with the horn switch as expected. However, this time, the actual car horn didn't sound at all.
The best I can work out is that there's a resistor in-line with the relay trigger (otherwise connecting it straight to ground would cause a short, right?) However, that resistor is just enough to allow the factory horn relay to trigger when connected to ground.
The way the car is designed, I can't splice into the wire coming out of the switch to detect when the horn is pressed, since it's a shared ground with other components.
My question is, is there such a thing as a relay with no resistor? Essentially all I'm looking for is a component that will "detect" current on the horn switch wire, and close a separate circuit. I'm not sure if a relay is even the correct way to go about this. Hopefully you guys can point me in the right direction.
You may be able to use a optocoupler to sense the horn then use that to trigger the relay. What are you trying to accomplish? Are you making like sensor to know when your car alarm has triggered?
Right now I'm trying to hook up a custom dashcam that can show an overlay when the horn is pressed. It works via GPIO, and essentially just checks to see if a specific pin is connected to the ground pin. I figured putting a relay between those two pins with a small resistor for safety would be the least invasive way to do that.
Why not just add a new switch or button to the car? Like a button for your left foot to hit like old headlight high beams were.
The idea is that the dashcam can show when the driver presses the horn in a car accident without the driver needing to do anything different. Anyone can drive the car without any specific knowledge, and the camera will just work in the background.
If you've even seen police dashcam video with the text overlays at the bottom reading "[HORN] [SIREN] [LIGHTS] [BRAKE]", this is essentially a custom implementation of that for more general use.