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.
Can you not just stick a diode in line with your trigger wire so you don’t get feedback from the relay?
It seems like the horn wire is normally at 12V, and pressing the horn brings it down to about 0V. I figure the horn switch it just shorting that wire to ground, which in turn triggers the factory horn relay. Would a dioide in like with my trigger relay stop the add-in relay from connecting the factory horn wire to ground?
Yeah. Put the stripe end towards the horn wire you are tapping into.
Edit: I just reread your post. If 86 is connected to your car’s horn wire, 85 needs to be connected to 12v, not ground. That’s probably the issue you’re running into (but I would still run a diode for isolation).
I think you may have just solved my problem. When I’ve used relays in the past, pin 85 was connected to ground, since I wanted the relay to close when the trigger went high. I’m not sure why it never occurred to me that I’m essentially trying to do the opposite thing here, since the horn is triggered when the trigger wire is connected to ground.
I’ve never worked with individual diodes, so I’m not sure about the correct terminology, but which way would I want the diode to “face”? Do I want it to allow current to run from the 12V source, through the added relay, to the horn switch wire, or the other way around?
Additionally, would I need to add an in-line resistor? It makes me a little nervous connecting the horn switch to 12V, given that I doubt it’s designed to carry a significant amount of current.