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.
Not sure I understand the problem fully, but you want a circuit to operate when you push the horn button, without affecting the horn operation.
Chances are the relay coil is drawing to much power.
Have you thought about adding a transistor to your circuit? It would draw very little current from the horn circuit but should allow you to drive something else. - such as your relay. It would of course require you to do some electronics.
I’ve honestly never worked with transitors. Basically all of my experience is with pre-made 12V electronics. The appeal of the relay method to me was that it felt fairly non-invasive, since my add-in circuit is essentially isolated from the car with the relay (for example, I can trigger the relay with 12V, and switch a 5V circuit without any issue). Would the same be true with a transistor?
Car horns are usually ground switched.
There’s no resistor in a standard 12v relay (other than the coil itself).
This should work with no issues, provided your car has a typical horn circuit.
Check a wiring diagram for the car. Cars used to run the horn circuit through the horn switch on the wheel directly, no relays or anything else - the power for the horn went through the horn switch (well , the ground side anyway, which still has to support the horn current load).
Automotive relays generally take very little current to operate (like 0.5 amp) - it’s kind of their function to switch a high-load circuit using a low-current circuit.
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.
This doesn’t add up. If it’s a shared ground that’s switched, then other things wouldn’t work if the horn switch wasn’t engaged.
This is a typical horn wiring.
You’d want to wire your relay the same as the horn relay (ground switched, tapped into the wire between the horn switch and relay), or use #87 as your trigger into your relay’s #30 (wiring your relay as positive switched, typical config), or even more easily, just wire your actual device to #87, as horn circuits generally have moderate current capability (the wires are often 14 gauge).
Essentially all I’m looking for is a component that will “detect” current on the horn switch wire, and close a separate circuit.
I think you’re describing a transistor
You need to find a schematic for your car’s electronics in order to get a proper solution for what you’re trying to do. I would be particularly cautious about connecting a relay coil to some unknown wiring. The back e.m.f. from a discharging relay coil could easily fry a microcontroller if it happens to be connected to one.
I’m confident the wire I’ve tapped into is the horn, since I’ve previously used it to install a horn override as part of a PA system. That being said, I hadn’t considered that I might be connecting to a computer rather than something more analog. When I press the horn, I can faintly hear a relay clicking in the engine bay, so I’m hoping it’s just a simple relay.
Horn switches switch to ground. Power for your original horn relay is supplied from a fused battery source, passes through the horn relay, and when you press the horn button the button completes the circuit to earth, triggering the relay.
So, you need to wire your relay coil like this -
12 volts from a fused battery source to:
Your relay coil, to:
The horn switch, which then switches to:
Ground.
Just like how your current horn relay works.
This also works for older cars that do not have the really. They supply power to the horn, and then a single wire runs from the horn back to the horn button, which then completes the circuit to ground when pressed.