Ask Electronics

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.

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

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.

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?

There are coils which can detect current through induction, so you can trigger on press without altering the resistance of the circuit

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.

Relays operate by flowing current through a coil. The coil generates a magnetic field, which attracts a ferromagnetic plate that then closes contacts. Without current flow, no relay can operate.
This coil has a standard resistance/impedance based off the length of wire coiled, ans is engineered depending on the size of relay, designed operational voltage, etc. This cannot be modified.

With a series of solid state op amps, one could turn a "no current/infinite resistance" signal into a relay switching effect, but it would require some other outside power source to run the relays or mosfet switching. Seems complicated. Maybe look for a "solid state relay" or "digital relay", I've not ever used one though.
I know you weren't operating a car horn directly through a relay coil either, so there must definitely be some other relay or more likely a BCM computer with a MOSFET on board.

What year/model of car? Is there any chance you can locate a oem wiring diagram to understand better how the horn system works? In older cars, the horn is a ground-switched circuit, where the horn unit is always hot +12v and then it is directly connected to ground through the steering column when the contacts on the horn pad are pressed together. No intermediate relay. On a system like this I would connect the relay effectively "in parallel" with the horn, with one side of the coil on +12v hot, and the other on the non-grounded, switched side of the horn pad. This way the horn and coil sit at the same potential and no current will flow anywhere except the horn pad ground circuit when pressed.

I cannot guarantee any newer car uses a system remotely like this though, especially through a computer so YMMV.

And realistically you could in a pinch pull coil power from the horn itself. Just connect a wire to either side of the horn so your relay coil is in parallel, one side switched and the other side always hot/ground. This may/may not be safe though as the horn circuit will likely be in a relatively large fuse and it'll be impractical to relocate the relay or run an additional 14 feet of small gauge sense wires from the cabin to the front of the car.

Can you not just stick a diode in line with your trigger wire so you don't get feedback from the relay?

You need to put in a double pole relay. Disconnect the horn from the original switch entirely and connect it to one of the relay poles, connect your circuit to the other.

Or just use a single relay and put the horn and your other load in parallel as long as they both run on 12V.

The horn button gets connected to the coil of the relay.

Relays don't have resistors necessarily in their coils, but the coil is designed to draw a certain amount of current at a certain voltage. In essence it is a resistor. If it were to pass the full current required to drive a car horn through the coil, it would catch fire.