cvieira

85 needs to be connected to 12v, not ground.

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.

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.

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?

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?

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.

This coil has a standard resistance/impedance

I see. I was under the impression that there's a seperate resistor to avoid shorts when you connect from the trigger to ground.

What year/model of car?

It's a 2014 Nissan Altima. I've spent quite a bit of time trying to find wiring diagrams, but I haven't gotten very far. I got to the point I'm at now by following some old forum posts and confirming with a multimeter. What you described sounds accurate: the horn wire under the steering column seems to always be at 12v. Pressing the horn seems to send it to ~0V, so I assume it's just jumping to the shared ground wire. I will say that when I disconnect to horn at the front of the car (to silence it), I can hear a relay in the engine bay clicking when I press the horn. I figure that implies it's a physical relay, rather than a computer somewhere.

And realistically you could in a pinch pull coil power from the horn itself

This is something I've considered doing. The circuit I'm adding with the relay is extremely low current, so I don't think it would be too tricky to run wires from a relay connected right next to the horn to the cabin. I can just ground the relay to the frame of the car right near the horn such that there's minimal chance of creating a short on the high-amp horn circuit.

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.

I develop a self-hosted service designed to do exactly this! It's not quite finished yet, but it's at the point where enough functionality works that it can be used for testing.

https://github.com/connervieira/HealthBox

The docs/USAGE.md file gives an overview of how HealthBox works. Feel free to poke around in the other docs/ files as well.

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Understanding of the "normalcy bias" changed the way I think a lot. The general idea is that you'll tend to explain away and dismiss unusual things you see. In other words, people are hesistant to acknowledge when something isn't normal.

When you see something that strikes you as odd, I think it's good to understand that this bias exists. Avoid saying "oh, it's probably just XYZ", and actually try to work out an explanation. You may end up solving a situation you otherwise wouldn't have even realized was about to be a problem.

In the worst case, you'll just realize your initial instinct was right, and move on. In the best case, you'll be able to help with a situation that otherwise would have been neglected, and been allowed to become a serious problem.