Here’s a demo of how the original car phone can beep the car’s horn to alert you to an incoming call when you are away from your car.
Could you imagine sitting in a restaurant eating lunch while listening for your car horn, then running out to answer an important call?
I’m considering replicating this feature in my Bluetooth conversion. It will be a pretty useless novelty demo of how old car phone tech worked. With my conversion, you’ll have your modern cell phone with you when you go away from your car anyway (no need to go back to the car to answer the call), and you would have to stay within about 20 feet of the car to maintain the Bluetooth connection. So there’s absolutely no practical use for this. But I still want to do it.
Technically, it is very simple: just a transistor used as a low-side switch for the horn relay circuit in the car. I have one unused pin on my MCU that I could use to trigger a transistor, and I found the specs of the (obsolete) transistor in the original car phone that I can use as reference to confidently select a suitable equivalent.
The only downside is that the factory car phone wiring in the 3000GT does not include the necessary wired connection between the car phone and the car horn relay, so I would have to dismantle the interior to run a wire from the trunk to the dashboard area, and splice it into original wiring. I’m conflicted between keeping my car original, with only the originally-supported car phone integration, versus adding a custom wired connection to fully take advantage of an original feature of the car phone. I’ll probably at least update the hardware design to support it, then I can decide later whether to follow through with the software implementation and custom wiring in my car.