so I've just built a circuit and some Arduino code (not shown here) which interfaces between a car steering wheel and its buttons to an aftermarket cruise control system (Command AP500). The stock keypad for this cruise unit has a weird LED arrangement and if the LEDs are not present, the AP500 cruise control system will refuse to engage.
I have mimicked the LED circuit and it works, but the question is how the microcontroller (unrelated to my Arduino) in the cruise control system is detecting the existence of the LED? Image of the LED circuit is attached. Note that the cathodes of the LEDs do not appear to be connected to a ground and both of the cathode circuits have a voltage potential between them and the DC ground. Somehow, the microcontroller in the cruise control unit is able to detect if one or both of the LEDs are present.
I'm assuming it is something to do with the use of a 7.5v Zener diode (and it is only able to detect the Red LED). Could the microcontroller be measuring the voltage on that wire and detecting a difference in voltage when the LED is lit? Why does the lead from the microcontroller to the Zener diode have 2.7v on it? Note that when the red LED is powered, the voltage between pin #2 and DC ground drops to 2.3v. This is all when the circuits are connected to the microcontroller of course.
Lastly, +12v is permanently on and is not used in controlling each LED.