Floating input trying to read on/off on isolated circuit

I’m trying to detect a car door unlock, the car does this by completing a circuit to ‘ground’ through a switch. Anytime this circuit is completed, it will actuate the door locks.

I had put this together which I thought would work.

On the right is the cars wiring to actuate the door locks, and on the left is the arduino hooked up to its own power source.

What seems to be happening is the input is floating on the arduino, plus a side effect is it’s also ‘completing the circuit’ on the car-side of things as soon as it’s connected, and tripping the locks. Have tried using the inbuilt pull-up resistor, but the trick is I cannot really add in anything inline as that will ‘complete the circuit’ on the car side, or it will start actuating the locks without being requested to…

so not quite sure what the solution might be here if anyone has done this before or has any suggestions?

Show the full circuit not just a section. Your current switch shorts out the 5v and the LED does not have current limiting.

Weedpharma

weedpharma:
Show the full circuit not just a section. Your current switch shorts out the 5v.

Weedpharma

Thanks - that pretty much is the full arduino side - but as for the car side, I'm not a huge expert - but I'm hooking into the 2 leads coming out of the 'remote central locking' box in this diagram.

What I've figured out so far is it simply 'connects' ground to trigger the lock or unlock, I'm trying to detect when that event happens.

(Full size pic: http://i.imgur.com/DqaAT3z.png)

Any door lock type circuit in an automotive application is going to involve 12 volts at some point - without input scaling and protection, the Arduino can NOT handle 12v to any of it's inputs and bad things will happen. As pointed out, you also need current limiting to the LED otherwise it will either pull the signal down to it's voltage (around 2v) or let the magic smoke out of the led depending on how much current is available to it. Neither of those cases is what you want.