Reading a negative and positive voltage through Arduino

Question

Currently struggling with a problem due to my limited electrical engineering experience (1 first year course).

I'm working on a project for central locking in the toolbox on my ute. The actuators within the car are sent a voltage that alternates between 12V and +12V for 'unlock' and 'lock'. I am trying to somehow convert this voltage into something that can be read by an Arduino as an input to ultimately activate the servos on the lock.

From what I have been able to find maybe a voltage divider circuit or an op-amp. The photo below is how the car's locking system works.

enter image description here

Answer 1

There is no -12V in the car.

In this circuit the switch either connect the blue wire to ground or the green one to ground in order to unlock or lock the doors.

This could be a simple way to read locking or unlocking signals if a common ground is possible:

enter image description here

(Image Reference: https://commons.wikimedia.org/wiki/File:DoorAnimation1.gif)

• SWSA: what values do you think i should use for the ciruit? or how could i work them out?
• Paul Ghobril: The pull-up resistor can be internal (by configuring the input port in arduino) or can be external and of quite large value like 100k to reduce consumption since the input impedance of an Arduino port is quite high. the capacitor must be high enough to filter noise but not too high to avoid distorting the lock or unlock pulse. If you chose a large capacitor think of putting a small one in parallel to filter spikes.
• Paul Ghobril: I would go for 100nF