Problem with reading LOW/HIGH Voltage from another device

Hi guys,
I have installed in my car a central lock system, that when i press the button the car locks, when it does so a led turns ON while the car locks, i tried to connect the pins from the led, from the car lock controller, to the arduino, i tried to do it like with the button “example” but it does not work.

I connected the arduino to the same power supply of the car locking system, so the ground should be the same.

I tried to find a solution on google, i typed “arduino reading data from another/foreign device” but i can t find anything that could help me.

What i am trying to do is, when i lock the car, the arduino reads it and turns on a relay module, the carl lock system has a 2 relay module, but the problem is that 12 volt is passing trough the COM and NO, and as a sygnal i think it would fry the arduino,

if someone has a code or a link to help me understand how to do the i would be grateful
thank you

If you just want to read the 12V signal with a 5V Arduino, a simple voltage divider will drop the 12V to a suitable level.

12V interface.JPG

Exactly. Use a voltage divider. Connecting to the LED could also 'fry' your Arduino, depending on how the LED is wired.

6v6gt:
Connecting to the LED could also 'fry' your Arduino, depending on how the LED is wired.

Yep. And even if the LED is connected directly to ground, a standard LED will only have about 2V across it when 'on' - not enough to register as a HIGH on a 5V Arduino. That's probably why it didn't work when 'Von-The' tried it.

@Von-The , note that the values in my schematic above are for a 5V Arduino, which has a threshold of 3V for a HIGH.

If your's is a 3.3V board, you'll need different resistor values. A 3.3V board has a 2V threshold for a HIGH, so a 47K resistor instead of the 27K would work fine. You'd get 2V at 9.8Vin, (the minimum that would work), 2.44V at 12Vin, and 3.25V at 16Vin, (the maximum safe voltage).

A car's battery will be at about 12.6V with the motor off, and about 14.2V to 14.4V with the motor running.