12v sensor output to arduino pin

Projects General Guidance
1

So, I'm just playing around with some old security items and the mega2560. I have a motion detector which has 4 wires. I can't even find this model in Google, scary :slight_smile: Anyway, it needs 12v dc input to operate, so I hooked up an old wall ward to power it. The positive to positive and negative to negative. The two outputs I'm using on the sensor are "com" and "alarm", fairly standard. It's a normally closed circuit that outputs 12v dc at 10ma.

This is where I confused. I feel like I should wire the "com" to the ground and the "alarm" to the pin. But then again, that doesn't seem right to me. Also, given that the pins can only handle 5v, do I need to use a voltage divider to get that down to an acceptable range? I guess what I really don't understand is what the input voltage matters at all if you can get the amperes at an acceptable level with a resistor.

Obviously I'm a novice but any directional links or advice would be greatly welcomed. I've done quite a bit of reading with similar projects but nothing was clicking.

Thanks!

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picture would be nice.

the comm and NC pins are most likely for a SPST relay.

so, you sould be able to put the sensor, facing away from you and then move your hand and hear a click of the relay.

connect your meter for continuity on the outputs.

if it goes between infinity and dead short you know you are seeing it work.

if it is a relay, then just run 5v to the comm and the other pin through a 330 ohm resistor back to a pin on your duino.

if it has power, then use it to power an opto.