12v sensor output to arduino pin

so, i'm playing around old security items , mega2560. have motion detector has 4 wires. can't find model in google, scary anyway, needs 12v dc input operate, hooked old wall ward power it. positive positive , negative negative. 2 outputs i'm using on sensor "com" , "alarm", standard. it's closed circuit outputs 12v dc @ 10ma.

this confused. feel should wire "com" ground , "alarm" pin. again, doesn't seem right me. also, given pins can handle 5v, need use voltage divider down acceptable range? guess don't understand input voltage matters @ if can amperes @ acceptable level resistor.

obviously i'm novice directional links or advice welcomed. i've done quite bit of reading similar projects nothing clicking.

thanks!

picture nice.

the comm , nc pins spst relay.

so, sould able put sensor, facing away , move hand , hear click of relay.

connect meter continuity on outputs.

if goes between infinity , dead short know seeing work.

if relay, run 5v comm , other pin through 330 ohm resistor pin on duino.

if has power, use power opto.

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