Connect 240v PIR motion detector to Raspberry Pi
There are three wires going to the detection board. It is very likely that there are two power lines (0 V and 12 V) and a signal for when something has been detected.

Your task is to trace the circuitry to figure out which is which. If you have a pinout of the IC then it should be fairly easy, but do remember you're looking at the IC upside-down from the bottom of the PCB.

If you can, confirm that any other devices (such as the IR detector) can work at 12 V. That's so that nothing breaks when you supply the board with 12 V. A voltmeter connected between the remaining wire and 0 V will tell you what voltage it gives out when it is idle and when it is activated, so then you can figure out if you need a simple voltage divider made of resistors to connect safely to the RPi, or if something a bit more complex is needed.