One day I was buying groceries at Lidl and while going to buy jogurt I spoted they were selling solar powered motion detector reflectors. That’s can serve as a nice asynchronous trigger for alarm system. You can just use python script to scan for motion but this is cool too.
So poped open it’s circuit box. Then mesured voltages between all the things. Then triggered the motion detector so that light turns on. Then measured the change. Then attached two wires two to two of the nodes there since input was just right for arduino or rpi. Hoocked it to arduino that signald the pi to take picture. It has rechargable solar battery.