I use a hard drive recorder like this. You can define an area on the screen to detect motion then if anything moves in that bit it will start to record for selected amount of time. You need a hard drive, but may be able to use an old pc one if you have any, and a camera. But it worked well for me when i needed it. Only thing was once had the cameras installed and recording the little scrots never came near again. Guess it's a good deterent when they see the camera...

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