Simple Phototransistor Motion Detector
Ambient light levels are always a problem. One possible solution is to use I(nfra) R(ed) at high frequency. It just happens that TV zapper technology is available at very reasonable cost. The detector IC does the work for you. A simple 38kHz IR oscillator made from the ubiquitous 555 (astable) with IR led will generate the signal.



A second 555 (wired as a re-triggerable monostable) is connected to a 38kHz IR sensor IC. This will keep the output high until it fails to detect the 38kHz signal.