Shadow sensors are widely used to detect the movement of a person in a confined area. Many circuits published earlier have a serious drawback that only one light dependent resistor (LDR) is used for shadow detection.
Needless to say, this is not a perfect solution!
Described here is a simple but improved circuit of a smart shadow sensor alarm, which can register a shadow when there is a light difference. Here two 5mm LDRs are used with the popular Op-amp LM741CN to drive an active piezo-sounder when a “valid” shadow is detected. The whole shadow sensor circuit can be powered from four 1.5V AA cells (6VDC), or similar dc supply sources.