smart shadow sensor alarm schematic

Shadow Sensor Alarm

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!

Shadow Sensor Circuit Schematic

smart shadow sensor alarm schematic

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.


  • Place LDR1 & LDR2 1 inch apart (LDR1 —— 1 inch —— LDR2)
  • Diode D2 is not a crucial component. It is for input supply polarity protection only
  • Current consumption of the lab prototype is 2 to 3 mA (standby) & 20 to 30mA (Active). Major “current consuming” component is the Piezo-Buzzer!
  • The Piezo-Buzzer can be replaced with a 6V low-current electro-magnetic relay to drive external loads like an electric hooter. Remember to add a freewheeling diode (in anti-parallel configuration) across the relay coil


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