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Last Updated on: Tuesday, June 01, 2021 03:06 PM

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More  Motion Detectors

Shadow Activate Motion Detector 

There are many ways one can detect nearby human or object motion.  This circuit demonstrates one way.  It detects motion by noting the changes in the shadow cast by the moving object.
The hobby circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both devices produce nearly identical current levels.  The current is converted to a voltage with a load resistor in parallel with the photodiode.  One resistor is adjustable so the two voltages can be carefully balanced under uniform lighting.  When a hand or an arm is moved over the sensors, casting a shadow, one device will detect more light than the second.  This triggers an imbalance. 
A voltage comparator connected to the two devices detects the imbalance and sends a high logic level signal to an n-channel FET, which can turn on a beeper or activate a relay.  If a low power voltage comparator is used, such as the LMC7211, a 9 volt battery will power the electronic circuit for many years.  The more popular LM358 would also work but would draw more current.  Click below to see a short movie (MPG File) of the device in action.

Demonstration AVI Files
(Double Click on the above link to view movie)


Click on Circuit Below to view PDF of Schematic

Shadow Activated Motion Detector designed

 by Dave Johnson, P.E. (Oct 24, 2005)

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