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Circuit Solutions Notebook
Last Updated On:  Wednesday June 02, 2021

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Solution to the Latest Technical Challenge


Short Range Hand Wave Detector


Design a battery powered circuit, which can turn something on or off, with the wave of a hand over a surface.



The 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 two load resistors in parallel with the photodiode.  One resistor is adjustable to 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, as the shadow moves over the two sensors.  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 circuit for many years.  The more popular LM358 would also work but would draw more current.  Click below  to see a short movie (AVI File) of the device in action.


 Shadow Activate Motion Detector Circuit 

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


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