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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, November 29, 2011 08:14 PM
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Motion
Detectors |
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Shadow Activate Motion Detector
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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. |
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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.
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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)
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Click on Drawing Below to view PDF version of Schematic |
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eMail David A.
Johnson, P.E. about this circuit |
