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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.
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) |
