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Solution:
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.
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