The circuit below works quite well in typical indoor room lighting. I would not recommend its use in direct sunlight. Two small PIN photo diodes 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 photo diode. 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.
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.
hello,i'm interesting about your project,can you show me where is 9v battery connected in the circuit?
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