LED DETECTS LIGHT
The LED in this circuit will detect light to turn on the oscillator. Ordinary red LEDs do not
work. But green LEDs, yellow LEDs and high-bright white LEDs and high-bright red LEDs
work very well.
The output voltage of the LED is up to 600mV when detecting very bright illumination.
When light is detected by the LED, its resistance decreases and a very small current
flows into the base of the first transistor. The transistor amplifies this current about 200
times and the resistance between collector and emitter decreases. The 330k resistor on
the collector is a current limiting resistor as the middle transistor only needs a very small
current for the circuit to oscillate. If the current is too high, the circuit will "freeze."
The piezo diaphragm does not contain any active components and relies on the circuit to
drive it to produce the tone.
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