Page 82 - Electronics Starter Kit_EN

Schematic

Wiring diagram

21.4 Circuit Explanation

In this circuit we used a passive buzzer, so we used NE555 to generate high frequency output
on pin 3. But we did not connect pin 1 directly to GND, instead we connected it to the collector
of TR1, so the buzzer will not generate a sound till TR1 be in the saturation mode.

We used the LDR and 10k (R3) resistor to make a voltage divider, and the output of the voltage
divider goes to the base of TR2 amplifies the signal then sends it through its emitter to the base
of TR1.

There are two situations for this circuit, the first one when nothing between the LED and the
LDR.
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