Piezoelectric Alarm Circuit

The alarm uses a fixed-frequency piezoelectric buzzer in conjunction with the cadmium-sulfide (CDS) cell and the two-transistor circuit to provide a unique effect. Whenever light reaches the CDS photo-electric cell, the alarm is silent. But when no light strikes the cell, transistor Ql turns on, and the circuit emits a high-pitched tone.The alarm consists of a piezoelectric disk that oscillates at the fixed frequency of 3.137 kHz, created by transistor Q2, capacitor CI and C2, and resistors Rl through R3. Transistor Ql is used as a switch. It is forward-biased on by R4;-however, the CDS coll turns Ql off when the light is striking it.
Piezoelectric Alarm Circuit - schematic

A CDS photo cell is made from cadmium sulfide, a semiconductor material that changes resistance when the light strikes it. The greater the amount of light, the lower the resistance. The low resistance conducts positive voltage to the base of pnp transistor Ql, keeping it turned off` when the light shines on the CDS cell. As soon as the light is removed, the CDS cell provides a resistance of over 100 kQ. That causes Ql to turn on, allowing a positive voltage to reach the emitter lead of Q2, which then begins to oscillate. That then causes the piezoelectric element (transducer) to produce a loud signal.

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