This circuit was obtained in a 1978 English magazine. We can easily assemble it, as all components are common. The transistors can be of any type of general use such as BC548 for Q1 and Q2 and BC558 for Q3. The sensor is a common LDR. You can replace R1 with a 10k potentiometer for a sensitivity adjustment. The power can go from 6 to 12 V, depending on the buzzer, but a relay can also be used.
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