Dark-Activated Alarm With Pulsed Tone Output Circuit

NOR gates a and b form a low-frequency oscillator that is activated when the CDS cell, under dark conditions, causes NOR gate a to see a logic zero at one input. This low-frequency (10 Hz) gates a high-frequency oscillator (c and d) to oscillate at around 1000 Hz. Rl can be varied to change the pulse rate and R2 to change the tone.

R3 sets the trigger point.

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