Precision Light Alarm With Hysteresis Circuit

  
The TL081 is used as a comparator in a Wheatstone bridge circuit. When the CDS cell resistance decreases due to exposure to light, the output from IC2 cause the low-frequency oscillator (a) and (b) to generate a 10-Hz square wave, gating the 1000 Hz oscillator (c) and (d) on and off. This signal drives an amplifier.
Precision Light Alarm With Hysteresis Circuit - schematic

R3 controls hysteresis, which reduces on-off triggering near the threshold set by R4.




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