Using an advantage of the photo-voltaic voltage from ordinary LED, this circuit can makes some different output recognize dark and light. The output voltage while in darkness condition is around 7 volts and fall to about 2 volts in bright light. The LED voltage is buffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain about 20. This produces a change about 5 volts at the output from the darkness and to bright light. You can adjust the 100K potentiometer to set the output voltage ranges.

source : bill bowden’s circuit collection
led-photo-sensor-circuit

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