This circuit give low level output, when exist enough lighting, that is a detector of light. It can give, say ,command turn on somebodies lights, when fall the dark. His exit is compatible with TTL level and it give low logic level, when phototransistor Q1, detect light. With adjustable resistor RV1, we can regulate the sensitivity that it will have.
R1=270ohm C1-2=100nF 100V ceramic IC2=74HCT13
R2-5=1Kohm RV1=10Kohm trimmer Q1=BP103 Phototransistor Siemens