Circuit Project Electronic

The LED voltage is buffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain of about 20. This produces a change of about 5 volts at the output from darkness to bright light. The 100K potentiometer can be set so that the output is around 7 volts in darkness and falls to about 2 volts in bright light.
source : http://ourworld.compuserve.com/homepages/Bill_Bowden/

Related Links
More circuit Lighting Senser
More circuit by IC LM1458
More about light sensitivity
Dark-Detecting LED
LED4 LED Sensor Electronic Tracker with Simple Solar Charge Controller
Audible Light Sensor by IC 741
Light Change Detector by IC CA3130