Light Dependent Resistor and Photocell Circuits
Photocell based night light - Many automatic night light circuits had been published here. This one uses a photocell for detecting the light intensity. At full light the resistance of the photocell will be few ten ohms and at darkness it will rise to several hundred ohms.
Light Sensing RF Transmitter - This Transmitter is very similar to the Simplest RF Transmitter. The only difference being the Photoresistor placed in series with R1. This could also be a thermistor or other variable resistance sensor. The frequency of the tone or "clicks" that is transmitted varies with the amount of light falling on the Photoresistor
Photocells and their Application
Programmable Light Activated Relay:
RF Transmitter, Light Sensing: