Translations:Adventist Youth Honors Answer Book/Vocational/Radio Electronics/56/en

In this circuit, R2 is a cadmium sulfide cell (CdS cell) - that is, a light-sensitive resistor. When exposed to light, the resistance of the cell decreases. In darkness, the cell has a higher resistance. R1 and R2 together form a voltage divider circuit. As the light striking CdS cell gets brighter, the voltage at the common terminal decreases. As it gets darker, the voltage increases. This voltage is applied to the positive terminal of a comparator IC. The negative terminal of the comparator is connected to the wiper of a potentiometer which forms a second voltage divider circuit. This voltage can be adjusted by tuning the potentiometer. When the light striking the CdS cell gets bright enough, it causes the voltage on the positive terminal to fall below the voltage on the negative terminal. This causes the output of the comparator to go to zero volts, thus lighting the LED. When it gets dark again, the LED will turn off.