A photocell (also light-dependent resistor, photoresistor or photoconductor) is a variable resistor whose value decreases with increasing incident light intensity. In this project, an LED is controlled by a photocell (for example: the LED could be turned on and off depending on ambient light).

Use a photocell (R1), a 1K resistor (R2), a resistor with a resistance equal to the resistance of the photocell in your normal ambient light conditions (R3), an LED, and a 220 Ohm resistor (R4). You will need a multimeter to measure the resistance of the photocell.