Photocells are sensors that allow you to detect light. They are small, inexpensive, low-power, easy to use and don’t wear out.
Size: Round, 5mm (0.2″) diameter. Resistance range: 200KΩ (dark) to 10KΩ (10 lux brightness) Sensitivity range: CdS cells respond to light between 400nm (violet) and 600nm (orange) wavelengths, peaking at about 520nm (green).
Power supply: pretty much anything up to 100V, uses less than 1mA of current on average (depends on power supply voltage).