A photocell is a solid-state device (it has no moving parts), which converts light energy into electrical energy in the form of current or voltage. Photocells are used in exposure meters and burglar alarms among other applications.
A photocell is essentially a type of electrical resistor. When light strikes the cell, its resistance falls, allowing electrical current to flow more freely. The operation of a photocell can be tested by connecting it to an instrument that measures electrical resistance, known as an ohmmeter.
Disconnect the photocell from the circuit to which it is attached. Connect it to the ohmmeter, set to measure on the 1,000 ohm (Ω) scale, and point the lens, or eye, of the photocell toward a bright light source. If the photocell is working correctly, the ohmmeter should read less than 500Ω.
Obscure the lens of the photocell. The resistance reading on the ohmmeter should increase dramatically. If it does not, the photocell is faulty and should be replaced.