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How to Know If a Photocell Is Bad

Photocells are semiconductors whose ability to conduct electricity is based on the amount of light they receive. When they do not receive enough light, they have a high resistance, and so do not allow electricity to flow when they are placed inside a circuit. When they are near light, their resistance falls and they allow currents to flow. They are also called photoresistors, light dependent resistors, or LDRs. They are normally used as detectors or switches. The ingredient most commonly used to make photocells is cadmium sulfide, or CDs. To know if a photocell is bad, use a digital multimeter. Another trick is to use one in a simple circuit.

Things You'll Need

CDs photocell
Light source
Mini-lamp
Hobby motor
Battery
Solderless breadboard
Jumper wires
Digital multimeter

Instructions

- 1
  Turn the multimeter on and place it on the resistance setting, which is usually indicated by the Greek letter omega. The resistance should be on a high reading, such as megaohms.
- 2
  Attach the red probe of the multimeter on either leg of the photocell, and the black probe on the remaining leg. Use a hand or book to block out the light from reaching the circuit, or move the circuit to a dark room.
- 3
  Observe the resistance reading on the multimeter. It will be very high, so adjust the knob on the multimeter if necessary.
- 4
  Place the photocell under a light source, such as a lamp or in the sunlight. If the multimeter is not automatic, adjust its knob so that the resistance setting is low. Observe the resistance reading. After a short interval, depending on the type of photocell, it should be a few hundred ohms.
- 5
  Create a simple series circuit by connecting a battery, photocell, and mini-lamp or hobby motor together on a breadboard. The lamp and the motor should be low voltage. When the circuit is covered, the device will not operate. When the circuit is illuminated enough, the lamp will light or the motor will spin.

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