Photocell

Location:

Concepts Shown:

semiconduction, conduction band, resistivity

Equipment:

Cd sulfide photocell, ammeter, voltage source (batter - 1.5V), on/off switch, copper wire, and wire stripper, electrical tape or solder. The photocell is available at any camera shop. Cost less than $20.

Procedure:

Setup:

Wire the photocell in series with the battery, the switch and the ammeter (see diagram below). If possible, secure all connection with electrical tape or solder. For efficiency, try to put the battery and switch inside the ammeter itself, having only one wire coming out the back to connect to the photocell.

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Presentation:

  1. Turn the switch to on.
  2. Make sure light is getting into the photocell, and take a reading on the ammeter.
  3. Place your finger in front of the photocell, blocking out the light.
  4. Watch the ammeter show a drop in current, by taking a new reading.
  5. Discuss scientific principles involved by drawing up a picture of the conduction band and gap. Show how the incoming photon strikes an electron, causes it to jump into the conduction band, thereby causing the material to be more conductive in the presence of light.
Science:

An ammeter simply reads the current flowing through the circuit, or basically, the resistivity of the circuit. When less current is read on the ammeter after the light is blocked out, that means that the circuit has become more resistive. The part of the circuit where the resistivity is increasing is in the photocell, and here's why. The reason is because of photoluminescence. When the light (photons) strikes the photocell, it excites the electrons into the conduction band, decreasing the resistivity of the material and increasing the current flow of the circuit. However, when you reduce the light, less photons strike the material and thus, less electrons are excited into the conduction band causing less conduction, higher resistance and decreasing the current flowing through the circuit.

References:

Author:
Tyler vanHouwelingen
Credits:


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