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What Is the Difference Between a Photo-Resistor & a Photo-Transistor?

A broad class of electronic devices, called optoelectronics, produces light or senses it. Photo-resistors and photo-transistors fall into the latter category; for these devices, light causes a change in electrical resistance or voltage. Combined with other components in a circuit, these devices can use light to carry data and control a wide range of equipment.

Photoelectric Effect
- A physical phenomenon called the photoelectric effect produces electrical changes when light shines on certain materials. The energy in light moves loosely-bound electrons in the atoms of the material, altering its electrical behavior. For a photo resistor, the electrical resistance changes. In a photo transistor, light produces a current inside the device that varies its electrical conduction in one direction.
Circuits
- A photo-resistor does not produce a current by itself; instead, it takes a current from a power source and increases or decreases it by varying its own resistance. A circuit for this device has a power source, such as a 9-volt battery, a current-limiting resistor and the photo-resistor connected in series, and other components controlled by the changing current. A photo-transistor circuit also uses power source and a series resistor, though you can configure them in three different ways: common-collector, with the resistor wired to the transistor's emitter lead; common-emitter, having the resistor in series with the collector; and common-base, which has resistors at the base and collector terminals.
Speed
- A photo resistor responds slowly to changing light levels. This makes it suitable for turning on street lights, automatic porch lights and other applications where low sensitivity keeps small changes in light intensity from turning the equipment on and off excessively. A photo transistor responds rapidly to changes in light intensity. Data communications devices use photo transistors to send computer signals via light impulses.
Materials
- Photo resistors use a chemical compound called cadmium sulfide. The material's resistance increases in darkness and decreases under bright light. A photo transistor, like a regular transistor, contains thin slices of treated silicon. Unlike a regular transistor, its case is transparent. When light strikes the silicon, it produces about one volt of electric potential, and a few milliamps of current when connected to a circuit.

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