PhotoResistors Very similar to Cadmium Sulfide Cells, except that they change their resistance with light rather than producing a voltage. They can be used in a Whetstone Bridge-type setup to detect changes in lighting conditions. Commonly used to detect the passing of objects, especially those that cause large changes in light conditions. Also can be used as electronic limit switches, or as the receptor for a light beam that, when broken, indicates a passing object Like CdS cells, photoresistors are very sensitive to ambient light conditions. LEDs Light Emitting Diodes can be obtained to produce light at many frequencies, including frequencies that are outside the visible spectrum.
Some typical uses are remote controls for televisions, etc.
Building the electronic circuit for an LED is very simple, but the receptor circuit can be very complex. Switches Convert mechanical motion to electrical motion. Used to detect the position of a machine, trigger events that are dependent on position or other mechanical inputs. A very simple to build and obtain solution for most applications. ElectroMagnets,magnets, coils, and motors Magnets or electromagnets close to coils of some conductor induce a current in the coil of the conductor when relative motion occurs.