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Switches, relays and transducers are mechanical or electrical devices which are used to connect and disconnect circuits (switches and relays) or to produce an output from a circuit by changing one form of energy into another (transducers).
Switches
Switches are devices for connecting and disconnecting circuits. Different types of switches have different numbers of poles [poles: output terminals or contacts in a switch ] and throws [throws: circuits or conducting positions controlled by each pole of a switch ].
There are four main types of mechanical switches used in circuits. The description given to these depends on the function of the switch.
| Switch type | Diagram |
|---|---|
| Single pole single throw (SPST) | |
| Single pole double throw (SPDT) | |
| Double pole single throw (DPST) | |
| Double pole double throw (DPDT) |
The single throw switches are either open or closed. Double pole single throw switches are often used to switch between two outputs. The double throw switches are used to change from one part of a circuit to another. The double pole double throw can be used to reverse the direction of a motor. The dotted lines on the symbols indicate that both sides move together.
Switches are available in a wide range of forms that include rotary, key, membrane, mercury, tremble and toggle.
Push switches
Push switches are a type of SPST switch. They can be either Push to make switches, which are normally open (NO) or Push to break switches, which are normally closed (NC).
Push switches
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