A Single Pole Double Throw (SPDT) switch is a switch that only has a single input and can connect to and switch between the 2 outputs. This means it has one input terminal and two output terminals.
A Single Pole Double Throw switch can serve a variety of functions in a circuit. It can serve as an on-off switch, depending on how the circuit is wired. Or it can serve to connect circuits to any 2 various paths that a circuit may need to function in. For example, a SPDT switch can connect to create a Ready Mode and a Standby Mode in a printer.
Single Pole Double Throw (SPDT) Switch Circuit
Below is an example of a circuit which utilizes a single pole double throw switch.
You can see above how a SPDT can be wired up to put a circuit in either one of two modes. When the switch is connected one way, the lamp will turn on, while the LED is off. When connected the other way, the LED then turns on, and the lamp shuts off.
This shows the dynamic 2-mode capacity that SPDT switches allow.