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A time switch (also called a timer switch, or simply timer) is a device that operates an electric switch controlled by a timer. Intermatic introduced its first time switch in 1945, which was used for "electric signs, store window lighting, apartment hall lights, stokers, and oil and gas burners." A consumer version was added in 1952.
Most cam timers use a miniature mains synchronous motor to rotate the mechanism at an accurate constant speed. Occasionally more complex timers with two motors are seen. A drum sequencer is a re-programmable electromechanical timing device used to activate electric switches in repetitive sequences.
A motor-driven cam (usually relatively slow-speed) and one or more micro switches form a timer mechanism. The snap-switch mechanism can be enclosed in a metal housing including actuating levers, plungers, or rollers, forming a limit switch useful for control of machine tools or electrically-driven machinery.
The dashpot timer is a fluid time-on-timer that can be used in definite time motor acceleration starters and controllers. A dashpot timer is a container, a piston, and a shaft. The dashpot timer functions when a magnetic field forces a piston to move within a cylinder when the coil is energized.
This type of timer often has a friction clutch between the gear train and the cam, so that the cam can be turned to reset the time. Electromechanical timers survive in these applications because mechanical switch contacts may still be less expensive than the semiconductor devices needed to control powerful lights, motors and heaters.
To provide longer lighting times when required the button may be pressed repeatedly. For this reason any timer should be a retriggerable monostable, i.e. the off delay lasts from the time of the last button press. There may also be a switch, sometimes a key switch, that can turn the lights on indefinitely for tasks such as cleaning.