A time switch (also called a Timer
switch, or simply timer) is a timer that operates an electric switch controlled by the timing mechanism.
The switch may be connected to a electric circuit operating from mains power, including via a relay or contactor; or low voltage, including battery-operated equipment in vehicles. It may be built into power circuits (as with a central heating or water heater timer), plugged into a wall outlet with equipment plugged into the timer instead of directly into the power point; or built into equipment as, for example, a sleep timer that turns off a television receiver after a set period.
The mechanism may be mechanical (e.g., clockwork; rarely used nowadays), electromechanical (e.g., a slowly rotating geared motor that mechanically operates switches) or electronic, with semiconductor timing circuitry and switching devices and no moving parts.
The timer may switch equipment on, off, or both, at a preset time or times, after a preset interval, or cyclically. A countdown time switch switches power, usually off, after a preset time. A cyclical timer switches equipment both on and off at preset times over a period, then repeats the cycle; the period is usually 24 hours or 7 days.