The switch may be connected to a circuit operating from mains power, or for lower-voltage circuits, including battery-operated equipment in vehicles. It may be built into power circuits (as with a central heating timer), plugged into a power point 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 an interval.
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. For example, a central heating timer may supply heat for a specified period during the morning and evening every weekday, and all day on weekends. A timer for an unattended cooker may switch on automatically at a time and for a period suitable to have food ready at mealtime. Timers may do other processing or have sensors; for example, a timer may switch on lights only during hours of darkness, using a seasonal algorithm or light sensor.
Time switches can be used for many purposes, including saving electric energy by consuming it only when required, switching equipment on, off, or both at times required by some process, and security (for example switching lights in a pattern that gives the impression that premises are attended).
Among applications are lighting (interior, exterior, and street lighting), cooking devices such as ovens, washing machines, and heating of buildings and vehicles. Built-in automatic washing machine controllers are examples of very complex electromechanical and electronic timers cycles, starting and stopping many processes including pumps and valves to fill and empty the drum with water, heating, and rotating at different speeds, with different combinations of settings for different fabrics.
- Walter T. Grondzik, Alison G. Kwok, Benjamin Stein Mechanical and Electrical Equipment for Buildings, John Wiley and Sons, 2009 ISBN 0-470-19565-7 page 1201
- http://www.hydro.mb.ca/your_home/home_comfort/block_heaters.pdf Car warmers, block heaters, and energy controls.[dead link]