A time delay switch or TDS is a switch which is linked to a timer, allowing the user to delay on/off cycles of the switch. There are a number of different styles of delay switch available for a variety of applications, and many are very easy to configure so that users can get the switch to do exactly what they need it to. Hardware stores, home supply stores, and electronics stores typically sell time delay switches along with useful accessories.
In the most basic form, a time delay switch is a switch linked to a timer which can be set by the user. When the user activates the switch, the timer goes into effect, determining when the light or appliance should be cycled on and off. In an example of how this type of switch might be used, someone could set up a time delay system on the lights of a house prior to a vacation so that the lights would turn on and off as though someone was home, deterring criminals.
Time isn't the only thing that can be linked to a delay switch. Some respond to the levels of light detected by the switch, as in the case of outdoor security lights which turn on at dusk and stay on until dawn. Others can be programmed to respond to humidity, as one might want to do with a bathroom fan to keep the bathroom fully ventilated so that mold does not occur.
There are a number of uses for a time delay switch. Some hotels and public buildings use these switches to turn hallway lights off when not in use, for example, giving people a set period of time after they turn the switch on to reach their rooms before the lights are turned off. Time delay switches are also used in bathrooms and supply closets to ensure that lights aren't left on continuously while no one is present.
Delay switches can also be used for very precise and finicky tasks, as for example in a research laboratory in which someone needs to create very controlled conditions for an experiment. In this situation, using a time delay switch gives the experimenter total precision, which means that the experiment is easier to repeat after the process has been documented. Time delay switches can also be built into automotive ignition systems, ornamental fountains, and virtually any other electronic item imaginable.