What is a Time Delay Switch?

Mary McMahon
Mary McMahon

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.

People can set up a time delay system on the lights in their house prior to a vacation.
People can set up a time delay system on the lights in their house prior to a vacation.

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 delay switches can work as a type of fail-safe for those who frequently forget to physically switch off a light.
Time delay switches can work as a type of fail-safe for those who frequently forget to physically switch off a light.

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.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a EasyTechJunkie researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


I've installed a bunch of these recently in hotel bathrooms to control the vent axia. I didn't build them like @NathanG though. I got them directly from MS Electronics.


@NathanG - Yeah, I had those kits as well. Of course nowadays you can buy a 12v timer delay switch and accomplish the same results, programming whatever interval you desire. I have several that I use for my attic fans.

These units also come with rechargeable batteries that kick in if there is a power failure, so the timer can continue to store your programmed information.


@David09 - Back in the day when I worked as an electronics hobbyist, I used to build delay timer switch circuits. I had a kit which included the circuit board, timer chip and other components and I just followed the diagrams to wire together a timer circuit.

I could set the delay anywhere from seconds all the way up to weeks and even a whole year if I wanted to, but I never took it that far. I think the most I did was to set it up for hourly intervals to switch little lights and relays on and off.


You might be surprised to learn that windshield wipers, which use time delay switches, were once manual in operation at one time. I still have an old Chevy truck with a manual wiper blade. Imagine the hassle of having to rotate that thing on your own, in the midst of a heavy downpour. It would be unsafe to say the least.

The wiper delay switch in modern cars is one of those small conveniences that I consider to be a life saver. I have 5 timer settings in my wiper, from slowest to fastest, so it can handle any rainstorm.


I worked in the electrical utility industry for some time. Time delay switches as an integral part of the test set equipment that we hook up to our relays. The test set sends timed pulses to the relay to run fault tests.

Of course these timer values are in the hundredths or even thousandths of a second, but the concept is still the same. A switch is being turned on and off at various intervals as it sends the currents to the relay.

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