In circuit design, there are a lot of valid reasons for choosing to control an event based on a fixed period of time. Timer relays are commonly used to achieve this in lots of electronic circuitry including housing, mobile equipment, buses, and even work trucks and off-road vehicles. The key to successfully designing a time delay circuit is choosing the right timer relays for your purposes.

To choose the perfect relay for your application, you must spend some time thinking through exactly how the circuit will operate and specifically what will initiate the relay.

• Do you want the delay to start counting when the contacts are closed and the voltage applied or when the contacts are opened and the voltage removed?
• Will the output be on or off while the timing is ongoing?
• What happens if the contacts are opened during the timing process, and will this cause the load to de-energise?

There are three main types of timer relays which are commonly available. These are:

• One-shot relays
• On-delay relays
• Off-delay relays

One-shot timer relays should be chosen when one function begins a second function which will need to remain activated for a fixed period of time. For example, you may want a heater to only operate for ten minutes to avoid overheating or damaging the surrounding area. In that situation, the operating switch for the heater will be connected to the timer input while the heater is connected to the output of the timer. Turning a switch will activate the heater for the specified period, and at the end of the period, it turns off automatically no matter which position the switch may be in.

On-delay relays are commonly used where a circuit function is needed to activate a delay once another function has taken place. Suppose you wanted a small delay between pressing the start button on a piece of machinery and the machinery beginning to work. By connecting an on-delay relay between the switch and the machinery, you can set a ten- or fifteen-second delay. In this scenario, once the machinery has started it will stay working until the timer input is taken away.

Off-delay timer relays can be used when one function starts a second function but the second function needs to remain on for a set time once the first function has turned off. Examples of this type of function can be seen everywhere. One use is in lighting where movement or opening an automatic door triggers a light switching on. It is common for the light to remain on for a set time after the door has closed or movement has been sensed.

Once you know the type of timer relays you will need for your project, you can begin to design your circuits and determine exactly which relays would be best for your situation.