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I am trying to use a push button switch to send 120VAC for a certain amount of time after it has been pressed. I want it to only be pressed once, and not have to be held in the closed position. Is there some sort of timer/timed relay I could use to accomplish this?

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  • \$\begingroup\$ Have you done any research? A simple google search "timer circuit" yields scads of results. See: electronics.stackexchange.com/help/how-to-ask electronics.stackexchange.com/help/dont-ask It is forbidden to write non-specific questions here. \$\endgroup\$ – Voltage Spike Aug 15 '16 at 22:59
  • \$\begingroup\$ I did look into timed relays and timer circuits but was confused on how to implement a single button push to activate the circuit. All the answers have been very helpful in understanding and solving my problem. \$\endgroup\$ – John Aug 16 '16 at 16:19
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There are industrial timing relays that can do this, in addition to the pneumatic ones that Majenko mentioned. From the outside, they appear to work similarly to traditional relays, except that there's a (usually programmable) delay between the "coil" receiving power and the contacts moving. Here's one possible way to wire one:

schematic

simulate this circuit – Schematic created using CircuitLab

  • When you push the button, the load and both coils get power. The normal relay closes immediately and maintains power when the button is released.
  • The timer relay does nothing for the programmed time. When it finally moves, it removes the normal relay's ability to hold the circuit on, and so everything resets.
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You can buy buttons that do exactly that. They are often pneumatic, holding themselves in until enough air has leaked in to release them. They're used for lighting stair wells, etc.

Alternatively a 555 timer in monostable mode driving a relay, or a small microcontroller doing the same job (easier to make configurable and get more precise times).

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You can also get various DIN and panel mount timer relays that do this.

Purely as an example:

http://au.rs-online.com/web/p/products/8285161/?grossPrice=Y&cm_mmc=AU-PLA--google--PLA_AU_EN_Automation_And_Control_Gear-_-Timers_And_Counters&mkwid=scbgdPzdy_dc|pcrid|99325705834|pkw||pmt||prd|8285161

This may not be exactly what you are after but you can generally get various timer schemes or multifunction units in various voltage ranges.

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  • \$\begingroup\$ I like that your answer offers an "Industrial", safe, plug-and-play solution. \$\endgroup\$ – Michael Molter Aug 16 '16 at 7:02

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