I am trying to find a circuit that will time itself out after being left on for a certain amount of time. For example if you leave the circuit on it will turn back off after say 20 minutes.

So I am not loooking for a time off delay circuit whereby if you remove power the circuit will turn off after a time delay but instead if you leave power on the power will turn off after a desired time period.

Thanks in advance for any suggestions or solutions!

 

Use a 555 timer one shot controlling a relay. 555 derives power from load side of relay. A momentary switch bypasses relay contacts so when pressed , power goes to load & 555 with short RC delay to pin 2 , which energises relay & starts timing 20 min.When timed out, load & 555 drop out. If load V not compatable with 555 , add another set of contacts to relay for load.

 

Im guessing we are missing something.

You also want the timer to reset if something is done?

For instance If this was a TV, and if no one changed a channel or volume in 20 minutes, it would shut off the TV.

So as soon as the tv was turned on, a timer would start counting down 20min.

If 18min later someone changed the channel, the timer will have to reset to start counting down from 20min again. Else, 2min later, the tv would shut off.

If you want a simple timer that turns a relay off after it has been powered on, 20min later, then Bernard's idea is about as easy as it gets.

 

That is a form of dead man switch - http://en.wikipedia.org/wiki/Dead_man's_switch