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Timer (pulse after x seconds)

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CJP

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I've been trying to find a circuit that is normally off, then when a button is pressed it will wait x seconds (say 10-20) and then give a pulse out (to a switch blah blah blah triggering a buzzer got that sorted), so I was wondering if it was simple enough, I've seen the monostable mode 555 but that starts the pulse instantly, or if sinking current it is normally on which triggers the buzzer circuit straight off which is undesired!
I would like to be able to use the 555 chip but other circuits are welcome.

Thanks everyone.
 
There are a few ways to go about it. Depending on the actual times involved one method would be a two stage timer using a pair of 555 timers or a 556 (dual 555 timer). The attached is one such method taken from an old Forrest M Mim's engineering notebook circa 1980.

Another simple method would be to program a uC.

There are also other circuits that could be used.

Ron
 

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  • Two Stage Timer.gif
    Two Stage Timer.gif
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Am I right in saying that while pin 2 is high on 555 number 2, the output stays low until pin 2 goes low again? And that's what enables the time delay?

EDIT: oh I've found a far simpler circuit that carried out the exact task I wanted with only a single 555 timer. Thanks anyway.
 
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The circuit consist of what becomes a pair of 555 timers configured as mono-stable one shot timers. Placing C2 (the trigger) at a logic low starts the first stage one shot which goes high on pin 5 for a time duration setup by R1 and C1. Following that period pin 5 will go low. When that happens it triggers the second stage through pin 8 starting the period of the second one shot. So we trigger a one shot which in turn triggers a second one shot when it times out. Once the second stage times out everything is back to where we began and waiting for another trigger. Thus a two stage timer. The pin not identified in the drawing (Output 2) is pin 9 on a 556 chip.

Ron
 
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