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Variable one-shot timer

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New Member

I am hoping to build a circuit to turn a standard desk light on for a given period of time, and then off.

I'd like the time to be variable, controlled by a dial, and the shot triggered by a button.

The time would be between 1 and 30 minutes.

It would also be handy to have a second button to terminate the light at any point.

Can you please point me in the right direction? I am clearly new... :0)

Can I generally replace the R1 resistor (in any of the many examples I can find on the internet) with a variable resistor to achieve this effect?

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If you are talking about a 555 type of timer, the delay is proportional to RC, so to get 30 min (1800sec), if the R is 10megΩ (10,000,000Ω), the C must be 1800/1e7 = 180uF. If you read the data sheet for the 555, it cautions against making the R bigger than about 10meg.

If the lamp you are switching is 120V ac, you should use a relay between the 555 and the AC line to do that switching.

Resetting the timing period early can be done with momentary push button. Post the schematic that you thinking of using, and we'll tell you where to put it (the switch, that is):D
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Well-Known Member
For $5.00 you can buy a timer than turns things on and off every 20 minutes. It simply plugs into the wall and you plug the device (lamp) into the timer.

schmitt trigger

Well-Known Member
The simplest -for long duration timers- is to run a 555 in astable mode, running at 1 hz, and then with a divider like the CD4060 or CD4020 to count down the pulses. You would need some gates at the counter's output to select the proper combination of outputs that will provide the delay you want.

And as MikeMi advised, you'll need a relay to perform the actual lamp switching.
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