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Electronics => Beginners => Topic started by: dteck on March 23, 2016, 06:48:29 pm

Title: 555 triggering?
Post by: dteck on March 23, 2016, 06:48:29 pm
Hi everyone,

So this is probably a silly question but I've never used a 555 before so I'm hoping someone who knows better can help me. I want to make a simple 5 minute timer circuit attached to a pir motion detector (Your basic security light setup) But what I want to know is so you pull the trigger low to start the timer. What happens if you pull it low again during its countdown? does it restart the 5 minute countdown? or does it simply ignore the second trigger while the countdown is running?  :-//

Thanks in advance
Title: Re: 555 triggering?
Post by: matseng on March 23, 2016, 07:25:26 pm
Nothing happens if you re-pull the trigger low during the 5 minute window.  The output will be off after 5 minutes regardless what you do with the trigger input - you can even keep the trigger low constantly and the output will go off when the time is up anyways.

Falstad simulation here: http://goo.gl/hVhuCq (http://goo.gl/hVhuCq)

On my machine the 555 in the simulation timeout is set to about 6 seconds.... Singe click (or click and hold the "H" do the left to take if low.
Title: Re: 555 triggering?
Post by: dteck on March 23, 2016, 07:34:53 pm
Nothing happens if you re-pull the trigger low during the 5 minute window.  The output will be off after 5 minutes regardless what you do with the trigger input - you can even keep the trigger low constantly and the output will go off when the time is up anyways.

Falstad simulation here: http://goo.gl/hVhuCq (http://goo.gl/hVhuCq)

On my machine the 555 in the simulation timeout is set to about 6 seconds.... Singe click (or click and hold the "H" do the left to take if low.

Thank you Matseng. That was very helpful.
Title: Re: 555 triggering?
Post by: Audioguru on March 24, 2016, 01:44:11 am
Nothing happens if you re-pull the trigger low during the 5 minute window.  The output will be off after 5 minutes regardless what you do with the trigger input - you can even keep the trigger low constantly and the output will go off when the time is up anyways.
Absolutely NOT. The Falstad simulator is wrong.

The datasheet says, "The trigger input must be returned high at least 10?s before the end of the timing interval." Or it will never time out.
An applications note from Philips (they bought Signetics who invented the 555) shows a coupling capacitor used to feed a negative-going pulse to the trigger pin and a pullup resistor: