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Old December 24th 20, 03:38 AM posted to sci.electronics.repair
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Default Monostable

I have built a 555 timer monostable circuit that works fine(when pin 2, the trigger is momentary grounded the output, pin 3 goes high for about 10 seconds before going low again) but now, i need a circuit that will work the same way even when pin 2 (trigger) is held low for say 5 minutes. Meaning when pin 2 is held low for a long time, the output will go high for the same 10 seconds and then will go off. it should stay off until pin 2 is removed from ground and sent back to ground again before the output can go high again.
The circuit is for a project that has an infrared obstacle sensor as its input and operates a light when it detects a hand. What I want is even if the hand stays there for a longer time the light should go off after 10 seconds until the person removes the hand and brings it back before the light will come on.

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Old December 24th 20, 03:00 PM posted to sci.electronics.repair
HW HW is offline
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Default Monostable

On Wed, 23 Dec 2020 18:38:29 -0800 (PST), Daniel Fynn
wrote:

Meaning when pin 2 is held low for a long time, the output will go high for the same 10 seconds and then will go off.


What you need is what is called edge triggering. A simple way to
implement this, is to use a capacitor in series with the trigger line.
You will also need a pull-up resistor and a diode.

Try this:

https://www.google.com/search?q=555+...e+edge+trigger
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Old December 24th 20, 11:12 PM posted to sci.electronics.repair
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Default Monostable

On Thursday, December 24, 2020 at 2:00:19 PM UTC, HW wrote:
On Wed, 23 Dec 2020 18:38:29 -0800 (PST), Daniel Fynn
wrote:

Meaning when pin 2 is held low for a long time, the output will go high for the same 10 seconds and then will go off.

What you need is what is called edge triggering. A simple way to
implement this, is to use a capacitor in series with the trigger line.
You will also need a pull-up resistor and a diode.

Try this:

https://www.google.com/search?q=555+...e+edge+trigger

Thanks
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Old January 11th 21, 02:44 PM posted to sci.electronics.repair
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Default Monostable

On 12/23/2020 8:38 PM, Daniel Fynn wrote:
I have built a 555 timer monostable circuit that works fine(when pin 2, the trigger is momentary grounded the output, pin 3 goes high for about 10 seconds before going low again) but now, i need a circuit that will work the same way even when pin 2 (trigger) is held low for say 5 minutes. Meaning when pin 2 is held low for a long time, the output will go high for the same 10 seconds and then will go off. it should stay off until pin 2 is removed from ground and sent back to ground again before the output can go high again.
The circuit is for a project that has an infrared obstacle sensor as its input and operates a light when it detects a hand. What I want is even if the hand stays there for a longer time the light should go off after 10 seconds until the person removes the hand and brings it back before the light will come on.


*How about a differentiator?

If the sensor input is changed slowly, (slow fall time) that might not
work and you would need a schmitt trigger before the differentiator.

http://evalidate.in/lab1/pages/RC/RC...ntiator_I.html

This shows a square wave, But your transition from B+ to 0V will give a
negative pulse to trigger Pin 2.

********************* Mike


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