Monostable

Dec 24, 2020 2 Replies

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 sec onds before going low again) but now, i need a circuit that will work the s ame way even when pin 2 (trigger) is held low for say 5 minutes. Meaning wh en 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 ag ain. The circuit is for a project that has an infrared obstacle sensor as its in put 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 second s until the person removes the hand and brings it back before the light wil l come on.

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:

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