Hello everyone!
I need a simple pulse activated circuit. I have set up an op amp comparator which output goes high just for a split second. I need a circuit that will hold that high output level for some time (minute or more). I thing something with a 555 timer will do the job, but i'm not very familiar with that IC so i can't choose the right circuit.
How about this circuit ?

 

If your trigger input is a negative-going one (e.g. 5v dropping to 0V then back to 5V) you can apply it directly to pin2 0f the 555. If not, you will need a transistor stage to invert the trigger first.

 

Why not reverse the input connections to the comparator so that it outputs a negative going pulse? DUH!

 

OK, so do pulse to pin2 must be going from positive to negative voltage. MikeML, this would be the simplest solution.
Is that all? So pin 5 is left unconnected and i just put whatever capacitor and resistor pair for the time i want the output of the 555 to be high?
Also, lets say i want to interrupt the positive output, will a button that shorts the resistor do it?

 

Can you follow this:?



does it do what you want? Look at the polarity of the inputs?

Obvious improvement: add hysteresis to the comparator.

Does the comparator need a pull-up resistor?

Did I forget the RESET?

 

Yes i can, thanks!

 

For minute-long timing periods, use the CMOS LMC555 version. It handles large timing capacitors better.

ak

 

In inverting comparator if i want to add a 1Mohm resistor for a hysteresis where do i connect it? Between the output and the positive input or between the output and the negative input ?

 

In inverting comparator if i want to add a 1Mohm resistor for a hysteresis where do i connect it? Between the output and the positive input or between the output and the negative input ?

Positive.