Pulse Generator

[guscrown]

Alright, I haven't done something like this since college, and I kind'a have the idea of what I need to do, but I can't seem to figure out.

I'm trying to generate a pulse from an input signal. The signal is normally high, and will go low to activate a device. The generated pulse should be HIGH for as long as the input signal is low, and after the input signal goes back to high, the pulse will remain ON for X seconds.



I started refreshing my memory about monostable multivibrators so I fired up Proteus to try and simulate something but it's just not working like the datasheet says it works, and I don't know if this is a simulator problem, or a circuit problem. Doesn't matter what values I set for R and C, the output pulse is always the length of the time constant.

[OM222O]

you first need an edge detector to find the rising edge and send a very quick pulse to reset a monostable timer. Can be easily done via a 556 timer (2 555 timers in 1 package).

Then you can OR the inverse of the signal with the output of the timer (timer should be active low as well).

[guscrown]

Thanks for the advice. I re-did the circuit, not using the 556 you suggested, but I arrived at something that appears to work as I intend.

[OM222O]

You can filter out the small "glitch" in the transition period using a very small value RC filter too.

[guscrown]

Thanks for the help.

Now I just need to figure out if Proteus' model for the 74HC123 is broken, and hence I can't really change the pulse width. The circuit looks correct, at least according to the datasheet, and other examples I've seen online.

[eblc1388]

Another way to achieve your goal, using CD40106B hex inverter. Change value of R1 for the length of "x seconds" after feed signal goes high.