Electronics > Beginners
Is there a way to prevent re-trigger on one-shot (555)?
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bitbanger:
Hi -

Been a while since I've dabbled with 555's but it's just the ticket for what I'm trying to do (I think).

I'm trying to figure out an easy way to prevent re-triggering within a certain timeframe on a monostable configuration. I don't mean debounce: I am triggering via a debounced input, however looking to 'lockout' retriggering a few seconds after firing?

The gear I'm controlling switches between two 'modes' with a pulse input, and it doesn't take well to switching back and forth more than once every 5 seconds.

Thoughts? Apologies in advance I don't have the exact circuit in front of me (I simulated in PSPICE and all is well for the basic operation), but similar to this topology except an an edge triggered configuration (DC blocking cap and snubbing diode attached to pin2)

Zero999:
A 555 monostable is not retriggerable. Once triggered, it no longer responds to further trigger pulses, until the delay has elapsed, so the input doesn't need to be debounced.

Are you referring to the issue that ff the input is held low for longer than the time delay, then the output will remain high, until the output goes high again? If so, that can easily be solved by AC coupling the input, via a small capacitor.

https://www.eevblog.com/forum/beginners/555-monostable-always-on/msg824998/#msg824998

If you want to prevent the 555 from being triggered again, after a certain period of time, then you need another 555 timer monostable circuit, with its output connected to pin 4 of the other circuit.
soldar:
So what I understand is that you have a monostable for 2 sec but want to prevent it from being fired for five seconds after it is fired. Then you can just have two monostables, one for two seconds and one for five seconds which disables the first one.  You can even find two 555 timers in one single chip.
schmitt trigger:
Now, if  one really wants to be able to choose between retriggerable or non-retriggerable options, able to trigger on either negative or positive pulses,  simultaneously have both an output and its complemented output, and two independent devices in a single IC package, you can always consider a CD4538.
bitbanger:
Hey thanks for the responses everyone - it hit me last night that I could stage two one-shots. This isn't though how I've seen it traditionally done, but simulates OK and I'll breadboard it to verify. Basically the first monostable fires on pushbutton and latches for the entire "lockout" duration (1-2S).

The second, 50mS one-shot is triggered from the first, and is edge-triggered so it doesn't re-trigger with the input still latched.

Schmitt that CD4538 is mighty interesting - I might have to play around with that!
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