Electronics > Beginners

Is there a way to prevent re-trigger on one-shot (555)?

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Ian.M:

--- Quote from: bitbanger on January 09, 2019, 12:28:19 am ---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?

--- End quote ---
Unless there's a safety certification reason to avoid a programmable device, this is probably a job for a small low in count MCU e.g PIC10 or PIC12 or ATtiny etc.   With no analog RC timing networks that are vulnerable to leakage currents, It will be far easier to enforce a reasonably accurate lockout period.   If you want it field-adjustable, a simple preset feeding an ADC input + code to handle it can add that.  It could also handle the debouncing.   

Even if you need to work with voltages above 5.5V so have to add a small regulator to power it and some sort of driver transistor or level shifter for the output its still a win over multiple monostables etc.

bitbanger:

--- Quote from: Ian.M on January 10, 2019, 09:16:43 am ---Unless there's a safety certification reason to avoid a programmable device, this is probably a job for a small low in count MCU e.g PIC10 or PIC12 or ATtiny etc.   [...] If you want it field-adjustable, a simple preset feeding an ADC input + code to handle it can add that.  It could also handle the debouncing.
--- End quote ---

Yeah you know this was not a decision/direction I made lightly! My background is small micros and I thought the exact same (defined settings via multi-tap resistor network and A2D). To be honest it was very tough to not go down the feature-creep rabbit hole with a micro ("well why not have a setting pushbutton that you press and hold to set the pulse width and lockout period"; "there are some super cool micro LCDs/character displays these days, why not eliminate the need for a scope",... and so on.

This is application is for a one-off need, with slight potential for re-use down the road. However at this time the other ranges aren't a *real* neccessity (really just to tune, but why not give an entire range), and timing doesn't have to be super precise. Ultimately the end user would be more comfortable with the ability to swap resistor/cap values if the ranges were outside of those they needed.

I guess with age comes more refined restraint?  :D

schmitt trigger:
"CD" is the old RCA's prefix. The important part is the number.

But a number of companies, OnSemi, TI, Nexperia still make them. ONLY the prefixes change.

Benta:

--- Quote from: bitbanger on January 10, 2019, 03:39:41 am ---Okay schmitt trigger, Benta, I'll concede, the CD4538 looks much simpler!

Unfortunately it's long-gone 4000-series logic.

I feel trolled.  ;D

--- End quote ---

4000-series logic is alive and kicking and will be for many years to come. Check TI and ON Semi.

Zero999:
Lots of it is discontinued though.

The good thing about the old CMOS is the wide power supply voltage, noise immunity and insensitivity to poor supply decoupling. I agree that the 4000 for awhile.

Do they still make microcontrollers with old 4000 series technology? I know lots of old mask programmable MCUs did, but don't know if they're still used.

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