Is there a way to prevent re-trigger on one-shot (555)? - Page 1
Forum section: electronics / beginners

author: bitbanger
author_rank: Regular Contributor
timestamp: January 09, 2019, 12:28:19 am
content: 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)

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2019, 09:48:14 am
content: 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/#msg824998If 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.

author: soldar
author_rank: Super Contributor
timestamp: January 09, 2019, 02:50:49 pm
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 09, 2019, 03:15:47 pm
content: 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.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 09, 2019, 06:11:23 pm
content: 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.Schmittthat CD4538 is mighty interesting - I might have to play around with that!

author: bitbanger
author_rank: Regular Contributor
timestamp: January 09, 2019, 06:12:15 pm
content: sim

author: Benta
author_rank: Super Contributor
timestamp: January 09, 2019, 08:08:03 pm
content: The very best solution is NOT to use the 555.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 09, 2019, 10:31:10 pm
content: Quote from: Benta on January 09, 2019, 08:08:03 pmAh, good thing it's a CMOS '556 then.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 10, 2019, 03:39:41 am
content: Okayschmitt trigger,Benta, I'll concede, the CD4538 looks much simpler!Unfortunately it's long-gone 4000-series logic.I feel trolled.

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2019, 08:56:22 am
content: Quote from: bitbanger on January 10, 2019, 03:39:41 amFarnell still sell them.https://uk.farnell.com/c/semiconductors-ics/logic?brand=texas-instruments&logic-family-base-number=cd4538But you need to look for the HEF4538, which is a bit better and more widely available:https://www.digikey.co.uk/products/en?keywords=HEF4538%20https://uk.rs-online.com/web/p/monostable-multivibrators/3559432/https://www.mouser.co.uk/Search/Refine.aspx?Keyword=hef4538There's also the 74HC123, which is even easier to get hold of, but it's limited to maximum voltage of 6V.http://www.ti.com/lit/ds/symlink/cd54hc123.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: January 10, 2019, 09:16:43 am
content: Quote from: bitbanger on January 09, 2019, 12:28:19 amUnless 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.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 10, 2019, 04:02:57 pm
content: Quote from: Ian.M on January 10, 2019, 09:16:43 amYeah 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?

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 10, 2019, 07:27:02 pm
content: "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.

author: Benta
author_rank: Super Contributor
timestamp: January 11, 2019, 07:59:53 pm
content: Quote from: bitbanger on January 10, 2019, 03:39:41 am4000-series logic is alive and kicking and will be for many years to come. Check TI and ON Semi.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2019, 02:02:00 pm
content: 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.

author: bitbanger
author_rank: Regular Contributor
timestamp: January 12, 2019, 02:40:36 pm
content: Seems the 14538 is functionally the same, if not a little better performance wise..is this numbering scheme common (adding the 1 out front)? A way for a manufacture to pickup the series and give resemblance?

author: StillTrying
author_rank: Super Contributor
timestamp: January 12, 2019, 05:55:15 pm
content: After a hour of fiddling I quite like this version.The input triggers can be any width, if the input is kept at the triggered level it outputs a 50ms pulse every 2 seconds. Slow edges on the trigger might be a problem, set to 5us in the simulation.

author: tautech
author_rank: Super Contributor
timestamp: January 12, 2019, 08:05:36 pm
content: A simple 4013 one shot can be useful, Google it.IIRC there are single flip flop IC's available rather than the 4013 dual data-type flip- flop.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 13, 2019, 01:12:20 am
content: Quote from: bitbanger on January 12, 2019, 02:40:36 pmMotorola semiconductor , nowadays OnSemi, does that