Input Lockout For 555 Monostable

[MrAl]

Hello there,


As you know, the 555 timer in monostable configuration is triggered by a low going pulse edge which makes the output go high and stay high for a predetermined length of time. And as you probably also know, if the input pulse STAYS low, the output pulse never goes low again. That is, it wont only stay high for the predetermined time, it will stay high until the input pulse goes back to a high logic level. The ways to get around this are collectively called simply, "Input Lockout". Input lockout prevents the input from affecting the output pulse after the first low going edge starts the timer even if it stays low. A side effect is that it may also mean that the low going pulse can not affect it either if it goes low a second time within too short of a time period. This second effect would also be a benefit to this particular application which would not like to see multiple trigger pulses within some definite time period like 1 second or something like that.

There are different ways to do this, including the simplest which is a simple capacitor and resistor with the right time constant.

But i would like to see other ways of doing this too from your experiences or web searches. These might include circuits that link the output back to the input via a logic gate or two, etc., but not too very complicated. Maybe another 555 even.

All ideas will be appreciated.


Thanks and take care,
Al

 

[alec_t]

An OR gate with one input fed by pin 3 of the 555 should do it,
.....or discard the 555 and use a non-retriggerable monostable (e.g. 74HC221)

 

[MrAl]

Hello,

Thanks alec i'll look into both of those soon. I like the 74HC solution because that seems the simplest.

Pin 3 is the output right? But that goes low again after the timer period has expired. Ideally, this should lock out the switch for maybe 1 second min up to maybe 10 seconds would be really nice. Maybe an additional flip flop in there?

 

[Mr RB]

The 555 reset pin?

Rewiring the connections to the threshold and trigger pins? The 555 is just a comparator and some voltage references, and you should be able to wire it up to give any type of monostable operation you like.

Remember too the trick where you can use the bipolar driver pin 3 output to operate the timing circuit, not just the open collector pin7 discharge pin.

 

[dougy83]

You can also use schmitt-trigger gates to get whatever function you're after. The one shown is for the nonretriggerable monostable.

 

[Ian Rogers]

Remember that a pic12f675 is the same size as a 555... Problem solved...

 

[jpanhalt]

High Tech Chips Inc (HTC) actually has published several 555-like applications. Here is the monostable. I have suggested before that a properly programmed 12F675, like Ian mentions, could probably substitute for the 555 except for: 1) You have to reverse the power connections; and 2) It cannot drive the same load without additional parts. In return, you use fewer parts for most other apps. No one seems to have picked up that suggestion as an FYP.

edit: As a direct answer to the original question, you can use capacitor coupling at the input. Then it won't matter how long you hold the switch down. Here is how Paisley shows it:

**broken link removed**

John

View attachment HTC550.pdf

 

[MrAl]

Hello again,


Thanks for the suggestions.

MrRB:
Well i was hoping you could show a rough diagram of the circuit you might think of

dougy:
Yes that seems like an option too. As long as it cant retrigger for a long time afterward.

Ian:
That's a good idea too, i'll have to see if the person doing this wants to use a uC or not. That would be my choice too i think, but also an alternate using non uC chips would be nice too just in case.

jpanhalt:
Yes, i thought i had mentioned that option in my original post, but maybe that's still the simplest. What worries me about it though is that if we release the switch the switch bounce might cause another trigger. This requires more thought i think.

 

[alec_t]

I posted a latching option in this thread
https://www.electro-tech-online.com...-to-trigger-solenoid-valves-questions.133154/

 

[MrAl]

Hi alec,

Yes i missed your post because i posted less than 2 minutes after you did. I'll go back to that thread and comment on that now. Looks good

 

[schmitt trigger]

A CD4538 has connections to wire it as retriggerable or non-retriggerable.
It has both true and complementary outputs
and has two timers in a single package.

 

[dougy83]

dougy:
Yes that seems like an option too. As long as it cant retrigger for a long time afterward.

The time that it won't trigger for after the initial trigger edge is set by the initial input blanking time (the leftmost RC). The blanking time will always be >= the output pulse width. If you wanted the blanking time to extend a certain time after the last trigger, then simply placing a diode between the first NAND inputs (pointing towards the trigger input) will accomplish that.

 

[Mr RB]

...
MrRB:
Well i was hoping you could show a rough diagram of the circuit you might think of
...

That means I have to design it for you! Do I get your homework marks too?

Seriously now, do you have an exact spec? A couple of timing diagrams would help too, showing exactly what output you want from any possible input, (ie input debounce, all input pulse lengths etc).

 

[MrAl]

Hello again MrRB,


Ha ha, sure, you get an A+ if you submit a really good design, which i know you could

We're also talking about this in another thread. It appears that the best solution is to use a latch on the input and provide the user with two pushbuttons, one to 'arm' and one to 'trigger' the output. That way there can be only one trigger no matter what happens. It's a sort of critical application that cant afford to go wrong.

 

[MrAl]

A CD4538 has connections to wire it as retriggerable or non-retriggerable.
It has both true and complementary outputs
and has two timers in a single package.

Hello there,


We have a slight problem with the choice of this part however: CD4538. Im not sure if you are aware of this or not, but that part appears to have been obsoleted. At least that's what all the online parts outlets are reporting. There are still some on the surplus market for cheap if anyone is interested, but not sure if i'd want to use one in a new design. Maybe there is a direct replacement somewhere?

I like this chip too. Maybe there is a replacement part number.

I found MC14538 but im not sure if it is available in DIP package, just small outline 0.05 inch pitch.
Ok nevermind, found some in the DIP 0.1 inch pitch package too now.
Also, CD74HC4538 by Texas Instr. in DIP package too.