How to Trigger 555 Timer SPST

Thread Starter

oKCfGYhmJQi

Joined May 3, 2019
60
I have another thread where some forum members have helped me formulate my thinking on a project, and as we've made our way through the "big picture" questions I've come to realize that I might be better served by asking some of the smaller questions in their own threads, so I'm creating a few. For reference, here is the "root" thread: https://forum.allaboutcircuits.com/threads/high-speed-actuation-sensing.159382/

Groundwork: I want to trigger a timed pulse in response to an SPST switch going low. The switch would stay low, and this needs to not affect the pulse (i.e., the pulse still needs to end, even though the trigger is still low). Currently I'm looking at the 555, mainly because it was the first thing suggested by a friend of mine, but in reading about it I'm coming to the conclusion that it won't behave this way on its own.

Questions:
  • Is it true that in monostable mode for the 555, if the trigger goes and stays low, the output pulse won't end after the timer runs out?
  • If the above is true, how can this behavior be made compatible with a trigger that stays low?
  • If it can't be made compatible, what are some other options (other approaches, other ICs, etc.)?
 

crutschow

Joined Mar 14, 2008
23,364
Questions:
Is it true that in monostable mode for the 555, if the trigger goes and stays low, the output pulse won't end after the timer runs out?
Yes.
If the above is true, how can this behavior be made compatible with a trigger that stays low?
You can use a capacitor in series with the trigger input, so only triggers on the negative edge of the input pulse (below).

upload_2019-5-8_13-40-58.png
 

Thread Starter

oKCfGYhmJQi

Joined May 3, 2019
60
Yes.You can use a capacitor in series with the trigger input, so only triggers on the negative edge of the input pulse (below).

View attachment 176893
Thank you, that is most informative. Let me ask, how robust is this solution? Would it be expected to work, for example, in a range of temperatures? What about stability? Could the 12v IN signal stay low indefinitely without retriggering the timer?

I'm also curious whether you think the 555 is a good choice in this scenario.
 

AnalogKid

Joined Aug 1, 2013
8,108
how robust is this solution?
Would it be expected to work, for example, in a range of temperatures?
What about stability?
Could the 12v IN signal stay low indefinitely without retriggering the timer?

I'm also curious whether you think the 555 is a good choice in this scenario.
Very.
Yes.
As long as the input pulse former always is shorter than the 555 pulse timer (R2-C1), there is no conflict. Both the trigger pulse and the 555 output pulse are as stable as their components' tolerances and behavioral characteristics.
Yes.

Not my favorite part in the whole world, but if you don't need any other combinatorial logic for other functions it is a good choice.

ak
 

Thread Starter

oKCfGYhmJQi

Joined May 3, 2019
60
Not my favorite part in the whole world, but if you don't need any other combinatorial logic for other functions it is a good choice.
I'm curious what you mean by "combinatorial logic" here. Would the choice to use the 555 be impacted by the presence of other components in this circuit? If so, how?
 

AnalogKid

Joined Aug 1, 2013
8,108
I'm curious what you mean by "combinatorial logic" here. Would the choice to use the 555 be impacted by the presence of other components in this circuit? If so, how?
If there were other CMOS gates doing other things in the circuit, and there was one or two spare gates floating around, I'd use them instead of the 555.

ak
 

crutschow

Joined Mar 14, 2008
23,364
That will continue to trigger if the input pulse stays low longer than the output pulse, which is what the TS wanted to avoid.

The 555 circuit I posted is a reasonably good solution to the problem.
The input pulse can stay low indefinitely without re-triggering the one-shot.
It produces a stable output pulse-width over voltage and temperature changes, and can output more current than most logic circuits can.
 

Thread Starter

oKCfGYhmJQi

Joined May 3, 2019
60
That is the standard circuit, and has all of the problems asked about in post #1. The thing to remember about the 555 is that it is *not* either of the two standard monostable functions, retriggerable or non.

ak
When you mention 'standard monostable functions,' it makes me wonder what the standard is. Are there other chips that do the monostable thing? What differentiates them as per suitability to particular circuits?
 

SamR

Joined Mar 19, 2019
1,170
The 555 has 2 standard operating modes. Monostable - one shot output. Astable - multivibrator/digital oscillator output. Look up the NE555 datasheet PDF. It is the most used Integrated Circuit chip ever made and still being made since it was invented in the 60's.
 

mvas

Joined Jun 19, 2017
538
That will continue to trigger if the input pulse stays low longer than the output pulse, which is what the TS wanted to avoid.

The 555 circuit I posted is a reasonably good solution to the problem.
The input pulse can stay low indefinitely without re-triggering the one-shot.
It produces a stable output pulse-width over voltage and temperature changes, and can output more current than most logic circuits can.
Yes, I know.
But you jumped directly a final solution.
So, you missed my point.
The TS needs to understand how the a falling edge triggers a 555 Timer.
The TS had questions about the 555, I had answers via an animation.
The animation may help the TS understand how a Falling Edge can trigger a 555 Timer circuit.
When that is understood, then we take the next step - ie Change the Input Signal to a "one-shot".
One step at a time, is a way of teaching, it promotes learning.
 

mvas

Joined Jun 19, 2017
538
That is the standard circuit, and has all of the problems asked about in post #1. The thing to remember about the 555 is that it is *not* either of the two standard monostable functions, retriggerable or non.

ak
Yes, I know.
It was step #1 of the learning process.
I don't just "shout out" the final solution.
The circuit does exactly what I said it does, nothing more, nothing less.
The TS had questions about the 555 Timer.
The animation is a learning tool.
Once the TS understand the basic 555 Timer operation then we can advance to making the input "One-Shot".
You are too impatient and think "handing out a fish" ... is teaching.
I prefer to ... "teach how to fish" which promotes learning.
 

Thread Starter

oKCfGYhmJQi

Joined May 3, 2019
60
Yes, I know.
It was step #1 of the learning process.
I don't just "shout out" the final solution.
The circuit does exactly what I said it does, nothing more, nothing less.
The TS had questions about the 555 Timer.
The animation is a learning tool.
Once the TS understand the basic 555 Timer operation then we can advance to making the input "One-Shot".
You are too impatient and think "handing out a fish" ... is teaching.
I prefer to ... "teach how to fish" which promotes learning.
Guys, it's OK. I appreciate both of you.
 

AnalogKid

Joined Aug 1, 2013
8,108
The TS had questions about the 555 Timer.
The TS had specific questions about the 555 timer; specifically, about why the standard circuit (that you posted) will not work for his application. This was answered in post #2 (complete with a schematic and simulation results) and the followup questions were answered in post #4.

I have other comments, but first - how old are you and where are you located?

ak
 
Last edited:
Top