Need a way to reset the timing on a Monostable 555 Circuit

Thread Starter

sciengart

Joined Mar 25, 2020
36
monostable 555 switch - Works - AB.gifHey guys, so this is the circuit I am working with.

I just simply want it, so that if I re-press the switch in mid cycle, the one-off timer starts again.
Any help would be appreciated :)
 

dl324

Joined Mar 30, 2015
11,526
I found using the reset to ground for switch terminals instead, and 100k as you suggested to replace the 8-4 connection was the solution.
Not sure what you're trying to say. Reset will only work if you ground the pin. That's what the bubble on the reset pin means; it's LOW active.
 

ElectricSpidey

Joined Dec 2, 2017
1,237
Grounding the reset pin during a one shot cycle will end the cycle but not restart it.

Look up re-triggerable 555.

This is done by bringing the trigger low and discharging the timing cap at the same time.
 

crutschow

Joined Mar 14, 2008
25,419
I'm not sure that using the Reset input will work for your purpose, since the output goes low when the reset goes low (or is that what you want?).

Here's the LTspice simulation of a circuit that provides the retriggerable function, which resets the timeout each time the PB is pressed..
The simulation shows the output staying high until the switch stays open for the timeout time (a little over 1 second for the component values shown).
Note that the timeout starts when the switch opens.

1585277411425.png
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
My configuration enables me to set output high for a predetermined duration.
My objective was to be able to retrigger that interval at any time during the high state, and start again which my revised schematic did well thanks to dl324! thanks all for your help though!
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
This schematic has been removed by sciengart to prevent future users from accessing a bad schematic.
See Below for a working diagram.
 
Last edited:

Thread Starter

sciengart

Joined Mar 25, 2020
36
If anyone knows how to get that switch to work from a positive supply rather than a negative, now, that would be a feat.
I so far cannot make that happen.

But this meets my needs.I have control of the length of the high, as well as complete re-triggering without an overly complex schematic.

Also a cool little thing, if you increase the resistor value between 4 and 8 to 100k, you get a voltage drop across the capacitor that causes a pretty cool pitch shift that you could sample if any of you are audio engineers who compose as well.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
I think we are all at lost and confused at what you are trying to do. Draw a pic, its worth a thousand words.
The circuit you post makes absolutely no sense. R3 has nothing to do with 'timing'.
I don't know how it makes no sense, it works absolutely perfectly.

When I press the switch, pin 3 goes high, for a duration of "x" milliseconds. Both the 10uf capacitor, and R3 can be varied to increase and decrease the length of the high output. After the set duration, it turns off.
Not sure how it doesnt make sense, I mean dl324 understood what I was asking for from the start and within minutes helped me solve the problem.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
Darn it, I see what you are saying. I have no idea why the cap is over there. Sorry, I must have switched that and forgot.
This is the fully working model, super sorry guys, and thanks!Resettable Monostable 555 - Works - AB.gif
 

dl324

Joined Mar 30, 2015
11,526
This is the fully working model,
At this point, I think it might be a good idea for us to tell us again what you're trying to accomplish.

The timing capacitor isn't normally put on the trigger input. Timing is controlled by the the voltage at the threshold input. When that voltage reaches 0.67Vcc, the timer resets and the output goes LOW.

By putting the timing cap on the trigger input, the output will remain high until the voltage at that pin reaches 0.33Vcc. Having the threshold pin tied to Vcc through the 100k resistor is unusual.

I'm not sure what you're trying to tell us with the labels on the battery. At first glance, I thought ground was at -6V, but then I thought you might be trying to indicate the terminals on a 6V battery.
 

Alec_t

Joined Sep 17, 2013
11,642
You might want to add some resistance (~100-300 Ohms, say) in series with the push-switch contacts to limit the cap discharge current and so prolong contact life.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
Top