Unable to get lower duty cycle for monostable w/ astable input via opamps

Thread Starter

vouzmevoyez

Joined Apr 30, 2024
7
Hi, I have to keep my frequency ~800Hz which means T = 1.25ms. Using a variable resistor, I have to get a duty cycle of 65% (which I achieved @ Rpot = 23.5k ohms) and 15% when the potentiometer is at 0. However when I try to trial and error simulation values, my output waveform doubles per period. I believe it has something to do with the capacitor charge time, but I'm not sure how to fix or account for it.

I am using an astable cascaded to a monostable

Link here for visualization of what I'm trying to say
 

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dl324

Joined Mar 30, 2015
17,096
Welcome to AAC!

Circuit for those who don't, or choose not to, use LTspice:
aacCircuit.jpg
I haven't attempted to analyze the circuit because I dislike reading LTspice schematics...
 

Papabravo

Joined Feb 24, 2006
21,301
Unfortunately I am limited to only using astable and monostable circuits
Is this an artificially imposed restriction to see if you can accomplish your goal without using other parts, or is it a more practical limitation? I'll try to help you in the latter case, but not in the former.
 

Thread Starter

vouzmevoyez

Joined Apr 30, 2024
7
Is this an artificially imposed restriction to see if you can accomplish your goal without using other parts, or is it a more practical limitation? I'll try to help you in the latter case, but not in the former.
It is something for my class, so I guess it's kind of the latter? Using other circuits would void my work, so yeah
 

crutschow

Joined Mar 14, 2008
34,817
You are using a slow op amp which causes the observed slow rise and fall times.
A wider bandwidth op amp with a faster slew rate would help that.
 

Thread Starter

vouzmevoyez

Joined Apr 30, 2024
7
You are using a slow op amp which causes the observed slow rise and fall times.
A wider bandwidth op amp with a faster slew rate would help that.
Would that be the only workaround for this? I thought it had something to do with the RC differentiator circuit, so I've been dabbling on it since the interval between my input pulses are shorter than my desired period + capacitor charging time but I can't seem to get it right. Am I missing another angle to this? Thank you hehe
 

Ian0

Joined Aug 7, 2020
10,232

Thread Starter

vouzmevoyez

Joined Apr 30, 2024
7
You should never us an op-amp as a comparator. Who is it that is teaching you such bad practice.

Look at the OP07 datasheet.
https://www.ti.com/lit/ds/symlink/op07c.pdf?ts=1714635590482&ref_url=https%3A%2F%2Fwww.google.com%2F
It has Back-to-back clamp diodes across its inputs. It is only is to be used as a linear amplifier. It will never work properly in your circuit.
Oh, thank you! I only used op07 since a friend told me it was better for simulation since the lm358 one was too slow on ltspice. My only other sources are from circuitdigest and electronics tutorials as it had worked with other people, but for different duty cycles
 

crutschow

Joined Mar 14, 2008
34,817
Not clear as what you want the output to be exactly (?).
First you say you achieved the desired output and then you say you didn't.
 

Thread Starter

vouzmevoyez

Joined Apr 30, 2024
7
Not clear as what you want the output to be exactly (?).
First you say you achieved the desired output and then you say you didn't.
I'm sorry, the output I achieved is only for a 65% duty cycle, the lower one which is 15% is still unattainable to me. I need to get both. The output I get when I try to attain a lower duty cycle by varying the potentiometer to a lower value is something like this, 2 PW @ 800Hz, but this only happens at certain potentiometer values below 10k(?). If I go above 10k, the PW is still small and it doesnt double, but not small enough for 15% Dc1714662362927.png
 

crutschow

Joined Mar 14, 2008
34,817
Try these changes--
  • Replace the op amps with an arbitrary faster device type to reduce their rise and fall times..
  • Move R_POT to the feedback for U3 (R4) since that is the best place to adjust the one-shot period.
 
Hi! I have the same problem!


OP, were you able to make it work? I can’t for the love of all things good, make my monostable multivibrator work at around <20% duty cycle. Does anyone have any suggestions moving forward?

p.s. just like OP, I am only limited to an astable to monostable multivibrator config

thanks!
 
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