Op-Amp Monostable Pulse Generator, won't trigger on pulse

Thread Starter

Mojo Pin__

Joined Apr 13, 2019
60
Hello! I've attached screenshots of my attempt to recreate a circuit in a simulator using everycircuit.com

Firstly, is a square wave the same as a trigger pulse?

The square wave on the simulation is 1V HIGH, 0V LOW with a period of 1ms (these are the default values, I'm not sure what to change them to)

I added a switch so that I could see the output at a steady state before the square wave should come in to change the output but unfortunately I can't seem to get it to behave this way.

I think I have to generate a negative input at the inverting terminal of the op amp in order to trigger the output to change from -15V to 15V but I'm not sure how to do that, is there a way to calculate what the value of Vin should be in order for this to work?

Any advice would be greatly appreciated,

Thank you

Op Amp monostable multivibrator.PNGOp Amp monostable multivibrator values.PNGOp Amp monostable multivibrator schematic EC.png
 

Alec_t

Joined Sep 17, 2013
11,295
Firstly, is a square wave the same as a trigger pulse?
Not necessarily. Although certain circuits can be triggered by a square wave, others might be triggered by non-square wave signals or just voltage levels.
Where did you get that circuit? It's not a common monostable.
Why is the V- input biased at 5V? A 1V amplitude pulse can't pull that down below the V+ pin voltage.
 
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Thread Starter

Mojo Pin__

Joined Apr 13, 2019
60
Not necessarily. Although certain circuits can be triggered by a square wave, others might be triggered by non-square wave signals or just voltage levels.
Where did you get that circuit? It's not a monostable.
Why is the V- input biased at 5V? A 1V amplitude pulse can't pull that down below the V+ pin voltage.
Thank you for your response!

I got it directly from the lecture slide from our Analogue Electronics class, I'm not sure why these values were chosen.

Should I make the pulse value negative so it will pull the +5V down?

Thanks for your help as always,

Keith
Op Amp monostable multivibrator slide.PNG
 

Jony130

Joined Feb 17, 2009
5,163
This circuit should get triggered at a falling edge. But I do not know your simulation time step. So at the beginning try to increase the Cin capacitor value, try 10nF at the beginning and C1 to 470nF.
 
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Thread Starter

Mojo Pin__

Joined Apr 13, 2019
60
This circuit should get triggered at a falling edge. But I do not know your simulation time step. So at the beginning try to increase the Cin capacitor value, try 10nF at the beginning and C1 to 470nF.
Thank you for your response,

I have messed around with the Cin value and it does seem to work eventually but I also need to change the period of the square wave and the amplitude of the square wave, it's quite odd. Perhaps everycircuit.com is not the best simulator to use.
 

Thread Starter

Mojo Pin__

Joined Apr 13, 2019
60
If Vref is 5V then your input pulse must be greater than 5V to trigger it. Try it with a pulse height of 6V.
This makes sense to me but for some reason I can't get the simulation to output the expected values. I tried to make the same circuit in falstad and it seems to work now. It's strange that it won't work on everycircuit simulator.
 

Thread Starter

Mojo Pin__

Joined Apr 13, 2019
60
Thanks for all the help everyone, can anyone recommend a good simulation software or online site to use for circuits?
 
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