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

#### Mojo Pin__

Joined Apr 13, 2019
83
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

#### Alec_t

Joined Sep 17, 2013
13,336
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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#### Mojo Pin__

Joined Apr 13, 2019
83
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.

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

#### Jony130

Joined Feb 17, 2009
5,439
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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#### AlbertHall

Joined Jun 4, 2014
12,202
If Vref is 5V then your input pulse must be greater than 5V to trigger it. Try it with a pulse height of 6V.

#### Mojo Pin__

Joined Apr 13, 2019
83
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.

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.

#### Mojo Pin__

Joined Apr 13, 2019
83
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.

#### Mojo Pin__

Joined Apr 13, 2019
83
Thanks for all the help everyone, can anyone recommend a good simulation software or online site to use for circuits?

#### AlbertHall

Joined Jun 4, 2014
12,202
Lots of folk on here use LTspice and it is free - but simple to use it isn't.