Online Multisim using a Schmitt Trigger

[joelsd]

I am a college teacher who is using the online Multisim since due to the pandemic I can't get access to my school's serial numbers. 

I have been trying to simulate an Astable Multivibrator using an op amp with positive feedback to yield a schmitt trigger (see file opamp schmitt trigger).  However, when I run a interactive simulation the circuit doesn't operate.  It appears that it doesn't know how to function in a positive feedback mode dynamically (see astableopamp). 

I can substitute a 555 timer and configure it as a schmitt trigger the astable works property.  However, I would like to demonstrate this with a op amp (see astable555).  I also tried the LM324 (which is the op amp we use in class) and that also fails.  Any help on how to get the op amp version. 

See the screen shots attached.

[GGoodwin]

Hi joelsd,

I have these Multisim Live circuits which are related to what you want to do: four Schmidt triggers, three astable multivibrators, and some pulse generators that use comparator / op amp. These circuits are working fine although they need to be enhanced. While I used virtual devices, these circuits are adequate in proving that Multisim Live "knows" positive feedback and can correctly simulate Schmidt trigger and astable multivibrators.

I can't view the pdf file (problem with handheld device and I don't have an alternative right now) but I have opened the jpeg circuit which uses an op amp. I can't see what op amp did you use as the part no. is behind a reading.

I don't exactly know your circuit's problem, it's just clear that the output is (stuck) at -1.6578 mV. I suggest that you scale up (all) the resistors to 10 kOhms. If you want to preserve the original frequency, scale down the capacitor to 0.1 uF.

If your circuit worked after doing my recommendation, the original resistances may have caused the op amp's overload protection to trip. The resistances are low for power  supply voltages +/- 12 V.

Best regards,

G. Goodwin

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[joelsd]

Thanks.  I tried your suggestion.  Unfortunately, no oscillations; now pinned at -2.6mV.  I am using the online version and maybe that's the problem.  I can't get the serial number for our school since we are sequestered at home.  

[GGoodwin]

I'm also using the online version, free subscription only, my circuits that I mentioned previously are working properly.

In my previous reply I thought of an overload condition which consequently resulted to shutdown. The grapher's time scale is from 9.99 to 10 s, I thought there might be event(s) near 0 s which were already obliterated.

You might want to try this:

Click (select) the capacitor, click Settings (gear icon), check the Initial voltage (IC) checkbox, leave the value at 0 V.

Click at a blank area of the schematic diagram, click Settings (gear icon) again, this time the Simulation settings will be shown. Click the Initial conditions drop-down list, change the option from Determine automatically to User defined.

Start the simulation.

[GGoodwin]

I remember that I also encountered a problem when I was building my first RC astable multivibrator (relaxation oscillator) using a dual-supply op amp in Multisim Live (online). I think the problem is similar to what you have. My circuits are working now although, as I mentioned previously, I used virtual op amp. I believe your circuit can be made to work as well, even when you used other op amp such as LM324, if you try my suggestion in my second reply.

[joelsd]

I have always been using the LM324.  However, initializing the capacitor and setting the simulation IC to user defined works.  Well sort of.  If you look at the attachments below Astable2good shows a symmetrical threshold Schmitt trigger with threshold at -6 and +6 since the supplies are at +12 and -12.  The traces are at the output of the Schmitt trigger and at the input to the Schmitt trigger where the capacitors are connected.  Seem here are the voltage go from +10 to -10 and the thresholds are +5 and -5.  The trace at the capacitor looks weird since its not exponential and from cycle to cycle the breakpoints at the thresholds differ  which make the period of the waveform differ from cycle to cycle.  

The second attachment Astable1good is even worse.  It a non-symmetrical threshold Schmitt trigger with threshold at 1/3VCC and 2/3VCC.  The traces show similarly as the previous case.

Maybe this is due digital simulation of an analog process and it being on the interactive version. 

Any comments?

Thank you for your help.

[GGoodwin]

I cannot view the pdf attachments as I stated previously. Due to community quarantine policies we are only allowed a limited area of activity and we can only go to nearby stores or market. I don't have access to places where I do electronics or simulation. We didn't had clear idea beforehand what and how policies are to be enforced so, currently, I only have a small phone for accessing this forum.

The LM324's output can swing close to Vee especially if lightly loaded. However, typically it cannot swing greater than 2 V below Vcc (Vout limited to Vcc - 2 V). Hence, asymmetry is expected from output voltage levels. On the other hand, if the output is also limited to Vee + 2 V, that needs to be investigated.

If the waveforms manifest noticeable variations in period you can try increasing the resolution of the simulation. Go to Simulation settings, click Maximum time step and/or Initial time step, enable Manual time step and decrease Time step. Note that this will slow-down the simulation because of the increase in data to be computed.

The deviation of capacitor voltage from exponential waveform may have already been solved by increasing the simulation resolution as explained in previous paragraph. If linear segments still manifest, the resistances may need to be scaled up as I described in my first reply.