Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Trying to simulate a square wave. NOT WORKING

  1. Jan 20, 2011 #1
    For my extra credit project i am building a simple square wave osc i am trying to simulate the schematics in Multism, a program i just learned today.

    Here are the original schematics:

    more info on the circuit is found here

    These are my schematics suppositly replicatinmg the ones above. The ocilliscope just shoots up then remains blank.

  2. jcsd
  3. Jan 20, 2011 #2
    You might try two 9-volt batteries, with the midpoint connected to the common ground.

    Bob S
  4. Jan 21, 2011 #3
    now i just get a straight line
  5. Jan 21, 2011 #4
    It seems to me there is a jumper where C1 should be. Did you intend that. If you replace it with a capacitor, does it work?
  6. Jan 22, 2011 #5
    no that didnt work i aslo saw that as a possible problem. It is now a cap
  7. Jan 22, 2011 #6
    It takes a long time to charge C1 before the multivibrator starts. Wait at least 0.25 sec before starting plot. See thumbnail.

    Bob S

    Attached Files:

  8. Jan 22, 2011 #7
    It also looks to me like you have the inverting and non-inverting inputs reversed from the example.
  9. Jan 22, 2011 #8
    omonoid, I just created and ran your circuit and get a beautiful square wave with a frequency of about 210 Hz. I think if you reverse your inputs it will work.
  10. Jan 22, 2011 #9
    You build the circuit yet to proof it works? I agree with Bob S that you need to put two 9V battery and put the middle to ground. In your simulation, you don't have a ground reference anywhere. Even if you build the circuit as in the simulation, it won't work. It is different from you drawing with two battery.
  11. Jan 22, 2011 #10
    I meant to say I created it in SPICE and I did use two 9 V supplies. Afterward I changed the circuit as little as possible to work off of one supply. See attached.

    Attached Files:

  12. Jan 23, 2011 #11
    Took me a few, LOL. You DO need two batteries, give this a try.


    Attached Files:

    • OSC.jpg
      File size:
      91.8 KB
  13. Jan 23, 2011 #12


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    With respect, the initial diagram tells you that you need a + and - supply. The input biasing is taken to the Earth point, which is half way between the two supplies. How could anyone be surprised that the circuit didn't work when connected 'lop sided'? These devices have pretty good common mode rejection but give 'em a chance!
  14. Jan 23, 2011 #13


    User Avatar
    Science Advisor

    This circuit works well in practice, but it does depend on the output either being positive or negative and a simulator can start up with the output being zero.

    I tried this recently and had to deliberately unbalance the circuit by adding a large value resistor from one of the supply rails to the non inverting input.

  15. Jan 23, 2011 #14
    thanks for all the feed back. I would like my final product to have a pot to vary the frequency and one to vary the amplitude. Would i just preplace one of the resistors with a pot to make the freq vary? Also for amplitude would i just wire in a pot before the output?
  16. Jan 23, 2011 #15


    User Avatar
    Science Advisor

    First try the extra resistor shown in the previous post to see if you get oscillation.

    If you do, then R1 and C control the frequency so it is easiest to make R1 variable.

    The voltage at the non inverting input sets the amplitude of the square wave, however adjusting it may cause loss of oscillation.

    You could just put a pot (maybe 10 K) from the output of the opamp to ground and take the output from the moving contact and ground. Try simulating it.
  17. Jan 23, 2011 #16
    ugh i must suck at simulating. My result was a flat line on the oscillospope

    I also noticed its a LM124 op amp. I only have a 741

    here is my schemtaics for the simulation

  18. Jan 23, 2011 #17


    User Avatar
    Science Advisor

    I get delays of a second or so before it starts oscillating.

    Can you set your time so that you watch it for 5 seconds?

    The actual opamp probably doesn't matter for this simulation.
  19. Jan 23, 2011 #18
    Hmm i started at the oscope for like a minute and it was a flat line

    Attached Files:

    • f.png
      File size:
      22.1 KB
  20. Jan 23, 2011 #19


    User Avatar
    Science Advisor

    I tried those values and I get a flat line, too. Try 10 K for R2.
  21. Jan 23, 2011 #20

    I mean no disrespect, but I downloaded multisim, leaned it and made it work in 2 hours using your OP schematic. I posted the screen shot. Exactly how are you having difficulty with the notion the 741 requires a dual polarity supply when it was part of your OP? Anyway, I hope you get your EC; I did my part for fun, LOL.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Trying to simulate a square wave. NOT WORKING