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Designing a monoflop

  1. Jul 27, 2010 #1
    Hi, I am a first year undergrad, and for my research, I was asked to design a monoflop (monostable multi-vibrator) which takes in a sine wave, and outputs two square waves which are corresponding to each other (one is on while the other is off, and vice versa)

    I was told to refer to the design in attachment. However, no values were listed in the sample design. I'm just wondering if someone could please help me calculate the values I would need for the circuit, since I haven't learned how to analyze transistor and diodes yet.

    I know from different readings that the collector for T1 and T2 is one output and the collector from T3 is the other output. I have tried arbitary values, and the problems I'm having are that a) the output from T1 and T2 is still a sign wave, and b) the output from T3 is not big enough. Since I have no clue regarding what are the appropriate values for each device, I cannot seem to get what I want on it.

    (Also, I have seen other monoflop designs online, however, the majority of them are creating square waves from a pulse, which is different from the sine wave. If there are any site that uses sine wave as input, I would glad to hear about them.)


    Attached Files:

  2. jcsd
  3. Jul 27, 2010 #2


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    Staff: Mentor

    Welcome to the PF. Schoolwork questions like this normally need to be posted in the Homework Help section of the PF. I'm going to let this thread stay here in EE for now though, as long as the help you are given conforms to the HH rules (hints and questions only, etc.).
  4. Jul 27, 2010 #3


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    The circuit in the figure looks to have issues. Where did you get it from? The diode arrangement especially looks wrong -- is there a connection missing to somewhere from the junction between the two diode cathodes?

    To deal with a sine wave input, you just need lots of gain. You want your output stages to go between cutoff and saturation, so they need to be driven with enough base current (and have a big enough collector resistor) to push them into saturation when they are on.
  5. Jul 27, 2010 #4
    Thank you so much for your quick reply!

    The circuit in the figure was from a paper. My tutor/phd referred me to it. The pic is a direct screen shot from the paper, so the chances of a missing connection is slim. however, if it's wrong, what would you suggest to change?

    cutoff and saturation, does that have to do with the transistor? So in order to get lots of gain, i need big collector resistors. Are there any specifications for other resistors?

    my tutor/phd also mentioned that the RC part controls the length of the square wave. I am confused with which R would that be, and when I tried to vary the C, it did not seem to work.

    I am also currently wondering whether or not the circuit that was given actually does what I require it to do, as in with sine input, outputs two corresponding square waves.

  6. Jul 27, 2010 #5


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    Staff: Mentor

    Since the schematic was from a paper, what did the paper say about the function of the two diodes?

    Do you have a circuit simulator that you can work with in designing your project circuit? Or do you have to do it by hand on a breadboard?

    BTW, I used Google Images to search for the term monoflop (which I hadn't heard before), and got a number of ineresting circuit implementations....
  7. Jul 28, 2010 #6
    Attached is a screen shot of a page of the paper describing how the monoflop works. it seems that it only described how the circuit works, however, no diode was mentioned.

    I am currently working on it on ADS, which allows simulation. I have tried arbitary values with all resistance set to 50 Ohms, and the capacitor set to 10pF, however, the result I got is a sine wave for one output (T1, T2 collector) and a smaller, rounded square wave for the other output at the T3 collector.

    I tried searching for monoflop on google as well. It seems that I should have mentioned that it's also called "monostable multi-vibrator". Have you heard of this before? If so, do you happen to know if the circuit works correctly?

    Attached Files:

  8. Jul 28, 2010 #7


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    So it turns out that the two diodes in this circuit are actually varactors (voltage variables capacitors). Not surprising then that no one could figure out how it was supposed to work without that crucial piece of information.
  9. Jul 29, 2010 #8
    Sorry, when I read it the first time, I didn't see any diodes mentioned and since I couldn't find the mentioned V2 on the diagram, I thought that the V1 for the other part of the circuit is V2.

    Let's see if I understand this correctly, so the varactors are controlled by the voltage source V1, and decides the width of the ouput pulse. But should the diodes be set to some specific value? (this is my first encounter with diodes, and when I simulated with different values fo the diodes, the pulses did not seem to change)? Also, I couldn't seem to get the normal diode to work on ADS, so instead I used a clamp diode, does it have the same function?

    Also, now knowing how the circuit works, does the circuit seem correct? And if so, what would be some of the suggested value for the resistors? for example, around what degree of magnitude should the resistance values be? I have tried different combination of resistance values, however I still can't get the right pulse, and throughout all of this, the output from T1 and T2 collector is still a sine wave.

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