Discrete NAND Synthesizer Help

Discussion in 'General Electronics Chat' started by lofiboy, Dec 26, 2010.

  1. lofiboy

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    Dec 26, 2010
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    I know it's possible to create an audible square wave generator using a 4093 or a 7400 by connecting one input to high and the other to ground using a capacitor, then feeding the output back into the grounded input through a resistor.

    Not having access to either of these, I made a (working) TTL NAND gate using c2240 transistors running on 6v. I can't, however, make it oscillate.

    I assumed it would work because the logic function is identical to that of a 4093, though the IC's design is much more intricate, but I just can't seem to make it work.

    When connected to an amplifier, all I hear is uneven crackling. Does the malfunction have something to do with the 7400/4093's totem-pole output (and my circuit's lack of one), my 6v power source, or am I just flat out wrong in my design?
     
  2. Ron H

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    CD4093 is a Schmitt trigger. This fact is essential to getting it to oscillate. A 7400 will not oscillate (except maybe at a very high frequency) when you apply the RC feedback, because the inputs have no hysteresis. In fact, it is not necessary that the device be a NAND gate. A Schmitt trigger inverter (such as a 7414) will work just fine.
     
    Last edited: Dec 27, 2010
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  3. lofiboy

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    Dec 26, 2010
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    Thanks for the advice. I didn't realize that the hysteresis was a vital part of the 4093 oscillator's function.

    Off to build a multivibrator!
     
  4. Ron H

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    I saw your thread on Electro Tech. A multivibrator is a good way to start. I could also show you how to make an oscillating Schmitt trigger using discrete parts (transistors, resistors, and a capacitor).
    BTW, you got some bogus advice on Electro Tech. Most CMOS gates do not have input hysteresis.
     
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  5. lofiboy

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    Thanks for rectifying that.

    As you probably read on electro tech, I can't obtain IC's here, so learning to make discrete Schmitt triggers would be a huge aid!
     
  6. Ron H

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    The one I post won't have the same circuit as a TTL Schmitt trigger, but it has a TTL-compatible output.
    Are you still interested?
    What are you wanting to connect it to?
    What supply voltage do you want to use?
     
  7. lofiboy

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    Dec 26, 2010
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    As long as it's frequency can be controlled through a potentiometer, it's more than useful to me as I'm planning on making a synthesizer with it. The oscillating frequency should also be pretty low as the audible spectrum only ranges from about 20Hz to 20KHz.

    I'm planning to use either a 6v or 9v supply.
     
  8. bertus

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  9. Ron H

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    I hope you aren't planning to change the frequency over that range without changing the capacitor. It can be done, but it requires a more complex circuit.
     
  10. lofiboy

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    Dec 26, 2010
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  11. lofiboy

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    Well not over the whole spectrum, no. I was planning on building several with different ranges.
     
  12. lofiboy

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    Ron H: Not over the whole spectrum. I wanted to build several (for polyphony) with different ranges
     
  13. Ron H

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    If you want to play with a basic circuit with limited frequency range, build the basic version. For wider frequency range (full audio), build the full version.
    In either one, you can change C1 to change the range.
    If you can get your hands on a 555, use it instead.:D
     
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  14. Jony130

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    Here it is another version of a Schmitt trigger oscillator.
     
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  15. lofiboy

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    Dec 26, 2010
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    Thanks guys.

    I've successfully built a TTL multivibrator but I'm having trouble actively modulating the frequency. Perhaps one of these Schmitt trigger osc's will bring more success.

    I'll attempt the basic one first as I'm still pretty new to this stuff. The darlington pair (i believe) version is also very interesting.

    As a side inquery, would it be practical/possible to operate a two transistor multivibrator using 2n222's at RF (AM) range? I don't know if the transistors are fast enough.
     
  16. Wendy

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  17. marshallf3

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    If you mean a 2N2222 it will work fine in the AM broadcast range. just pull up a data sheet.
     
  18. Ron H

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    I suggested that in post #13. In post #10, lofiboy had said
     
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