7404 oscillator

Discussion in 'The Projects Forum' started by lab-specialist, Nov 25, 2009.

  1. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    question:

    i have constructed a simple logic oscillator from 3 NOT logic gates and a resistor and capacitor.

    see attached .png file

    i am using a 1.0 microfarad cap and a 1k potentiometer as the resistance

    i want a 1.0kHz freq, so i have set the pot to @ 330ohms

    when i use the pot, the circuit produces a beautiful square wave...

    the problem:
    if i remove the pot and replace it with a 330ohm resistor, the circuit no longer oscillates.

    i have tried many different resistors and no oscillation, however, the moment i hook it back up with the pot set at 330, it works again.

    strange...what am i missing
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I learned a different configuration. Here are the ones I know.

    [​IMG]
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    The illo shows a fixed resistor - how is your pot in the circuit - as a pot or as a variable resistor?
     
  4. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    it is as a variable resistor

    i have the ends of the pot connected across pins 1 & 3 of the 7404, with the wiperarm of the pot connected to one of the ends of the pot...i.e. failsafe mode
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Placing the resistance between pins 1 & 3 will not work. It is always best to use schematics with the pin numbers called out, as in the attached scribble.

    The other input pins (9, 11, 13) should be tied low to prevent destructive oscillations.
     
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  6. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    according to your diagram, that is exactly how i have it wired...sorry i left off the pin#'s

    i guess i am missing something here
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Did you ground the inputs to the unused gates?

    I've never seen the configuration you're using. I can't help but be a little suspicious of it.

    The problem with my 7404 schematic is I don't know how to calculate the resultant frequency. It is stable, but hit or miss as far as the oscillator freq goes.
     
  8. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    i know that this works....

    this oscillator works like a charm if the resistor is the pot put in failsafe mode (see file below)

    but if i swap the pot for just an equal resistance...it doesn't oscillate....very strange

    the pot is a ten (10) turn wire wound potentiometer.....so it is almost like it is needing the small "inductance" provided by the pot inorder for it to oscillate
     
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  9. Wendy

    Moderator

    Mar 24, 2008
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    I prefer the 7414 schematic overall, it is more stable and predictable. The 40106 is a direct equivalent chip (same pinout). I don't know your electronics background, you can study the theory of operation here.

    555 Hysteretic Oscillator

    Bill's Index
     
  10. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    this is a project that my students are working on...i instruct an introduction to electronics course for the physics department of my university

    the entire circuit is an ADC using only discrete components when possible

    this 7404 is more than stable enough as the sampling rate / timing circuit

    they have built this circuit for 6 years now and it has always worked as expected

    then for some reason, when they started to wire this circuit this semester, this strange behavior began

    it made no difference if pins 9, 11, & 13 where set low or not

    if i connect the pot to the circuit, it produces a beautiful squarewave (0 to 4.3V) @ 1kHz....try it

    however, when i replace the pot with a resistor...it doesn't oscillate

    there has to be something fundamental that i am missing
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    A new batch of TTL chips perhaps?
     
  12. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    i have tried that....plus different resistors, wires, even different locations on the bread board
     
  13. steveb

    Senior Member

    Jul 3, 2008
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    It's definitely a good idea to use the 7414, Bill! If you are using capacitors, the input voltages will change slowly, which is not ideal on the 7404s.
     
  14. MaxSmoke

    Active Member

    Oct 29, 2009
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    The circuit shown by lab-specialist is correct, the frequency is approximates to the following equation:
    \frac{1}{3(RC)}

    I am not sure why this circuit only works when the variable resistor is used. Could be stray inductance or capacitance, especially if the pot is wire wound.

    I would replace the 7404 with a CMOS version, for example a 74AC04. The 74AC04 will work from 2V0 to 6V0 and is far more likely to oscillate and having a CMOS threshold will given a better mark-space ratio.


     
  15. Wendy

    Moderator

    Mar 24, 2008
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    The design I drew has a nice square wave, except every third wave has a slight distortion (not quite flat) across the top of the waveform. The sides look excellent though.
     
  16. lab-specialist

    Thread Starter New Member

    Nov 25, 2009
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    my circuit produces a great squarewave and a frequency @ 1kHz

    the rest of the circuit contains 741 Op-amps and driving them much above 10kHz is not good
     
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