555 Timer Frequency

Discussion in 'General Electronics Chat' started by Sparky49, Jun 10, 2014.

  1. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
    834
    417
    Hi all.

    I've been bread boarding a circuit to allow me to control frequency and pulse width of a square wave independently. To do this, I have been using this circuit:

    [​IMG]

    However, whilst I can control the pulse width, I cannot seem to be able to control the frequency. I am sure I have built the circuit correctly, however, I have attached a picture below.

    I am sure I am missing something quite obvious. :rolleyes:

    I you would like any measurements, scope pictures, videos, etc, feel free to ask.

    Many regards, and thanks for your time.

    Sparky
     
  2. samuel.whiskers

    Member

    Mar 17, 2014
    95
    2
    I just built something similar - I used three 555's - one for pulse frequency, one for pulse duration and one for the pulse tone frequency.

    Are you not using pin 3 of your 555?? What about pin 4 - isn't it supposed to be held high for your astable oscillator to run??
     
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  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Although it may have nothing to do with your issue, pin 4 of the 555 should be tied high, and (of lesser importance) pin 5 should be tied low through a .01uF cap.
     
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  4. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    417
    Thanks guys. I'm using the charging curves of the cap to create the frequency, and a comparator to alter the pulse width, the 555 is just discharging the capacitor, so there is no need (I believe) for the reset and control cap as I'm not using the output (3).

    I've added them in just to check, and alas, no difference.

    I've also hooked up a speaker, and I can confirm that it is not the measurement at fault. The volume does change, but the pitch does not.

    Sparky
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    767
    The NE555 Clock and PWM Adjustable circuit.

    [​IMG]
     
    Last edited: Jun 10, 2014
  6. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    417
    Hi Scott, I'm afraid I cannot see the image.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    Try exchanging the positions of R1 and VR1.
    Also, do you have VR1 correctly wired as a rheostat?
     
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  8. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    417
    Turns out I had incorrectly connect VR1. Doh.

    I can now change the frequency, however the pulse width section seems to be causing problems. :\

    Below is a picture of my circuit as it is now, and the output signal. Before, I was getting a nice (relatively) clean square wave, but now this has quite a bit of noise. Any ideas?

    Regards,

    Sparky

    EDIT: Seems I can 'fix' it with putting a resistor in series with the speaker. However, the overall volume is much lower, which kind of defeats the point. Any ideas how to get around this?
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    Did you see the circuit that I attached on #5.

    I was used the dip switches to switching the clock and pwm functions, when you understand the theory then you can adding them to your circuit.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,853
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    The waveform of the picture is not clear, and the waveform seems look not normally, does the swp(sweep) var didn't adjust to a stable situation?
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    You still have pin 4 wired to gnd. Pin 4 is a reset pin, unless you need to reset thr 555 it must be high.

    You have pin 3, the output, wired to the pot. Huh?

    VR1 should never be allowed to go tp 0 ohms. Put a 100 ohm resistor in series with it.

    9V batteries are notorious wimps. Add a 220 uF cap across the battery to suppress surges.

    Try using the schematic you posted. The circuit will work as predicted if wired correctly.
     
    Last edited: Jun 11, 2014
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