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555 oscilator oscilates at wrong frequency

Discussion in 'Electronic Basics' started by asdf, Feb 11, 2004.

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  1. asdf

    asdf Guest

    I have built 555 oscilator, and I need it to oscilate at about 50-100 Hz
    (Hertz, not kilo-Hertz) range. I am just a beginner.

    It should mimic my car engine ignition since I would like to use it as a
    test input for a tachometer I am building. So, 50-100 Hz range corresponds
    to 3000-6000 RPM of the engine.

    R1 is 100 k trim pot (changeable resistance) and C is 0,22 uf. R2 is 1k.
    Scheme is classic, with only one capacitor.

    According to the calculations, this should give me 65 Hz to over 500 Hz
    range which would correpsond to the

    But, when I measure it, my DMM says frequency is in kHz range! When I try my
    DMM on the power outlet, it says correctly 50 Hz, when I connect it to my
    computer and use some program to generate freqeuncy, it shows perfect
    results.

    Could it be that the duty cycle at about 85-90% tricks my DMM into false
    reading? All other tests I have made with md DMM were with 50% duty cycle.

    Is the missing C1 from the Cont pin a problem (I do not use it according the
    the scheme I had).

    What am I doing wrong?
     
  2. Your resistor and capacitor values should give the frequency range you
    are expecting, so the narrow pulse may be confusing the meter. Try
    removing the 1k resistor and reconnecting the 100k variable to the
    output (pin 3) instead of to the positive supply. This will give a
    more nearly 50% duty cycle and a lower minimum frequency. You should
    also connect a fixed minimum resistor in series with the variable, to
    not let you adjust the total all the way to zero resistance.
    Something in the 1k to 10k range, depending on the highest frequency
    you need.
    I doubt this is the problem. The timer should work all right without
    it as long as the DC supply is clean, except for a little more cycle
    to cycle jitter.
    Without seeing your connections, I can't say that you are. But I
    think you may have made a specification error, unless you are
    simulating a 2 cylinder, 4 stroke engine (or a one cylinder, 2 stroke
    engine). There may be more than 1 spark per rotation.
     
  3. You haven't said whether R1 is the upper or the lower resistor. Nor
    whether it's in series with an additional low value resistor to avoid
    the extreme R1=0 state. But from your figures, I'm guessing R1 is the
    upper, and has 10k in series with it. (If it was the lower, your duty
    cycle would be roughly 50% for most of the pot's range.)

    If so, my calculations show a higher duty cycle than you said.

    R1 R2 Low time High time Freq Duty
    Upper Lower ms ms Hz cycle
    ----- ----- -------- --------- ---- -----
    100k 1k 0.152 15.4 64 99%
    50k 1k 0.152 7.78 126 98%
    0k 1k 0.152 1.68 546 92%

    I suppose it depends greatly on your DMM, but FWIW mine copes OK with
    that sort of signal. (And its inverse, a very low duty cycle.)

    Are you confident your circuit is wired correctly? You could compare
    it with this, for example:
    http://www.terrypin.dial.pipex.com/Images/555Multivibrators.gif

    BTW, I think it's always worth adding the cap at control pin 5 for
    peace of mind. I usually use soemthing in range 10nF or 100nF. Your +V
    supply is also decoupled with a large cap too I assume? And another
    100nF for good measure.
     
  4. Hi,

    Just listen to it!

    Connect an earpiece or small speaker to it through a low
    value capacitor and the tone you hear should tell you
    whether it is generating Hz or kHz. You need to load it
    only lightly or the test circuit will alter the oscillating
    frequency too much.

    I bet that you are suffering from a common problem with
    counters that if the waveform is none too clean it sees
    multiple zero-crossings and clocks up the extra cycles.


    Cheers - Joe
     
  5. Jamie

    Jamie Guest

    its possible that you do not have your components correctly selected
    to give you a 50% duty cycle for the +/- side. this would cause the
    meter to not correctly read if it was using a Pulse width method of
    measuring the time in which case the unit may either show you very
    high or very low frequency results.
    if your using it for a Tach signal it is most likely the wave form
    that you need. in which case you may need to use a different Frequency
    counter that actually counts external events per second or use a
    scope to view it.
     
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