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555 timer circuit; frequency variation

  1. Aug 11, 2010 #1
    I conducted an experiment involving a 555IC timer circuit and the hypothesis investigated the frequency and period of the LED connected.

    Using the formula for frequency; 1.44/(Ra + 2Rb)C
    where R1= 1000R
    R2= 10000R
    C = 10uF

    the frequency equals 6.857 (rounded value)

    now using data studio (a reliable computer program which can graph the frequency) the frequency equals 1.429 (rounded value)

    Now, ive taken into consideration the 5% tolerance of the resistors used, and the variation on the capacitor as these may account for the changed frequency but this still doesnt explain this rather large difference in expected frequency and observed frequency....

    Any explanations for this large difference????
  2. jcsd
  3. Aug 11, 2010 #2
    I've never trusted that new-fangled math stuff, but the 555 is so old school that that formula should get you into the right ballpark. Have you tried a couple more chips to see if they are all the same? What is your power-supply voltage and regulation? I could see a low power voltage or a lot of ripple fooling the chip's comparators, as they fire at 1/3 and 2/3 of the supply.
  4. Aug 11, 2010 #3


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    If the supply voltage is not figured into the formula, which I don't believe it is, then it should make no difference as long as it is within the specs on the data sheet. I would suspect capacitor tolerance.
  5. Aug 11, 2010 #4
    I checked the power supply voltage and regulation and it is 6V and the regulation is very high. So the ripple is minimal.

    Also i have tried a few chips all returning the same results.

    Thanks for the other advice about the capacitor tolerance. I measured it with the capacitance meter and it seemed fine, but I'll try a few more capacitors.
  6. Aug 12, 2010 #5
    I was thinking if the supply voltage was quite low, the tolerance between the switching voltages on the capacitor (dis)charging circuit might swamp the calculation. But the OP seems to have good power so that's not an issue.

    It's been a while since I used a 555 and I think I just poked capacitors into the circuit until I got the time I wanted, so I don't remember how accurate the calculation is...
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