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Is there a frequency formula for this 555 oscillator?

Discussion in 'General Electronics Discussion' started by Calem, Jun 19, 2015.

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  1. Calem


    Jun 18, 2015

    Yesterday I posted this schematic of a simple 555 oscillator. I've built the circuit and it works fine, I'm just wondering if its possible to tell the frequency equation of a circuit like this so that I can determine the effects of changing the capacitor. I know there are frequency equations for circuits similar to this however I've been unable to find the equation for this particular one.

    Any help would be much appreciated.
  2. Supercap2F


    Mar 22, 2014
    Yep, there is a formula for that configuration of the 555. Have a look at pages 9-10 of the datasheet. But the frequency that you calculate and the actual output frequency might be quite a bit different, this is because of large tolerances in the component values.

    When it's hooked up like that it's said to be in astable mode, because the output is constantly changing from high to low.

    Note: it's not a good idea to leave pin 4 (the reset line) floating, instead hook it up to VCC for normal operation. And it is also a good idea to have about a 0.01μF capacitor on pin 5 (see datasheet).

    There is a pretty good explanation of the 555 timer here.
  3. Calem


    Jun 18, 2015
    Thanks so much! :D
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