LM358 Square Wave Oscillator Help

Discussion in 'Homework Help' started by RKG, Apr 5, 2011.

  1. RKG

    Thread Starter New Member

    Apr 5, 2011
    I am going to be using a LM358 as a Square Wave Oscillator for a college project but am having some trouble in calculating what Resistor and Capacitor values I'll need to use to produce a frequency of 1Hz. I'll be using the oscillator on pg 14 of this document: http://www.national.com/ds/LM/LM158.pdf

    I've found many equations during my research but am unable to find one that relates to the LM358.

    The most common equation I have found is: T=2RC ln[(1+λ)/(1-λ)] Where λ=R1/R1+R2

    Of course this does not take into account R3 of the LM358 circuit. Can I leave R3 out of the equation or do I change the equation to this: λ=R1/R1+R2+R3 ?

    Any help would be much appreciated.
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    For your circuit supply form single supply voltage equation for T1 is equal

    T1=R1*C*\ln \frac{Vcc - VL}{Vcc - Vh}

    T2 is equal

    T2=R1*C*\ln \frac{Vh}{VL}

    Vh = Vcc*\frac{R3}{R3 + R4||R2}

    VL = Vcc*\frac{R3||R4}{R3||R4 + R2}
    Of course I assume that opamp gives VCC in high state and 0V in low state.

    So if we assume R2=R3=R4 we have

    Vh = Vcc*\frac{R3}{R3 + R4||R2} =\frac{2}{3}*Vcc

    VL = Vcc*\frac{R3||R4}{R3||R4 + R2} =\frac{1}{3}*Vcc

    So finally
    T1=R1*C*\ln \frac{Vcc - VL}{Vcc - Vh} = R1*C*\ln2 = 0.693*R1*C

    T2=R1*C*\ln \frac{Vh}{VL}=R1*C*\ln2 = 0.693*R1*C

    Time period

     T=T1+T2 = R1*C*\ln4 = 1.386 R1*C

    So for 1Hz R1 = 220K C = 3.3uF

    And if you choose R2 = R3 = R and R4 = 0.52*R then

    T = 2* R1 *C

    For C = 470nF; R1=1M
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    Last edited: Apr 6, 2011
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