Audio Oscillator: Sawtooth Waveform with a 555

Discussion in 'Analog & Mixed-Signal Design' started by drmanmachine, Dec 22, 2017.

  1. drmanmachine

    Thread Starter Member

    Mar 12, 2016
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    I did some research and found this schematic online. I wanted to get your thoughts on whether or not this would actually work. I would try it myself, but I don't have the dual op-amps LMC662 used on this schematic. Are there comparable op-amps that would work? Anyway, just wanted to get some perspective on this schematic. Thanks.
    [​IMG]
     
  2. MrChips

    Moderator

    Oct 2, 2009
    16,281
    5,004
    The "sawtooth" waveform is taken from pin-7 of the 555-timer. This is the charge/discharge voltage of the timing capacitor C and hence is not a smooth linearly rising ramp. Use it if that is good enough for your purposes.

    The timing resistor R should have an additional resistor (about 1kΩ) placed in series with the variable resistor in order to limit the total resistance to a minimum value. If you set the total resistance to 0Ω you could destroy the 555-timer chip.

    The opamp is simply to buffer the waveform and level-shift the signal with a adjustable offset. Just about any single supply opamp would work. You can try an LM324.
     
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    An LM358 is the dual version of the LM324. For lower noise and distortion try something in the TL07x series.

    The voltage at pin 7 swings from 1/3 Vcc to 2/3 Vcc. The 2nd opamp is a gain stage as well as a DC offset adjustor, and the gain varies slightly as the adjustment changes. To me it looks like the gain is too high and the output waveform sill be clipped, but I haven't messed with the circuit. With the pot in the center of its range, the gain is 5.7, too high for that input signal. Where did you get this schematic?

    ak
     
  4. drmanmachine

    Thread Starter Member

    Mar 12, 2016
    65
    0
    I found this schematic online. Just did a google search for a Sawtooth Audio Oscillator. Would a voltage divider be a possible resolution to reducing the input signal down?
     
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    You don't say how large an output signal you need, but for starters, eliminate the 2nd opamp and all of its resistors. Take the output from the output of the first opamp through a coupling capacitor.

    To determine the coupling capacitor value, decide on the lowest output frequency you need (freq), determine the resistance or impedance (R) of whatever is attached to the output, and calculate the capacitor value using this:

    C = 1 / (2 x pi x freq x R)

    Double the result, then round it up to the next largest standard capacitor value. This sets a lower limit to the capacitor value. If you use an electrolytic cap, connect the + end to the opamp output.

    ak
     
    Last edited: Jan 12, 2018
    absf likes this.
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