Sine wave with 555 timer and RLC circuit

Discussion in 'The Projects Forum' started by peter_morley, May 22, 2011.

  1. peter_morley

    peter_morley Thread Starter Member

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    A 555 timer creates a high low pulse. What I want to do is create a sine wave from the output of the 555 timer. I have looked online and it involves a bit of calculus so I wanted to see if it was worth my time. I realize the RLC circuit connected to the output of the 555 needs to be over-damped to create my sine waveform. I'm not doing any transmitting for radio waves at this point I'm just trying to learn how create oscillator circuits. Its a useful tool to have and most of all cool!
  2. Kingsparks

    Kingsparks Member

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    I don't know about useful, that depends on you. As far as cool, well, I think so but then I like almost anything having to do with the 555. It won't be a true sine wave but you can get it pretty close. It is real easy to breadboard then "play with," "try it, you'll like it." :cool:
  3. DickCappels

    DickCappels Well-Known Member

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    It can be pretty simple. Just set up the 555 to make a square wave (approx 50% duty cycle) at the desired frequency. Then make a RC low pass filter using a 1K resistor and a capacitor calculated to give you a corner frequency at about 1/2 the frequency of the square wave. After that, place another RC low pass filter with the same corner frequency, but using a 5.1k resistor (so you don't load down the first filter section too badly), then follow it by a third stage with the same corner frequency, this time using a 27k resistor. That will get you a pretty good looking sine wave. You can lower the corner frequency or add more stages to get better suppression of the higher order harmonics.
  4. peter_morley

    peter_morley Thread Starter Member

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    Would the filter look this this?...attached schematic and graph. A to D are assigned as larger amplitude to smaller amplitude of waveforms.

    Attached Files:

  5. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Pretty much. LC works much better, but RC gets the job done. One problem though, it still won't be a pure sine wave, just very close.

    You could also use the input to the 555 (where it is a triangle wave) to remove one layer of RC circuits.

    You can also use integrators to make a better approximation, instead of a sawtooth you would have a true triangle waveform for example.

    My Cookbook

    http://forum.allaboutcircuits.com/showthread.php?t=10936
  6. peter_morley

    peter_morley Thread Starter Member

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    Bill_Marsden - Ok so no matter what the output will not be a perfect sine wave. Can you show me how to make a sine wave the LC way with the 555 timer...and is it possible to make a sine wave with just an inductor, capacitor and resistor without the 555 timer? So two requests here.
  7. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    There are lots of ways to make high quality sine waves. Pick a frequency.

    Actually, with enough filtering, you can make a very good sine wave, one for all practical purposes is pure.

    The old LC resonance formula is your friend, F=1/(6.28*(LC)^0.5). I'd LaTeXt except I don't have a lot of time for this post.

    Just build a LC low pass followed by a LC by a LC band pass, not necessarily in that order. LC circuits are much more precision than RC versions.

    Sine Wave Oscillators A PDF abstract by TI
  8. THE_RB

    THE_RB AAC Fanatic!

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    The big question is; do you want to change the frequency?

    It's much easier to generate (and filter) sine at a fixed frequency, if you want to be able to vary the sine frequency that changes everything!
  9. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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  10. Ron H

    Ron H E-book Developer

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    It needs to be underdamped.
  11. CDRIVE

    CDRIVE Senior Member

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    Though I haven't changed my opinion of using a square wave oscillator as the basis for RF communications, I felt compelled to contribute this option. The schematic shows a square wave generator feeding a common emitter (resonant tank) amplifier.

    Since this is a spice simulation I'm able to create Q factors that are not realistic in the real world. Non the less, the schematic and plots clearly show how effective resonance can be in cleaning a square wave, while inherently tuning out extraneous frequencies. Realistically though, it would take more than one stage to get a very clean sine wave.

    The plots starting from the top include:
    (1) Oscilloscope view of Q1 collector (Out) sine wave voltage vs. VG1(square wave generator input)
    (2) Fourier Amplitude: Plots (Out) amplitude of all frequencies up to 12MHz. Note how clean it is.
    (3) Fourier Amplitude: Plots (VG1) amplitude of all frequencies up to 12MHz. Note how filthy it is!

    Attached Files:

  12. halfknot

    halfknot New Member

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    Would the expected generated sinewave have a DC bias? If so, how would you eliminate this bias?
  13. ScottWang

    ScottWang Well-Known Member

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    I did the similar test before, but just using two stages, because the amplitude is too small, so it is need the op amp to amplify the signal, but why is 722Khz, many years I met someone asked for help is 700Khz?
  14. BillB3857

    BillB3857 Senior Member

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