Design for Variable Frequency and D.C /w LM555

Discussion in 'The Projects Forum' started by franzschluter, Jun 17, 2009.

  1. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Hi guys.

    I know it is possible for example using 74LS123..Those with dual package.. First set to produce desired frequency and doing it variable and then using the second set for adjusting and triggering the PWM.
    I have a working circuit for this also...

    But what I have been wondering is if it would be possible to build this using LM555?

    Frequency Range 1kHz-60kHz.
    D.C: 10%-80%

    How would this look like? I have some experience using LM555 and know a bit of the basics about it...But there are for sure people here who know them inside and out...Output should be able to trigger a transistor. Perhaps using two potis.. 1 for frequency and 1 for PW/D.C. I have yet no idea how to make a schematic for this.

    I'd like to connect them to some loudspeaker/oscilloscope and just for fun listen to how this would sound like.

    I think I am also afraid if frequency is varied over a great range it would affect the outputs amplitude? Would it not?

    Cheers
    Franz
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The frequency range will be to large for using the LM 555 or 74 123.
    Better make a triangle oscillator and use a comparator for the PWM.

    Greetings,
    Bertus
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Actually I think it may be possible with the 555, but it will not be very stable, given the wide range.

    It uses a pot, but this comes pretty close.

    555 Hysteretic Oscillator
     
  4. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    You mean real triangle oscillator? Or sawtooth using capacitors? I am trying to figure out how I should go about this.. Any other suggestion? Would a PIC be able to do this variable Frequency output at a stable rate with respect to amplitude?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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  6. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Will it still be stable if I set this to 60kHz? My range should be from 1kHZ to 60kHZ by only adjusting the potentiometer. I know by adjusting R we could vary RC constant.. Question is if the amplitude will not have any effect when varying to these ranges... Comparing afterwards is a different story though..

    Cheers
    Franz
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    My suggestion, try the 555 Hysteretic Oscillator , it is an extremely simple experiment, and adjust the capacitor for the low frequency setting. Use the 10 turn pot recommended.

    RC oscillators are inherently inaccurate, and drift over time. Not by huge numbers, but you can't set a watch by them. Figure 2 digits of accuracy long term.

    If you are needing a pure triangle wave I've been doing some experiments on those too. I've run into crossover distortion and some other problems I'll eventually get resolved. The 555 will come out with a nice sawtooth wave form, which is an appoximation of a triangle wave, but not a clean one.

    http://forum.allaboutcircuits.com/showthread.php?t=12405
     
  8. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Okay I'll try to use that circuit for test run on a breadboard first. 1 LM555 for frequency then its output to trigger another LM555 that will be Duty Cycle adjustable. Does this sound feasible?

    Cheers
    Franz
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Yes, but there are a lot of ways to do this. For example, once you have a triangle or sawtooth wave, you can feed it into a comparitor and vary the DC voltage (from a pot, say) into the other input on the comparitor, and instant PWM, fully adjustable, mostly linear. If you need a sketch to see what I'm talking about let me know.
     
  10. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Hmm.. I kinda understand the concept.. But I am unsure if this will change the Duty cycle? Since a comparator is only high or low a squarewave will be at output. The trigger when comparator goes High is when offset voltage = (where we at the slope/line of the triangular wave). This will change the frequency or duty cycle? I can't imagine it and I suck at imagination. I'll try drawing it on a math paper first... LOL

    1. Triangle must be really a triangle and not sawtooth?
    2. I think it can't go 100% DC but this is ok since I do not require 100% DC.

    So in short..
    Step 1 generate triangle wave using LM555.. Place a pot and simply compute RC to get a variable range of roughly 1kHz to 60kHz. Then this output goes to a comparator. 1 pin to it's input the other input to ground or vcc and a pot in between to make reference variable.. This will serve as D.C adjust.. Hmmm.

    You know this might actually work. :D

    It might be nice to put some sketch later on.. I might put up one perhaps. It is good reference material for people who might be looking for this stuff.

    Cheers
    Franz
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Something like this...

    [​IMG]

    Note: if the + input was biased with 6V, and an audio signal were input there, it would come out PWM modulated audio, the basis for Class D amplifiers. The sawtooth is not linear, so it would not be perfect, but it would be very close.
     
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  12. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Will ceramic disc perform better than electrolytic in this case?
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    I don't think it makes much difference. Ceramic discs overall are closer to ideal, less leakage, higher voltage (for the size).
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Note that comparators typically have open-collector outputs.

    You'll need a 1.5k Ohm pull-up resistor from the output to Vcc in order to observe an output.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Good point. An op amp and a comparitor are cousins, the inputs are the same. Most op amps can't quite reach the power supply rails, either ground or the + end, while comparitors are digital outputs.
     
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