In search of a poor quality clock pulse

Discussion in 'General Electronics Chat' started by zane9000, Oct 31, 2007.

  1. zane9000

    Thread Starter Member

    Mar 16, 2007
    I am trying to make a circuit that (pseudo-)randomly turns a switch on or off. To do this I need a high speed clock pulse. I was thinking however, that if I use a very inaccurate/instable source for a clock pulse it will only help my circuit be more random. I am planing on just using a simple two inverter pulse generator with a very small/old cap.

    I guess my question is: Are there any notoriously poor designs I can use for a clock pulse? I havent heard of any, but Im sure there must be something out there.

  2. cheddy

    Active Member

    Oct 19, 2007
    Here is a kinda silly suggestion that just might work (although not very practical). Perhaps you could use the seemingly chaotic amplitudes of voltages from the stereo output of an iPod?
  3. Gadget

    Distinguished Member

    Jan 10, 2006
    How about a white noise source hooked up to a binary counter like a CD4040...
  4. Papabravo


    Feb 24, 2006
    I think you may have some confusion. There is nothing really random about a clock source. The variations are entirely deterministic and depend on such things as temperature, supply voltage, and component aging. Beta decay is the only natural source of random events that I'm aware of. Anything else will be pseudo-random, that is apparently random but with a deterministic and repeatable pattern.

    You should google "Linear Feedback Shift Register" for maximal length sequences. Knuth, D.E., wrote a whole chapter about random number generators in Volume 2 of the Art of Computer Programming.

    Good Luck
  5. zane9000

    Thread Starter Member

    Mar 16, 2007
    Any suggestions for a source? I need something that would always produce noise. I had originally thought of an inductive pickup run through an amp, but then I realized I may hit a "quiet" spot in some areas.

    No, no confusion. Thats why in the original post I said I was building a pseudo-random switch. And while I dont think that I will be able to find a truly unpredictable clock source, I was hoping that people might recommend some more unstable components to use (some that are more susceptible to temp variances etc.) to I could increase the appearance of a random setup.
  6. GS3

    AAC Fanatic!

    Sep 21, 2007
    I am not sure I understand. The switch needs to randomly be turned off how often? thousands of times per second? A few times a week? What are you trying to do?
  7. zane9000

    Thread Starter Member

    Mar 16, 2007
    I suppose I should give a bit more detail about the project. Basically there are two clock circuits and two decade counters. The first clock has a user definable speed so it will take anywhere from a few seconds to a few minutes to get the first counter from Q1 to Q9. The second counter will be set to a fairly high rate (compared to the first) 60Hz or so it will cycle through Q0-Q9 of its counter many times between each of the first clock's pulses.
    When the first counter reaches Q0-Q8 it will send a signal to the second counter's enable input so it will pause. Then, when the first counter reaches Q9 it will release the enable and let the second counter cycle through many times before the enable is set again when the first counter reaches Q1.
    The half the outputs of the second counter will send a signal turning a switch on, the other half will turn it off.

    The reason I wanted poor quality clocks is so it would just make the circuit a bit more unpredictable. This is not for any serious application where it has to be truly random, or even close really... just the appearance of being random at a glance.
  8. GS3

    AAC Fanatic!

    Sep 21, 2007
  9. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    The white-noise source that is used in the old Moog and Arp synthesizers produces
    noise by avalanching a transistor. The circuit consists of a transistor a couple resistors
    and capacitors and an op-amp.

    A picture of the PCB layout I did for a white-noise is located at the bottom of the following page ---

    If you decide to use this type of noise source I could post the schematic on my

    You could create the noise with a small uC as well. National Semiconductor
    use to make a digital noise source the MM5437. Also in the "Art of Electronics"
    there is a few pages on digital noise generation.

    (* jcl *)
  10. chesart1

    Senior Member

    Jan 23, 2006
    I don't know if this idea is valid but...
    If the counters trigger on the rising edge of a clock pulse, you can create a clock with a rising edge that barely meets the specifications for that particular counter.

    You can make the circuit unstable by adjusting the width of each clock pulse to barely meet the specification for counters not triggered by the rising edge of the clock.

    You can add to the instability by attenuating the amplitude of the clock pulse so it barely meets the specifications for the counter.

    I think that if the rise time, the pulse width and the pulse amplitude barely meet specifications of the counter, you will get random pulses.

    My idea comes from a circuit I encountered where the capacitor was missing from the 555 timer. The timer generated random transients due to stray capacitance.