555's PWM trouble

Discussion in 'The Projects Forum' started by whelktrim, Apr 5, 2011.

  1. whelktrim

    Thread Starter New Member

    Apr 5, 2011
    Hi - this circuit sounds like it should do exactly what I need (variable frequency, variable duty cycle). Problem is, I've just got it hooked up on the prototype board and it's not behaving properly.

    Should the LED be flashing at the rate of the PWM signal? Mine is either fully on or fully off, depending on the position of either of the variable resistors.
  2. Wendy


    Mar 24, 2008
    You really should have started a new thread for this, instead of hijacking the existing thread. A thread belongs to the OP (original poster).

    Did you use the exact values for the components?

    The 555 is a fast oscillator, it can be run between 700Hz to 70Khz, depending on R1 setting. It sounds like you have a much larger capacitor, as the frequency of the oscillator is:

    f = 0.7 / (C1(R1+R2))

    The slowest frequency 700Hz should not be visible at all. Neither will the 700KHz.

    This circuit and more can be found in LEDs, 555s, Flashers, and Light Chasers, Chapter 5 - The 555 and PWM.
  3. bertus


    Apr 5, 2008

    As Bill_Marsden said, you hijacked an existing thread.
    Now you have a thread of your own.