555 PWM problem

Discussion in 'General Electronics Chat' started by somlioy, Sep 22, 2012.

  1. somlioy

    Thread Starter Member

    Jul 28, 2010
    I'm trying Bill Marsdens circuit (http://forum.allaboutcircuits.com/showthread.php?t=24028), but it dosent seem to work properly at 5V vcc.

    Instead of the 9V battery I've put in a 7805. Also omitted R1, R2, D1, D2, C2. And C1 is changed to 0.1uF.

    For the 7805 I got a 0.33uF on the input and a 0.1uF on the output.

    Problem is the circuit only works between 30% and 100% duty cycle. But works perfectly fine with a 12V supply.
  2. Wendy


    Mar 24, 2008
    The hysteretic oscillator, which this is based off, is only good to around 6VDC. Add diodes and it goes to 7V, but this can be minimized with Schottky diodes, which have a much lower dropping voltage.

    A CMOS 555 does not have this problem, and can go down to 3VDC. The reason is the 1.3V drop on the plus side of the signal caused by Darlington outputs. This was explained in the theory of operation in the article.

    A standard 555 design can go down to 4.5VDC, but is not so convenient.

    I have some other ideas in LEDs, 555s, Flashers, and Light Chasers, Chapter 5, The 555 and PWM.
    somlioy likes this.