Smoothing the output of a 555 timer

Discussion in 'General Electronics Chat' started by Spence, Apr 18, 2011.

  1. Spence

    Thread Starter Member

    Apr 23, 2010
    I need to illuminate a led very slowly and smoothly, as part of a voltage controlled filter. I suppose I'm looking for a sine wave and there are better alternatives than the 555 but wondered what's the closest to a sine wave that could be obtained with a 555.

    I can get a very smooth transition with a two transistor flip flop but I can't seem to get a smooth on and a smooth off at the same time, plus there's a dead zone between flip and flop.

    Any suggestions appreciated.
  2. Wendy


    Mar 24, 2008
    You can get a decent triangle wave off a 555. Does that count?



    The best way is a standard sine wave oscillator followed by a PWM modulator. This will do what you want, but it needs 9V.

    My blog has a lot of links for this kind of stuff.

    Bill's Index

    LEDs, 555s, Flashers, and Light Chasers Lots of alternate ways to do the same thing. Chapter 5 and chapter 12 especially apply.

    My Cookbook

    Sine Wave Oscillators A PDF abstract by TI
    Last edited: Apr 18, 2011
  3. MattQ87

    New Member

    Apr 5, 2011
    Out of curiosity, how many LEDs are you looking to power up? You could use an LED driver (available from Linear, National, and a number of other suppliers) with PWM input for a current controlled solution (that would probably give you the smoothest output).
  4. Wendy


    Mar 24, 2008
    Or you could look at the circuits I have posted in Chapter 12. :D It will work down to within a volt of the Vf of the LEDs, or to higher voltage. Nice think about PWM, there are no real LED limits except for the power supply.
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    I am not sure a square wave would not be fine. Why not try it first unless you have a really trustwothy ref. that says it will not.
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    You can make nice linear triangle waves with a 555 by using constant current diodes to charge/discharge the timing cap.
  7. Spence

    Thread Starter Member

    Apr 23, 2010
    I solved this using a 555 and a dc to dc converter (toshiba k2611).

    The output from the 555 pin 2, goes to the gate.
    The drain goes to Vcc.
    The source goes to ground via a 470 Ohm resistor and the led.
    Rail voltage is 12v

    I have never seen a smoother transition.

    Thanks everyone for the advice