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
    49
    3
    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

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You can get a decent triangle wave off a 555. Does that count?

    [​IMG]

    [​IMG]

    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
    13
    0
    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

    Moderator

    Mar 24, 2008
    20,766
    2,536
    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
    818
    47
    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
    770
    90
    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
    49
    3
    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
     
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