LED pulsing with stepped output?

Discussion in 'The Projects Forum' started by Journeyman, Jun 29, 2011.

  1. Journeyman

    Thread Starter New Member

    Jun 29, 2011
    OK, I've been playing with 555 and 556 circuits, PWM, etc. to pulse LEDs. I figured out how to do "comet trails" using capacitors, but am having trouble getting this stepped pulsing effect (see ? below).

    My current theory is to use a 555 w/ capacitor to pulse the LED with comet trails, and a second 555 w/ transistor to "chop up" the comet trail from the first 555.

    Working on a diagram/breadboarding at the moment, so any help is appreciated.

    And yes, I have read LEDs, 555s, PWM, Flashers, and Light Chasers.
  2. #12


    Nov 30, 2010
    I don't think you can do that with (2) timer chips. You need something to lower the voltage 3 times, then reset to full voltage on the 4th pulse. I'm thinking a "count to 4" (or zero, one, two, three) circuit with a different resistance in series with each output. Digital gates can be used to make that kind of circuit.
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    You can use 4 timers (1 astable, 3 monostable) running on the same trigger and sum the outputs. By adjusting the value of each summing resistor, you can tailor the steps in brightness. I did this with a 558 once. The 558 is much more difficult to use than 2 556's though.
    Last edited: Jun 30, 2011
  4. Journeyman

    Thread Starter New Member

    Jun 29, 2011
    OK, I had more success than I anticipated, but it still feels.. I don't know, sloppy.
    I like the "fluting" effect, as seen in the attached picture. The actual strobe pattern is the bands on top and bottom, the middle is just glare from my crappy phone's camera.
    I think I need resistors on the NPN inputs, but haven't burnt out anything yet. I used TIP31s but switched to 2222s due to their size, they work but the TIP31s were "crisper" I guess.
    Any suggestions, comments, etc. is appreciated.

    Also, at one point while breadboarding I had something crossed, and it the pattern was cyclic between different pulse widths. It was awesome, but couldn't reproduce it. I think it was a cap/resistor bridging something somewhere?
    Anyone know how you would get a 555 to cycle back and forth like that?
  5. JingleJoe


    Jul 23, 2011
    Transistors always need a resistor on the base or they go boom. (Note: that's an exaggeration, they just break)

    Pulse width modulation! That's what I'm working on now, you may be right about the capacitor being bridged but also pin 5 may cause this, poke pin 5 with your finger for some fun effects!
    Pin 5 is the control voltage input, it chages the point at which the capacitor charges to and subsequently the frequency. I find that when poking pin 5 I am sort of acting as an æriel, if I put my other hand around a mains power cable (insulated!) I can modulate it with the frequency of the mains AC.