Throbbing Blue LED

Discussion in 'The Projects Forum' started by MrBeta, May 24, 2009.

  1. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    Hello everyone. Fairly new to electronics and stumbled on to this site on accident. I'm currently working on a throbbing led which fades on to off in about 5 seconds.
    I'm using a 555 timer with a resistor between 555 and NPN (for pulse time)and a cap for fade time. I think that's what each component does. I was sent the schematic to try it out.
    I have a working fade on my breadboard but was wondering if it was possible to alternate another led. Ex. fade up/down blue, fade up/down pink, repeat.
    Looking for a slow 5ish second fade between leds.
    Once I get it completed, I want to etch some pcbs and put them in some frosted vases for an upcoming wedding.
    Sorry for my ignorance.
    Thanks for any input. I'll try to edit in schematic when I get back to my normal computer.

    Dumb question but isn't this a version of PWM with a set cycle?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Bill Bowden's circuit, "Fading Red Eyes" (the 2nd version) will do just what you're looking for.
    http://ourworld.compuserve.com/homepages/Bill_Bowden/page5.htm

    Scroll about halfway down the page.
    This is the schematic I'm referring to:
    http://ourworld.compuserve.com/homepages/Bill_Bowden/eyes1.gif

    Well, no it's not a dumb question, and no, this isn't exactly PWM. :)

    In Bill Bowden's circuit, the LEDs get current flow through them that looks like a triangle wave over time; ramping up and down.

    Note that in Bill's circuits, he's using red LEDs; they have a Vf much lower than your blue and pink LEDs. You won't likely be able to run pairs of them in series.
     
  3. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    Is there a way to add on to my current 555 circuit or is the LM1458 the only way to go? I've got a bunch of 555 and misc caps, resistors but no 1458s.
     
  4. HarveyH42

    Active Member

    Jul 22, 2007
    425
    5
    I used an LM358, worked fine. Pretty much any common type opamp will work, not a picky circuit.
     
  5. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    I have to see if I have any opamps. I think I have some LM741s but i'm not sure.
    So the only way to achieve this is to use an opamp?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, opamps aren't the ONLY way - but it's a fairly low-parts-count way to do it.

    I suppose you could use just the emitter-follower portions (transistors, resistors and LEDs) of Bill Bowden's circuit attached to the timing capacitor of your existing 555 timer circuit.

    If you want more specifics, you're going to need to post your schematic, and the specifications of your LEDs.
     
  7. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    Here's the schematic he sent me. I've got it hooked up with a blue led forward voltage 3.2V. IIRC it's a 100uF cap and a 33k resistor from pin 3.
    Thanks for the continued help.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, it looks like your circuit is ALREADY using an NPN transistor in an emitter follower configuration for the one LED.

    Your schematic isn't complete though; it got "cropped" too much.

    Use a PNP transistor for the other LED, wired up like Bill Bowden's circuit.

    A 2N2222 is a good candidate for the NPN transistor.
    A 2N2907 is a good candidate for the PNP transistor.

    [eta]
    You're going to need to use 33k resistors on the bases, otherwise there will be too much base current to allow the timing cap to charge.
     
    Last edited: May 24, 2009
  9. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    I do see the missing lead from the led but is there anything else missing?

    [eta]
    So to extend this circuit, I would just take a 33k resistor and jump from base of npn to base of pnp and then attach the emitter to the proper resistor for the led?
    Also, am I reading his schematic correctly when I see the resistor attached to the annode? I thought they were supposed to be attached to the cathode. The top set of leds have +9V-resistor, but bottom leds have the leds then the resistor.
     
    Last edited: May 24, 2009
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Nothing that can't be figured out pretty quickly.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, use a separate 33k resistor for each transistor's base.
    As far as the LED and resistor goes, you can put the resistor on either the anode or cathode. It's a series circuit.

    My personal preference is to have the resistor on the LED's anode where possible, but it certainly doesn't make a difference electrically.
     
  12. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    I'll have to give it a shot when I get back to my desk. Thanks for the info on the led resistor thing, I never knew that.
    I'll have to see if I've got some PNPs.
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
  14. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    Alright everyone. Thanks for all the help. I've got both circuits breadboarded and now off to make a final schematic/pcb for these guys. Pretty neat little project from scrap parts.

    One last question though that I have not tested. I've got the fade time worked out well and they're alternating to plan but the red led doesn't dim all the way like the blue led does. Is this the forward voltage difference or am I missing something simple. I'd like the red to fade to off as the blue does.

    Hope that makes sense and thanks loads for everyone's assistance.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's because the red LED has a lower Vf than the blue LED does, and the default range for the timing capacitor is 1/3 Vcc to 2/3 Vcc.

    You could add a common diode or two in series with the red LED to cause it to fade nearly completely out.
     
  16. MrBeta

    Thread Starter New Member

    May 23, 2009
    8
    0
    Perfect. The diode worked wonders. Doubt I'll use that secondary color but the blue is there to stay.
    Thanks everyone for their help. I'll have to post a picture of these centerpieces I'm "building".
     
Loading...