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LED 'smoothing' question

Discussion in 'Electronic Design' started by frenchy, Mar 7, 2007.

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  1. frenchy

    frenchy Guest

    If a regular incandescant bulb (like a #44) is replaced with an LED,
    and the original bulb was made to sometimes have a slow on-to-off
    effect (by flashing it very fast for different amounts of time for
    each pulse), is there a simple way to give the same smooth look to the
    led, like with a capacitor or something like that? The bulb strobes
    smoothly since the filament does not cool off between the pulses, but
    the led is either totally on or off thus it has a flickering
    appearance when this strobing is being used. (When on solid, the led
    looks normal of course.) This application is a pinball machine, 6v
    ac, using one of the new LEDs specially made to be directly
    substituted for the #44. Thanks!
  2. D from BC

    D from BC Guest's not basic but I used a PIC microcontroller to solve that
    D from BC
  3. frenchy

    frenchy Guest's not basic but I used a PIC microcontroller to solve that
    To add - this would need to be something connected directly to the
    bulb, or between the bulb and the voltage going to it, I can't modify
    the circuitry that's sending the power to the bulbs.
  4. Joerg

    Joerg Guest

    I did the same thing with the MSP430, resulting in an aircraft beacon
    type light that blinks "softly". It can also be done with a cap but
    since the LED is basically a current driven device with a (somewhat)
    fixed voltage across it the cap would have to be larger than with a bulb
    of same current level. However, one has to carefully check whether the
    electronics can drive such a concoction. Opamps can oscillate, other
    stuff could fry when it sees a huge capacitive load.
  5. Joerg

    Joerg Guest

    Do you know what kind of circuitry that is? Reason I ask is that it
    might not like a capacitive load.
  6. Rich Grise

    Rich Grise Guest

    The electronic pinballs & such that I used to fix used SCRs and half-wave
    rectified AC. To turn the light on, you'd bias the SCR's gate, to turn it
    off, you'd ground the gate.

    So I don't think an LED circuit would hurt it - what I'd try is replace
    the dropping resistor with two of half the value, in series, with a cap
    from the junction to power return.

    Have Fun!
  7. Rich Grise

    Rich Grise Guest

    Nothing to do with the OQ, but I wouldn't "softly" blink an aircraft
    beacon - I'd use strobes, because they're so much better at catching
    your eye in your peripheral vision.

  8. Joerg

    Joerg Guest

    AFAIU he wants to smooth the flicker when the drive is PWM'ed. If it's a
    SCR as you said maybe they do that by skipping half-waves unless it's a
    dimmer circuit. A SCR might not like the current spikes a "smoothing
    cap" causes. It probably depends how much margin it has. Worst case
    there could be a loud pop and whiff of smoke.
  9. Joerg

    Joerg Guest

    On many aircraft you have both types. Soft blink is also needed for
    obstruction lighting where you are often not allowed to strobe. You'll
    see that on some of the radio towers, for example.
  10. frenchy

    frenchy Guest

    I definitely don't want to have a risk of damaging the circuitry, this
    pinball is not cheap and this isn't really some
    big problem, just makes the leds stand out when they are being pulsed
    so they are noticeable as being leds.
    I wouldn't mind trying it on one of my older pinballs but none of them
    do this pulsing, only my very new (and expensive) ones do.
    You are correct they are just powered by a transistor either on or
    off. They are slowly
    strobed by turning on and off and changing the length of each pulse.
    I wouldn't have any
    idea of what cap or resistor values to use for this app.
  11. Joerg

    Joerg Guest

    You'd have to experiment. Some data would help, like voltage and wattage
    of the lamp that was in there before, and the peak current of the new
    LED. The resistor in front of the LED works to protect the circuitry but
    only if the lamp voltage had been much higher than the LED voltage.

    The cap would have to be experimented with. Make it as large as it takes
    to remove the flicker. But have others look at it, too, because one
    person might see flicker that others don't.

    If the lamp voltage had been so high that the resistor has to drop
    several volts life gets easier: Then you can use two resistors in series
    and tap off for the cap in the middle. -> Smaller cap.
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