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LED driving circuit help

Discussion in 'Electronic Basics' started by Jeff Zimmerman, Dec 29, 2003.

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  1. First a desciption of what I'm trying to do (sorry if it is
    I'm trying to make an LED driving circuit that will change (8) groups
    of (8) LEDs (turn one group on and another off) whenever a beat is
    detected. To drive the LEDs, I'm using (8) ULN2803's (darlington

    For the beat detection, I was thinking about using a LM3915 VU meter
    chip, and the audio input is from an amp has a low pass filter at
    about 150 HZ.

    To change the groups of LEDs, I was going to use a 74393 dual 4 bit
    binary counter, with it's clock signal connected to the highest output
    of the 3915. There would also be a pull up resistor on the clock

    Now for the questions:
    I tried this and it did work, but there were a couple of problems.

    The first was that the output from the 3915 isn't steady, the counter
    would count more than once for each beat.
    Is it possible to clean this output up?

    Also, if anyone has any suggestions on a better way to accomplish
    this, I would greatly appreciate it.

    Alittle extra info:
    I have (4) 5 volt reg's, 3 are used to drive the LEDs (20 leds per
    regulator), a .1 ohm 5w resistor is placed in series with the
    regulator to limit the current.
  2. Richie

    Richie Guest

    just looked at the datasheet for LM3915.. this should be pretty basic
    stuff but have u tryed this (pasted directly from datasheet)

    Outputs may be run in saturation with no adverse effects,
    making it possible to directly drive logic. The effective saturation
    resistance of the output transistors, equal to RE plus
    the transistors’ collector resistance, is about 50W. It’s also
    possible to drive LEDs from rectified AC with no filtering. To
    avoid oscillations, the LED supply should be bypassed with a
    2.2 µF tantalum or 10 µF aluminum electrolytic capacitor.


    so is the clock oscilating slightly and not staying at one freq?

    The most difficult problem occurs when large LED currents
    are being drawn, especially in bar graph mode. These currents
    flowing out of the ground pin cause voltage drops in
    external wiring, and thus errors and oscillations. Bringing the
    return wires from signal sources, reference ground and bottom
    of the resistor string to a single point very near pin 2 is
    the best solution.
    Long wires from VLED to LED anode common can cause
    oscillations. Depending on the severity of the problem
    0.05 µF to 2.2 µF decoupling capacitors from LED anode
    common to pin 2 will damp the circuit. If LED anode line
    wiring is inaccessible, often similar decoupling from pin 1 to
    pin 2 will be sufficient.
    If LED turn ON seems slow (bar mode) or several LEDs light
    (dot mode), oscillation or excessive noise is usually the
    problem. In cases where proper wiring and bypassing fail to
    stop oscillations, V+ voltage at pin 3 is usually below suggested
    limits. Expanded scale meter applications may have
    one or both ends of the internal voltage divider terminated at
    relatively high value resistors. These high-impedance ends
    should be bypassed to pin 2 with at least a 0.001 µF capacitor,
    or up to 0.1 µF in noisy environments.

    try the steps mentioned above.. all seem like very easy to make errors..

    Could you do me a favor and make a schematic for this? sounds like an
    interesting circuit! that is, if you get around to making a schematic
    it would be cool if u posted it. I'm a musician so any kinda circuits
    that make noise or react to sound im always interested playing with :)


    hope i was of some help!



    "The only thing better than sitting outside and
    playing a banjo is sitting outside playing a banjo
    made of the skulls of people that made fun of you in
    elementry school."
  3. Rich Grise

    Rich Grise Guest

    My first suggestion is, if you have a suitable supply,
    drive banks of 8 LEDs in series, so you'll need only
    one ULN2803 to drive all 8 banks.

    You really don't need an LM3915 just to catch the
    downbeat, albeit it does make a handy peak indicator,
    with a visual "readout." What needs work is conditioning
    the signal that's going to it, like maybe a low-pass
    filter, or a differentiator to catch the rising
    edge; or mic the bass drum or whatever causes the
    actual audible beat.

    Good Luck!
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