LED driving circuit help

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

1. Jeff ZimmermanGuest

First a desciption of what I'm trying to do (sorry if it is
confusing):
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
arrays).

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

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
input.

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. RichieGuest

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

thanks

hope i was of some help!

richie

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Richie086
http://www.richie086.com

"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 GriseGuest

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!
Rich