VU-meter

[The Dark force of dance]

Y-ellow.
        I'm back. It's been a while.
At 01:13 PM 9/15/97 +0200, Erik Schuijers wrote:
>Hi everybody,
>
>today I'm going to start building a VU-meter (LED-display) using National's
>LM1916 (if I'm correct). It's very easy to build. Only very little
>components without the LEDs (17 LEDs in total). It should be very easy to
>build. I'm going to buy red, orange, yellow, green and blue LED's to give it
>a nice display! This module is also intended for a FracRac panel.
>
>I'm also thinking of some options like a peak detector, bar mode/led display
>etc...

Been there. Done that. The 3916 can be multiplexed. This revelation
shouldn't be lost on y'all. I've done 4 bargraphs with a single 3916. It's
possible to do more. Oh actually when I think about it I've done 8 bar
graphs with a single 3916. I'll elucidate.

My favorite tick is to display both peak and average on the same display. To
do this you need a clock. This drives a multiplexer such as a 4066 or 4053.
In the latter case with suitable BCD decoder of course. Each channel of
audio is split into a pair of precision rectifiers. One set to peak and the
other to average. The circuses for which appear in the 3914/15/16 data
sheets. You then run the output of these rectifiers through your multiplexer
and into the 3916 input. At the same time you switch between bar and dot
mode. The peak is selected in dot mode the average selected in bar mode. So
you get a display with a dot that appears to be pushed up by the bar. It
eventually decays away of course due to the time constant of the peak hold cap.

When doing multiple displays with a single 3916 you can simply use a bunch
of transistors which pull up to the positive rail when selected. Each bar of
10 LEDs is connected between the output of the 3916 and one of the
transistors. The one that pulls up activates or selects the bar of LEDs in
question. Thus a multiplexed LED display.

As I mentioned. I've done 4 bars each with peak hold. However I'm not sure
at what point the speed of the 3916 starts to run out. There is no mention
of this parameter in the 3916 data sheet. I don't think they envisaged
people doing this with them. But I'm sure you could get away with perhaps 10
bargraphs from a single 3916. Making a graphic analyser for an equalizer
entirely possible. Given that Exar make some nice little single chip
solutions for multiband filters, starting with the xr-1091, this would seem
an excellent idea.

Your clock frequency needs to be suitably high so as not to cause flicker.
Between 200 and 400 hz usually does the trick. Depending on the number of
bars. Somewhere in there though there's going to be a trade off. Where the
3916 can't respond fast enough to that many bars.

However. These days I'm trying to do multiple bar graphs with a
microprocessor and an A-D converter. My problem will become, working out a
suitable dB scale from an 8 bit linear A-D. Of which I'll be seeking help on
when I get to that bit. Being the rather empirical person I tend to be.

Anyway Hope this helps. If yur real lucky I might be able to dredge up a
circus for a multiple display bar-graph using 3916s.

Be absolutely ICebox.
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