[Re: 74LS90 replacement]

[Harry Bissell]

Harry Bissell suggests: If you series connect several LM3914's the resistor
tolerances can get out of hand. The individual resistors in a chip are well
matched (ratio) but vary in absolute value. Mage a resistor divider of 1%
values with a tap for the top-bottom of each chips resistor string, and buffer
these points with an op-amp voltage follower. This will source/sink extra
current to force the internal voltage drops to be equal.
Been there, done that. As you noted, the supply for the led driver needs a
HONKIN big cap or the noise is a killer.  :-)  Now I'll steal the idea for
feeding the ramp to make a frequency multiplier to digitally divide and get
"real" square waves (a perfect 50%) All I need is an 8 stage chip.  :-) Harry


"The Dark force of dance" <batzman at all-electric.com> wrote:
Y-ellow Y'all.
	I haven't exactly done this and I'm only jumping in half way through a
thread I haven't kept up with, however.

At 10:02 AM 03/25/99 +1000, Paul Perry wrote:
>At 08:01 AM 24/03/99 -0700, Ian ijfritz wrote:
> One pretty interesting way is to use the LM3914 LED
>>driver chip. Feed a saw in and you get ten separate pulses out. You can
>>then weigh and sum them to get an approximation to any waveform.
>>
>Has anyone actually done this?
>Because I think the stages do not go on and off exactly sharply, so that
>the eye is led to think the dot is moving smoothly across.
>It might still be a good thing, but you might not be getting what you 
>would from a more 'precision' setup.
>Still, would be a great mod for the MadMouse!

I have muxed LM3916s up to 8 bargraphs wide. Actually 4 graphs switching
between bar and dot mode alternatively. (Peak/avarage) My original 4 x
bargraph had problems with noise and I added caps which slowed the response
down but this was on the output not the input. My mistake but without the
caps it works pretty well. The LM391x family are virtually just 1 of 10
flash converters but I don't know what the upper frequency limit is. There
is no data available on this.

However I've heard where someone used a 3914 to produce a little
oscilloscope display with 100 leds. Probably pretty dinky but if you can do
that, surely you could do a complete graphic analyzer or 2 with a single
LM391x chip.

And speaking of Oscilloscopes. Over on the ATMEL list they're all agog
about this.

http://www.zmd.de/mixed/m_scope.htm

A German company called ZMD are making a complete Oscilloscope on a chip.
Minus the display 'n' stuff obviously. Worth checking out if you can handle
the software.

Be absolutely Icebox.

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