[sdiy] led bardriver with hysteresis?

[WeAreAs1 at aol.com]

In a message dated 10/15/04 6:28:25 PM, pfperry at melbpc.org.au writes:

<< Those led bardrivers that you can make into a 

quantiser.. how can I put some hysteresis into it, so that if I 

adjust the CV in to a new position, it will keep a new output until the CV 

input changes by a 'significant' amount (so that small jitters on the

input won't change it?)

Or, how can I do a similar thing with any A to D converter? >>

Hello Paul,

An alternative to using the 3914-type LED drivers would be to build a string 
of LM339 comparators.  The 339 is a quad comparator, so you'd get 4 steps for 
each IC package -- all you'd need to do is add a bunch of resistors.  LM339's 
are cheap, so buying three of them (12 steps) would probably still be cheaper 
than buying one 3914.

The typical LM339 LED bar graph circuit operates in "bar mode" (as opposed to 
the 3914's "dot mode").  If you require dot mode, then there's probably also 
a way to configure each stage as a sort of window comparator, so it turns on 
only if the input voltage is within the window that you'd set with resistors.  

I think it's also possible to add hysteresis to a 339 comparator circuit 
(probably by using a little bit of feedback through a resistor or two).  Another 
nice thing about using this type of comparator ladder is that you can build the 
ladder as long or short as you need, and you have total control over the 
voltage level and weighting of each individual step.

If you wanted it to have binary output (like a typical DAC), then you'd need 
to add some additional logic to the output of the ladder -- you know, some 
kind of decimal-to-binary decoder chip.  I'm pretty sure there's are a couple of 
4000-series CMOS IC's that could elegantly handle that function.  Now that I'm 
thinking about it, there might even be a way to use some simple diode logic 
(Don Lancaster's "Mickey Mouse Logic") to decode the decimal output into binary.

Here's a link to the data sheet for the LM339:  <A 
HREF="http://www.national.com/ds/LM/LM139.pdf">http://www.national.com/ds/LM/LM139.pdf</A>

Best regards,
Michael Bacich