[sdiy] The PPPPPulser

[jhaible]

> I took an hour yesterday and white-boarded the variable multi pulser I
> proposed a while ago.  To review, it uses an LM3914 bar graph chip with a
> constant (0-1V) saw drive and a variable (5-.5V) control drive on the
> comparitor resistor chain.  The odd-numbered 3914 outputs are summed into
a
> comparitor to make a pulse train. The output starts out as a square wave
> and continuously adds more pulses, up to five, as the external CV varies
> from 0 to 5V.  The whole circuit just takes the 3914 and two op amps.
>
> The results are pretty interesting, I think.  The unit actually has a lot
> of the characteristics of a filter.  There is a lot of phase shifting
going
> on, along with large and rapid changes in the harmonic content.  A sweep
> through control voltages sounds somewhat similar to a sweep of a high-Q
> bandpass filter.  Here's an MP3 demo, which is just a sweep up and down in
> CV, with no other processing.
> http://home.earthlink.net/~ijfritz/xfer.htm

Sounds nice!

I have a similar circuit, built from LM339 quad comparators, in
my JH-720:
http://home.debitel.net/user/jhaible/jh_720_vco_scanner.gif

I've used a 3080 to control the input level. Your method
of applying a variable Voltage to the resistor divider chain
is certainly more elegant!

As an ordinary comparator chain produces a "bar" rather than a
"dot" code, I had to find a way to convert the bar to dot.
At first I thought about some logic after the comparator outputs,
but as I'm also only taking every second output (to get that
multiple pulse waveform), I could simply wire the comparator
outputs together in pairs, with the comparator inputs having
opposite polarity. As we're going up the "thermometer code",
the first comparator of a pair "sets" the dot, and the second
"clears" it. Then the same thing will start again with the next pair
of comparators.

Of course these LED driver chips have all the required logic
already built-in. But if someone is looking for a "standard chip"
version, you may find the LM339 version interesting.

JH.