[sdiy] gate sequencer

[Richard Wentk]

At 14:37 18/11/2004 -0500, Roy J. Tellason wrote:
>Easiest thing I can think of would use a 4017 chip,  or similar.  I remember
>running across a page that did exactly that but can't remember who it was,
>perhaps somebody else will jump in with that URL.  Anyhow if you find the
>data sheet on that chip you should get some ideas.

You can't go wrong with a 4017 - or two - for a basic sequence trigger.

>If you want to be able to fiddle with the timing on each output,  then 
>maybe a
>555 timer chip on each output would be the way to go (or use the dual 556,
>quad 558,  etc.).  If you don't know this chip then you ought to -- and
>there's a whole mess of stuff out there on the web that talks about it.

Two problems: the first is that you'll always be some fraction of a step 
behind, so unless you add extra start-up logic you have to wait half a step 
or so after you hit play before you start getting clocks out of it.

The scheme would be to put a timer (like the 555) on each step output from 
the 4017. The timer delays the trigger by a set amount. By default it's 
half a clock, with half a clock variation either way.

This start-up delay might or not might be an issue, depending on your 
application.

The other problem is the timing offsets won't track the tempo, so you'd 
need to set them every time you change the main tempo setting.

A trigger with +/- offsets that track tempo would be rather more 
complicated to build. There are analogue solutions that rely on things like 
switching matrices or voltage controlled resistors, but it's round about 
here that you start heading into Big Board 'O Stuff territory.

The best way to build a rock-solid design would be to do it digitally, 
where you'd run the clock some large factor higher and then count and 
calculate multiples of these timing clicks to give you each output step. 
But that's getting into the kind of territory where you need to start 
programming a microprocessor to do the job.

For now I'd start with the 4017 and work your way up. :) Ken Stone's circuit:

http://www.cgs.synth.net/modules/gate_sequencer2.html

  would be an excellent place to start.

Richard