[sdiy] ...dumb question (CD4017 sequencer)

[Electronium]

First of all, thanks everyone for the great info on my 4017 sequencer
question.  Sorry for the slow follow-up, but things have been a little
hectic lately.  Here's a few more questions I have:

> ---Well "A" is not a "normal" sequencer function in most cases. When you
> stop a sequencer, it usually puts out the step voltage. The desired
> silence is typically a function of turning off the VCA in a patch (via a
> gated ADSR, for example). So you may wan tto think of having a gate out
> if you don't already.

Oh.... duh.
Thanks John, I guess I could have thought a little further than the VCO ; )

OK, now, I was thinking of using a latched flip-flop to the 'Clock Enable'
(or 'Inhibit') pin to turn on & off the clock, but I was also planning on
tapping directly off the 555's clock output to use as the gate signal (gate
time isn't critical, since I'm using A/D envelopes)... is this how to do it?
but if so, how do you stop it from outputting a gate when the 4017 clock
input is disabled?  Is there a better way?

Also, I wanted to use the sequencer clock as an optional master clock for a
few other clocked modules... how do I turn off the clock output at the same
time as the sequencer?

> You *could* use an analog switch connected to the seq out and control it
> via the seq start/stop to remove any voltage out when the seq is
> stopped.

Actually, I was thinking of doing this... but I thought it might be going
too far.  However, I suppose I could use a 4066 to shut off the gate/clock
output simultaneous to the 4017 inhibit signal.

> Having the 4017 *reset on start *requires a bit of external logic
> between the start/stop and the 4017 reset pin. You might want to try a
> very short pulse from the rising edge of the "start".

So it seems to me that what I need is a positive pulse to the reset pin,
simultaneous to an off state (0v) to the inhibit pin, in order to start the
sequence from the beginning.  Does that sound right?  And then I need an on
state (+v) to the inhibit pin to stop the sequence.

So, can I take the output of a latched 4013, trim off a short pulse, and
then invert it?  (the gate, not the pulse)  I am making any sense?  Can
someone set me straight?

> While the 4017 looks like a sequencer on a chip, getting it to do the
> usual seq functions is a bit complicated. You will probably find some
> circuits on the web if you do a search.

Well, I found yours along with the Blacet MIDI Sync stuff... ; )  I have one
of your ICs, but strangely enough, one of the design requirements of this
system was NON-midi compatibility.  The idea was to get away from my PC &
even any kind of conventional controllers.  But I was able to gleen (sic?)
some ideas from your schematics, as well as several others, to come up with
the basic design.

> I'd like to mention that you'd probably want this feature to be selectable
> with a switch. One of the benefits of having the sequencer output the CV
> while stopped is for tuning the pitch of the steps.

Thanks Dave, I was planning on having 2 separate switches, a SPST for
start/stop & a rotary for 'clock select' (internal/external/step).  However,
I guess the start/stop switch would have to be 'on' in order to use step
mode.

> This is cool because you can put a switch in between the steps of the
sequencer
>(rotary sp12t one for each step) and then choose how many sequences your
>sequence will have.

Thanks Gavin, I was planning on a SP6T switch to select between 3 to 8
steps.

>To stop the sequence and have it stay wherever you stopped it you can put a
>switch in between whatever you used as a clock source and the 4017.

This is how I have it wired up now on my breadboard.  seems to work OK, but
is it better to use the 'clock enable' pin on the 4017?  It seems that's the
way it's designed...

> ...which would go from a +5V source to pin 15 of the 4017 across a .001uF
>cap followed by a 1Meg resistor to ground. This should work.

This is to trim off a short pulse for the 4017 reset pin, correct?

> I wanted to be able to run two chips in parallel (2x8 steps) or
> cascade them for one long sequence (1x16 steps) and control the
> length of the sequence either way (2xN, 1xN), all at the flick of
> various switches. Cascading the two sequences introduced the problem
> that when the first 4017 cascades to the second and is reset, it
> triggers step 0 again (naturally). Since I was only using 8 outputs
> of the 4017, I tied the 9th step (output 8) to INHIBIT, using it to
> "park" the counter outside the visible 8 steps. Hitting RESET returned
> the counter to 0 and the sequence started anew. This worked well for
> an alternating sequence of 16 steps, but not so well for a shorter
> sequence length since the counter would never reach the park spot.
> So, I bumped the active outputs up to 1 through 8, leaving output 0
> for parking the counter out of sight. Unfortunately, this meant that
> when running the counters in parallel I always had an unavoidable
> "invisible" extra step in the sequence.

Thanks Robert, wow... this is an interesting idea.  Although this goes way
beyond the intended purpose of my sequencer, I think the key to cascading 2
CD4017's might be taking the 'carry out' & running it through a flip-flop
(or two) and then use that to switch back & forth between the 2 chips... I'm
pretty naive about these things, but I think that must be what it's there
for.  Anyone know about this?

Thanks again everyone!
Mike Brown
mike at electronium.com