[sdiy] demuxing 4017 inputs

[Tim Parkhurst]

On 7/28/07, bill bigrig <billbigrig at yahoo.com> wrote:
> Howdy,
>
>  I have 4 outputs from and inverter(just after a
> scmitt trigger), and can anyone tell me how these
> inputs, all tied together, can be separated and mixed
> to a single clock input of a 4017? Diodes seem to
> throw capacitance everywhere and the thing just
> triggers like a random machine gun. Should i try
> resistors on the individual inverter outs? I'm running
> the system at 5 volts. Even when I unplug the
> inverter, providing no inputs, the thing just runs
> like crazy. Are the summing diodes causing a
> capacitance thing to the inputs? I just need to know
> how to make 4 different signals work together to clock
> the 4017 input. No more than 1 line is high at the
> same time. I'm at a loss. Thanx
> Rig
>

Yeah, I think Daryl and Tom are correct. You basically want to OR the
inputs together. If you use a standard OR gate, then the 4017 should
operate correctly. If you're using diode OR, then you definitely need
to have a 10K to 100K pull down resistor on the 4017 clock input
(after the diode OR). That's one thing you have to watch out for on a
diode OR - they don't allow a path to ground for the input they're
driving and so the driven input floats when none of the OR inputs is
high. Floating inputs is A Very Bad Thing for CMOS, but hanging a pull
down resistor after the Diode OR should fix the problem. For more info
on implementing things like diode OR gates, check out Ray Wilson's
primer on "Mickey Mouse Logic"
http://www.musicfromouterspace.com/analogsynth/mmlogic.html

Note that Ray's OR gate shows a Schmidt trigger inverter on the
output. This also works to provide a ground path for the logic gates
following the OR, which means you don't need a pull down R with this
design. The Schmidt trigger also cleans up the waveforms, but it does
require an extra chip (7414, or I think you might want to use a
40106). Hopefully this will get you set up.


Tim (doesn't have both ORs in the water) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein