At 07:21 PM 9/20/00 -0400, Chris Eddy wrote:
>I am trying to sketch a circuit that has four select push buttons for
>inputs, and four seperate digital outputs.
>I want to push a button and latch that output, and unlatch any other
>selected output which was previously selected.

Does it matter what happens if 2 or buttons are pressed at the same time?

1) it does not matter: use a 4 bit SR latch (4043, 4044).  Tie all the
reset lines together and to the reset circuit as follows: one side of all
the buttons connected together and to the base of a transistor.  resistor
from base to emitter.  emitter to rail.  collector to reset line and
another resistor to the other rail.  Other side of each pushbutton to the
set inputs of the latches, with parallel RC network to non-active
rail.  Note that I am saying 'rail' instead of gnd and vcc - this is
because those latches are either NOR or NAND and you can select whether you
want active HI or LO output depending on which chip you use.  The choice of
chip determines whether the inputs pull UP or DOWN for control.  Its a
nice, simple 1 chip 1 transistor solution.

2) You cannot tolerate 2 or more outputs active at the same time.  You have
several choices - all require either lots of passive components or more
than 1 chip unless you use a PLD.  The easiest is probably an 8 input
priority encoder driving a latched decoder with a simple timer network on
the latch enable / clock line.

I have a single sided board layout for a 16 channel version of (1) above,
using 74HC564 latches.  Its part of our museum / interpretive centre line
of modules and has been used all over western Canada for years.  I don't
consider it proprietary and have no problem sending you a copy.  The PCB is
4"x4" and contains: PSU, 16 channels of lamp driver for illuminated
pushbuttons, adjustable timer (1 sec to forever), 16 sinking outputs
(2803), 16 active HI logic level outputs, selectable timer lockout (don't
allow any button presses until last button pressed has timed out).  I don't
have any PCBs in stock right now but could have one or more run off if you
need.

You know, though, that a simple processor based unit is gonna be cheaper,
easier, smaller.  I can think of a real easy solution that uses all 6 pins
on a 12c508 (int osc) and lets YOU decide how you want the outputs to
behave.  And I'll bet that the EMI considerations of a properly bypassed
12c508 running with the internal oscillator selected are not a problem.

dwayne



Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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