5 Dec
2007
5 Dec
'07
11:53 a.m.
This should be a much simpler problem than I'm making
it, but at the moment my brain doesn't want to come up
with a solution.
Picture this - a number of switches, say, eight.
Momentary contact, normally open switches. And then
eight outputs, one for each switch, TTL level. I want
to be able to push one switch and bring it's
corresponding output high, and have it stay high even
when the switch is released. But, making this more
complicated, I only want to be able to allow one
switch to be active at a time. So, say switch four is
pressed, output four is high. Then, switch one is
pressed, and output four goes low, and output 1 goes
high. Timing is not critical.
I know this should be a simple array of flip flops, in
some fashion, but at the moment, the solution eludes
me. I feel like there is probably even a single/two
chip solution to this.
Now, I know that this is probably off-topic, but I'd
be using this in on-topic hardware...
-Ian
5 Dec
5 Dec
12:05 p.m.
Date: Wed, 5 Dec 2007 11:53:11 -0800> From:
ian_primus at yahoo.com> To: cctalk at classiccmp.org> Subject: Almost OT:
Pushbutton switch latching> > This should be a much simpler problem than I'm
making> it, but at the moment my brain doesn't want to come up> with a
solution.> > Picture this - a number of switches, say, eight.> Momentary contact,
normally open switches. And then> eight outputs, one for each switch, TTL level. I
want> to be able to push one switch and bring it's> corresponding output high,
and have it stay high even> when the switch is released. But, making this more>
complicated, I only want to be able to allow one> switch to be active at a time. So,
say switch four is> pressed, output four is high. Then, switch one is> pressed, and
output four goes low, and output 1 goes> high. Timing is not critical.> > I know
this should be a simple array of flip flops, in> some fashion, but at the moment, the
solution eludes> me. I feel like there is probably even a single/two> chip solution
to this.> > Now, I know that this is probably off-topic, but I'd> be using
this in on-topic hardware...> > -Ian
What pops into mind, but would need a bit more thought,
8 D flipflops with common clock.
The 8 switches are connected to the 8 D inputs, but also
tied together with a small capacitor and connected to the
clock. When you press a switch, the D input is set to the
level of the pressed switch, while the other D inputs are
at the levels from the not-pressed switches.
Pressing any switch will produce a pulse on the clock
(with a small delay). The levels of the 8 D inputs is latched
in the 8 D flipflops.
I have build that circuit, but can't remember exactly how
the 8 switches connect combined to the single clock input ...
Henk
12:17 p.m.
Henk Gooijen wrote:
I have build that circuit, but can't remember
exactly how
the 8 switches connect combined to the single clock input ...
This is job for Superman ... err one-shots.
The first one-shot triggers reset and when that times out it
tiggers a second one-shot for the clock.
Henk
1 p.m.
Henk Gooijen wrote:
I have build that circuit, but can't remember exactly how
the 8 switches connect combined to the single clock input ...
How about a MM54HCT149 8-line to 8-line priority encoder?
http://eshop.engineering.uiowa.edu/NI/pdfs/00/53/DS005364.pdf
Mike Loewen mloewen at cpumagic.scol.pa.us
Old Technology http://sturgeon.css.psu.edu/~mloewen/Oldtech/
12:29 p.m.
Picture this - a number of switches, say, eight.
Momentary contact,
normally open switches. And then eight outputs, one for each switch,
TTL level. I want to be able to push one switch and bring [its]
corresponding output high, and have it stay high even when the switch
is released. But, making this more complicated, I only want to be
able to allow one switch to be active at a time. So, say switch four
is pressed, output four is high. Then, switch one is pressed, and
output four goes low, and output 1 goes high. Timing is not
critical.
Do you care what happens if two switches are pressed at once? In
particular, do you care what the outputs are like during the time when
both switches are pressed? Do you care what the state is like on
powerup, before any switches are pressed?
My first cut would be to set up a bunch of R-S flipflops (each one
being two cross-coupled NAND gates, probably). A switch would then
drive low the S input for its flipflop and the R input for all other
flipflops. (How this is done is more or less irrelevant. Personally,
I'd probably put pullups on the inputs (omittable if they're true TTL,
as opposed to something like HCT which is TTL-level but not real TTL)
and then have each switch ground the cathode side of a diode matrix
which then pulls the relevant inputs low. If you want to "do it right"
you'd need a bunch of 8-input AND/NAND gates - you might even make the
flipflops out of those, with one input of each flipflop dedicated to
each switch.)
If you care about power-up state I'd probably add some kind of
discrete-component circuit to fake pressing a switch for a brief time
during powerup. I can see a possible way to do it with a diode, a
transistor, a resistor, and a capacitor....
Another possible solution is to feed the raw switch bits into a
priority encoder chip, latch the resulting encoded bits, and feed the
latch outputs into an N-to-one demux chip.
/~\ The ASCII der Mouse
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12:49 a.m.
Picture this - a number of switches, say, eight.
Momentary contact,
normally open switches. And then eight outputs, one for each switch,
TTL level. I want to be able to push one switch and bring [its]
corresponding output high, and have it stay high even when the switch
is released. But, making this more complicated, I only want to be
able to allow one switch to be active at a time. So, say switch four
is pressed, output four is high. Then, switch one is pressed, and
output four goes low, and output 1 goes high. Timing is not
critical.
Generate a pulse when any switch is pressed (AND them together and
drive a one-shot) - use this to latch the inputs from the switches to the
outputs. So any time a switch is pressed, the current state of the switches
is latched. The latch should be triggered at the trailing edge of the pulse.
Side effects which may or may not be significant:
- The timing of the pulse will provide debounce - the switches will
have this amount of time to settle before they are latched.
- If you press (and hold) two switches within the period of time of the
pulse, you will get two outputs latched.
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html
11:56 a.m.
Look for row of mechanical pushbuttons that you only can push any of
button while any one of other 3 buttons even is pressed pops out.
I see this on older audio stuff. Usually 3 to 5 ganged together and
stays on but you could use tact button (they come in different length
of button itself) touching the rear end of the pushbutton (these
ganged buttons have open frame on back btw.) so you get momentary
aaction when pressed but button that was pushed stays in but not
closing the tact switch because of play in the latching mechanism.
Got it?
Cheers, Wizard
1:10 p.m.
Picture this - a number of switches, say, eight.
Momentary contact,
normally open switches. And then eight outputs, one for each switch,
TTL level. I want to be able to push one switch and bring [its]
corresponding output high, and have it stay high even when the switch
is released. But, making this more complicated, I only want to be
able to allow one switch to be active at a time. So, say switch four
is pressed, output four is high. Then, switch one is pressed, and
output four goes low, and output 1 goes high. Timing is not
critical.
How much overlap or "dead space" between switch depressions? In
other words, can any two outputs be simultaneously high, even for a
few nanoseconds? If a switch is pressed, can your application
tolerate a brief "nothing is pressed" period of a few nanoseconds
between cancelling the old depression and instating the new one?
Will your application be synchronous (i.e. depressions synchronized
to the transitions of a clock signal) or asynchronous? Would you
rather have your output as a 3-bit encoded binary number?
There are several solutions, depending on your needs.
The solution with the lowest parts count would be a PIC or AVR
microcontroller (might also be the least expensive). My next choice
might be a registered PAL. But if you want to stay vintage, there
are low parts count combinatorial solutions.
Cheers,
Chuck
1:29 p.m.
I see active low press.
1 8 input nand gate to generate a high clock
to all the D F/F with '0' as input. The active low press sets the FF.
Reset is left for power on clear of the FF's.
So 1 8 input NAND and 4 dual FF's are needed.
1:49 p.m.
On 5 Dec 2007 at 14:29, woodelf wrote:
I see active low press.
1 8 input nand gate to generate a high clock
to all the D F/F with '0' as input. The active low press sets the FF.
Reset is left for power on clear of the FF's.
So 1 8 input NAND and 4 dual FF's are needed.
My point was that there are many unanswered questions. Two keys down
at the same time would set two FFs, for example. If two keys are
down and one must decide, which gets priority? The first key
pressed, or the last key released? Is debouncing part of this
circuit? Since the "none pressed" never occurs after the first
keypress, is there an initial state?
Isn't this what engineering is about--to define one's requirements--
and then develop a solution?
Or am I just a geezer and too old-school?
Cheers,
Chuck
4:35 p.m.
--- Chuck Guzis <cclist at sydex.com> wrote:
On 5 Dec 2007 at 14:29, woodelf wrote:
I guess I should have been more clear... And everyone
has brought up some good points. I initally didn't
think about the initial power-up setting, but in this
application it doesn't much matter - I guess
defaulting to none pressed (or #1 pressed, even
better) would be good. I don't intend to press two
switches at once, but if it were to happen, it doesn't
much matter - either one could take priority. I'd
rather not have more than one select output active at
a time, but it's not a huge problem if it happens.
Timing is totally non-critical. Two outputs can be
active for a tiny period, no outputs active during
switching is fine. The idea is to simply control some
CBT3244A bus switching chips to switch some analog
signals. I want to avoid having each chip controlled
by a toggle on/off switch, because then in order to
change signals you would have to manually turn off
one, and turn on the next.
The main thing I'm trying to achieve is simplicity.
Nothing fancy. No PIC's, no microcontrollers, etc. I
initially thought of using flip flops and a bunch of
inverters to reset all the non-selected flip flop, but
couldn't work out in my head a good way to do it... I
hadn't taken switch debouncing into the equation - but
aren't flip flops commonly used to debounce switches
anyway?
I see active low press.
1 8 input nand gate to generate a high clock
to all the D F/F with '0' as input. The active low
press sets the FF.
Reset is left for power on clear of the
FF's.
So 1 8 input NAND and 4 dual FF's are needed.
My point was that there are many unanswered
questions. Two keys down
at the same time would set two FFs, for example. If
two keys are
down and one must decide, which gets priority? The
first key
pressed, or the last key released? Is debouncing
part of this
circuit? Since the "none pressed" never occurs
after the first
keypress, is there an initial state? Isn't this what engineering is about--to define
one's requirements--
and then develop a solution?
Or am I just a geezer and too old-school?
No, you're absolutely correct. I just didn't properly
define the requirements. It's been a rough day.
-Ian
4:43 p.m.
On 5 Dec 2007 at 16:35, Mr Ian Primus wrote:
The main thing I'm trying to achieve is
simplicity.
Nothing fancy. No PIC's, no microcontrollers, etc. I
initially thought of using flip flops and a bunch of
inverters to reset all the non-selected flip flop, but
couldn't work out in my head a good way to do it... I
hadn't taken switch debouncing into the equation - but
aren't flip flops commonly used to debounce switches
anyway?
How about an 8-position rotary switch?
Cheers,
Chuck
4:59 p.m.
Chuck Guzis wrote:
On 5 Dec 2007 at 16:35, Mr Ian Primus wrote:
and skip the FF's ... all manual :)
The main thing I'm trying to achieve is
simplicity.
Nothing fancy. No PIC's, no microcontrollers, etc. I
initially thought of using flip flops and a bunch of
inverters to reset all the non-selected flip flop, but
couldn't work out in my head a good way to do it... I
hadn't taken switch debouncing into the equation - but
aren't flip flops commonly used to debounce switches
anyway?
How about an 8-position rotary switch? Cheers,
Chuck
5:11 p.m.
--- woodelf <bfranchuk at jetnet.ab.ca> wrote:
Chuck Guzis wrote:
and skip the FF's ... all manual :)
Yeah, I thought about that. But then I'd have to find
a rotary switch with the proper number of positions...
And besides, what's the fun in that? I also wanted to
be able to use an arbitrary number of inputs. Eight
was just an example :)
-Ian
On 5 Dec 2007 at 16:35, Mr Ian Primus wrote:
> The main thing I'm trying to achieve is
simplicity.
> Nothing fancy. No PIC's, no
microcontrollers,
etc. I
> initially thought of using flip flops and a
bunch
of
> inverters to reset all the non-selected flip
flop, but
> couldn't work out in my head a good way
to do
it... I
> hadn't taken switch debouncing into the
equation
- but
> aren't flip flops commonly used to
debounce
switches
anyway?
How about an 8-position rotary switch?
5:22 p.m.
On 5 Dec 2007 at 17:11, Mr Ian Primus wrote:
Yeah, I thought about that. But then I'd have to
find
a rotary switch with the proper number of positions...
And besides, what's the fun in that? I also wanted to
be able to use an arbitrary number of inputs. Eight
was just an example :)
My take on this is that if you employ pushbuttons, you'll need some
sort of indicator as to which one's been pressed. So, that's a bunch
of LEDs or a digital display and all the gallimaufry associated with
that. If you want to extend the design, consider stacking a number
of binary-encoded switches. Readily obtainable with direct (i.e.
foolproof) readout.
But nothing beats a rotary switch for simplicity. A quick search of
the Jameco catalog shows no shortage of them.
Cheers,
Chuck
5:35 p.m.
> > How about an 8-position rotary switch?
On Wed, 5 Dec 2007, Mr Ian Primus wrote:
Yeah, I thought about that. But then I'd have to
find
a rotary switch with the proper number of positions...
And besides, what's the fun in that? I also wanted to
be able to use an arbitrary number of inputs. Eight
was just an example :)
. . . then I guess that a rotary telephone dial is out of the question
6 Dec
6 Dec
1:08 p.m.
Yeah, I thought about that. But then I'd have to
find
a rotary switch with the proper number of positions...
Over heere you can easily get single pole, 12 position rotary switches.
There's a little key plate, retained by the fixing nut, that you can
insert into different holes in the switch to select fewer than 12
positions. So you could quite easily have 8 postiions.
-tony
5 Dec
5 Dec
4:51 p.m.
On 5 Dec 2007 at 16:35, Mr Ian Primus wrote:
The main thing I'm trying to achieve is
simplicity.
Nothing fancy. No PIC's, no microcontrollers, etc. I
initially thought of using flip flops and a bunch of
inverters to reset all the non-selected flip flop, but
couldn't work out in my head a good way to do it... I
hadn't taken switch debouncing into the equation - but
aren't flip flops commonly used to debounce switches
anyway?
...or a 0-7 BCD switch connected to a demux chip? Small, direct
readout.
Cheers,
Chuck
6 Dec
6 Dec
1:05 p.m.
On 5 Dec 2007 at 16:35, Mr Ian Primus wrote:
Or a normal rotary switch and a bunch of pull up/down resistors (I don't
know if the enable lines are to be active high or active low)? Even more
direct. For that matter, what about a bank of mechancially-interlocked
buttons (I think you can still get such swtiches and the bracketry to
interlock them).
-tony
The main thing I'm trying to achieve is
simplicity.
Nothing fancy. No PIC's, no microcontrollers, etc. I
initially thought of using flip flops and a bunch of
inverters to reset all the non-selected flip flop, but
couldn't work out in my head a good way to do it... I
hadn't taken switch debouncing into the equation - but
aren't flip flops commonly used to debounce switches
anyway?
...or a 0-7 BCD switch connected to a demux chip? Small, direct
readout. 
1:39 p.m.
Tony Duell wrote:
Or a normal rotary switch and a bunch of pull up/down
resistors (I don't
know if the enable lines are to be active high or active low)? Even more
direct. For that matter, what about a bank of mechancially-interlocked
buttons (I think you can still get such swtiches and the bracketry to
interlock them).
It just occurred to me that there's a possible problem with the
rotary switch plan. If I've selected "3" and I then want to select
"7",
then "4", "5", and "6" are briefly selected. Well, or
"2", "1", and
"0".... :)
If the selection sequence matters, you'd need some sort of delayed
response.
Doc
5 Dec
5 Dec
1:01 p.m.
der Mouse wrote:
Couldn't one just hypothetically feed all of the switch outputs through
a bunch of XORs and then AND the output of the XOR array with each of
the outputs of the flip-flops you suggest below? Or am I
overcomplicating the problem?
Picture this -
a number of switches, say, eight. Momentary contact,
normally open switches. And then eight outputs, one for each switch,
TTL level. I want to be able to push one switch and bring [its]
corresponding output high, and have it stay high even when the switch
is released. But, making this more complicated, I only want to be
able to allow one switch to be active at a time. So, say switch four
is pressed, output four is high. Then, switch one is pressed, and
output four goes low, and output 1 goes high. Timing is not
critical.
Do you care what happens if two switches are pressed at once? In
particular, do you care what the outputs are like during the time when
both switches are pressed? Do you care what the state is like on
powerup, before any switches are pressed? My first cut would be to set up a bunch of R-S
flipflops (each one
being two cross-coupled NAND gates, probably). A switch would then
drive low the S input for its flipflop and the R input for all other
flipflops. (How this is done is more or less irrelevant. Personally,
I'd probably put pullups on the inputs (omittable if they're true TTL,
as opposed to something like HCT which is TTL-level but not real TTL)
and then have each switch ground the cathode side of a diode matrix
which then pulls the relevant inputs low. If you want to "do it right"
you'd need a bunch of 8-input AND/NAND gates - you might even make the
flipflops out of those, with one input of each flipflop dedicated to
each switch.)
If you care about power-up state I'd probably add some kind of
discrete-component circuit to fake pressing a switch for a brief time
during powerup. I can see a possible way to do it with a diode, a
transistor, a resistor, and a capacitor....
Another possible solution is to feed the raw switch bits into a
priority encoder chip, latch the resulting encoded bits, and feed the
latch outputs into an N-to-one demux chip.
This is the method I would normally choose.
Peace... Sridhar
3:16 p.m.
Well, . . .
You could do it purely in mechanical hardware. Look at an old 5 line
office phone, or an old car radio (sorry, Jay) with pushbuttons.
Or, . . .
you could do it entirely in software, using a junk PC.
Rip the tops off of all of the keys except the numerals.
NXT2: MOV AH,0
INT 16h
; Your most recent key is in AX (ASCII in AL, scanciode in AH)
. . .
JMP NXT2
If you want a multi-megabyte solution, you could use the "radio buttons"
in WINDOZE.
But you'd probably rather do it with relays
6226
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6227
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23 comments
12 participants
participants (12)
-
ard＠p850ug1.demon.co.uk -
bfranchuk＠jetnet.ab.ca -
cclist＠sydex.com -
cisin＠xenosoft.com -
dave06a＠dunfield.com -
doc＠mdrconsult.com -
henk.gooijen＠hotmail.com -
ian_primus＠yahoo.com -
jpero＠sympatico.ca -
mloewen＠cpumagic.scol.pa.us -
mouse＠Rodents.Montreal.QC.CA -
ploopster＠gmail.com