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'[PIC]: Touch sensitive switches?'
2004\09\06@173524 by Philip Pemberton

face picon face
Hi,
 I've just been looking at <http://www.bytecraft.com/touchsw.html> - has
anyone tried out the circuit on that page? Does it work?
 Even better - does anyone know if there's a way to reduce the number of I/O
lines in a similar way to a matrix of pushbuttons? I want something small,
with at least 12 keys, hooked up to a 16F628A (eventually). The whole
device - including the 3x4 keypad array - is going to be around the size of a
credit card.
 Am I stuck with normal mechanical switches?

Thanks.
--
Phil.                              | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB,
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2004\09\06@180209 by Engineering Info

picon face
I wouldn't recomend it for what you need.  The whole idea of that is
that you change the capacitance of the capacitor.  Just putting it in
your shirt pocket will probally trigger things.  Try membrane switches
instead.

Philip Pemberton wrote:

{Quote hidden}

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2004\09\06@180718 by mmynsted_news

picon face
Did you see (Touch Switch) on
http://members.cox.net/berniekm/tricks.html

Sounds like the guy has done something like what you want.
http://members.cox.net/berniekm/

On Monday 06 September 2004 4:35 pm, Philip Pemberton wrote:
{Quote hidden}

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2004\09\06@180751 by Support - KF4HAZ

flavicon
face
We use a 3x4 matrixed keypad that has 8 pins,
pin8 is grounded, 1 - 7 are pulled high through 10k resistors (sip).
When any one key is pressed the row and column read low.
This is read in as an 8 bit value, the 8th bit is stripped of by anding with
$7F,
then the key is read by using a lookup table, even got fancy and included a
couple of 2-key combinations.

KF4HAZ - Lonnie

----- From: "Philip Pemberton" <philpem@
> Hi,
>   I've just been looking at <http://www.bytecraft.com/touchsw.html> - has
> anyone tried out the circuit on that page? Does it work?
>   Even better - does anyone know if there's a way to reduce the number of
I/O
> lines in a similar way to a matrix of pushbuttons? I want something small,
> with at least 12 keys, hooked up to a 16F628A (eventually). The whole
> device - including the 3x4 keypad array - is going to be around the size
of a
> credit card.
>   Am I stuck with normal mechanical switches?
>
> Thanks.
> --
> Phil.                              | Acorn Risc PC600 Mk3, SA202, 64MB,
6GB,
> .....philpemKILLspamspam@spam@dsl.pipex.com              | ViewFinder, 10BaseT Ethernet,
2-slice,
> http://www.philpem.dsl.pipex.com/  | 48xCD, ARCINv6c IDE, SCSI
> ... The current limits placed are based on resistance

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2004\09\06@185909 by Bob Axtell

face picon face
One of the best ways to connect switches to minimize PIC pins is to wire
the switches so that when pressed, they present a different voltage
level on a
PIC analog input pin. I've sucessfully used 8 switches on a single PIC
analog input. Just hook 'em up so that a each different resistor value
causes the
voltage at the analog pin to be 1/8FS, 1/4FS, 3/8FS, etc etc. A reading
of FS means NO switch is pressed. If I had 12 switches, I'd connect them to
2 PIC A/D pins, 6 on each pin.

I've tried those touch switches. The reliability changes quite a bit
with temperature. They are NOT  a workable solution.

--Bob

Philip Pemberton wrote:

{Quote hidden}

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2004\09\06@205435 by Cwebber

flavicon
face
At 05:59 PM 9/6/2004, you wrote:
>One of the best ways to connect switches to minimize PIC pins is to wire
>the switches so that when pressed, they present a different voltage level on a
>PIC analog input pin. I've sucessfully used 8 switches on a single PIC
>analog input. Just hook 'em up so that a each different resistor value
>causes the
>voltage at the analog pin to be 1/8FS, 1/4FS, 3/8FS, etc etc. A reading of
>FS means NO switch is pressed. If I had 12 switches, I'd connect them to
>2 PIC A/D pins, 6 on each pin.

If I remember correctly microchip has an appnote called "trick's and tips"
for the 12F628 that includes the method that Bob describes above.  It gives
a schematic and maybe a little bit more detail.  It has a handful of other
techniques to maximize the usefulness of low pin count PIC's.  You should
be able to find it on the product page for the 16F628 or other 8pin PICs.

Chip


>I've tried those touch switches. The reliability changes quite a bit with
>temperature. They are NOT  a workable solution.
>
>--Bob
>
>Philip Pemberton wrote:


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2004\09\07@021947 by Robert Rolf

picon face
Philip Pemberton wrote:
> Hi,
>   I've just been looking at <http://www.bytecraft.com/touchsw.html> - has
> anyone tried out the circuit on that page? Does it work?

I haven't used that specific circuit but I see no reason why it
wouldn't work as described.

I think the 2nd version (500k pullup to Vcc) is the more pin
efficient. Just idle the pin high to minimize power waste.

Don't forget to add clamp diodes to Vcc and Gnd for EACH input pad
since the pin is capacitively couple to a huge static source (finger).

The 10k pullups or the mondo cct will only work if directly
connected to pads (thumbtacks) but there really should be 1k
series protection resistors and clamp diodes.

>   Even better - does anyone know if there's a way to reduce the number of I/O
> lines in a similar way to a matrix of pushbuttons? I want something small,

Use a pair of 4051 analog muxes for the finger pads.
You'll need 5 or 6 pins (4 address using an inverter to handle *E line
and 1 I/O or 3 address and 2 muxed I/O with *E grounded).

You should have a pullup resistor for each mux input, rather than
just one on the uP side since you don't want the inputs floating
particularly with external inputs.

> with at least 12 keys, hooked up to a 16F628A (eventually). The whole
> device - including the 3x4 keypad array - is going to be around the size of a
> credit card.
>   Am I stuck with normal mechanical switches?

No, but I'd make the touch pads interdigitated (two comb traces
interleaved) to ensure that you have a reliable ground.
I have found that you cannot rely on 'virtual ground' of a
human when they're not holding the case of something so small.

I would also suggest you look at snaptron
http://www.snaptron.com/
domes that can mount directly to your board. They are mechanical, but
they are very low profile (0.2mm) and give a nice tactile feedback.

Been using them for years, and as long as you gold plate the pads,
you get great reliability and life.

You'd use the usual key pad matrix scan cct's with them.

12 keys on a credit card? What the heck are you designing?

Robert



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2004\09\07@023231 by Jan-Erik Soderholm

face picon face
Cwebber wrote :

> If I remember correctly microchip has an appnote called
> "trick's and tips"  for the 12F628 that includes the
> method that Bob describes above.

Hi.
Maybe this one :

DS40040B, "Tips ‘n Tricks, 8-pin FLASH"

http://ww1.microchip.com/downloads/en/DeviceDoc/40040b.pdf

"TIP #7, 4x4 Keyboard with 1 Input"

Regards,
Jan-Erik.
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2004\09\07@054834 by Philip Pemberton

face picon face
In message <.....200409070632.i876WUq21213KILLspamspam.....d1o270.telia.com>>          "Jan-Erik Soderholm" <EraseMEjan-erik.soderholmspam_OUTspamTakeThisOuTtelia.com> wrote:

> DS40040B, "Tips ‘n Tricks, 8-pin FLASH"
>
> ww1.microchip.com/downloads/en/DeviceDoc/40040b.pdf
>
> "TIP #7, 4x4 Keyboard with 1 Input"

Thanks for posting the link, Jan-Erik - certainly beats digging around
Mchip's website :)

Now I need to find 16 switches and select the resistor values. I've been
thinking about ways to prototype the switches, too, and I found something in
one of the RadioShack "Engineer's Mini Notebook" books ("Electronic Sensor
Circuits and Projects", part number 620-5034, page 14) that seems to be quite
similar in concept to a membrane switch.
Basically, you've got a PCB with "fingers" etched into it, like this ('scuse
the ASCII art)
  ____
 | ___
 |___ |
--| ___|---
 |___
 Next you have a piece of plastic with holes cut into it - one per switch.
Finally, you have a piece of plastic with tinfoil squares stuck on the bottom
and your labelling on the top. It looks like it might work - making flat
solder joints to link up the matrix will probably be pretty difficult though.
OTOH, RoadRunner (wiring pencil) wire might work...

Later.
-- Phil.                              | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB,
philpemspamspam_OUTdsl.pipex.com              | ViewFinder, 10BaseT Ethernet, 2-slice,
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2004\09\07@062752 by Jan-Erik Soderholm

face picon face
Philip Pemberton wrote :

> Basically, you've got a PCB with "fingers" etched into it,
> like this ('scuse  the ASCII art)
>    ____
>   | ___
>   |___ |
> --| ___|---
>   |___
>  
> Next you have a piece of plastic with holes cut into it - one
> per switch.
> Finally, you have a piece of plastic with tinfoil squares
> stuck on the bottom and your labelling on the top.

Maybe some of the 0.05 mm double sided copper laminate I have
would work ? It's a 0.05 mm fiber glass core with one 18 mu (1/2 oz)
copper layer on each side. One could etch the "keys" directly from
the copper and put a plastic sheet in between. The "wiring" would be
etched directly from the laminat at the same time, of course.

Send me your address off-list and I'll send you a couple of
sheets (6" x 9"). I have a few uncut 18" x 24" sheets of the
same material also if you'd like to make some *large*
keyboards :-) :-)

Regards,
Jan-Erik.

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2004\09\07@070915 by olin_piclist

face picon face
Cwebber wrote:
>> One of the best ways to connect switches to minimize PIC pins is to
>> wire
>> the switches so that when pressed, they present a different voltage
>> level on a PIC analog input pin.
>
> If I remember correctly microchip has an appnote called "trick's and
> tips"
> for the 12F628 that includes the method that Bob describes above.

That would be a good trick, considering that there is no 12F628 in the first
place, and neither the 12F629 nor 16F628 have A/D inputs anyway.


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2004\09\07@084230 by hael Rigby-Jones

picon face


{Quote hidden}

Nothing to stop you using a homebrew single slope ADC with an R and a C and
a port pin.  Granted the 12F628 is something of a rarity :)

Mike

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2004\09\07@091755 by Russell McMahon

face
flavicon
face
> Nothing to stop you using a homebrew single slope ADC with an R and a C
and
> a port pin.  Granted the 12F628 is something of a rarity :)

Is that the one they are bringing out in TO92 ?
(Power version in TO220 with 2 amp I/O)



       RM


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2004\09\07@095511 by Cwebber

flavicon
face

At 06:09 AM 9/7/2004, you wrote:
>Cwebber wrote:
> >> One of the best ways to connect switches to minimize PIC pins is to
> >> wire
> >> the switches so that when pressed, they present a different voltage
> >> level on a PIC analog input pin.
> >
> > If I remember correctly microchip has an appnote called "trick's and
> > tips"
> > for the 12F628 that includes the method that Bob describes above.
>
>That would be a good trick, considering that there is no 12F628 in the first
>place, and neither the 12F629 nor 16F628 have A/D inputs anyway.

That would be pretty impressive, in the meantime i'll just use an existing
PIC - no need to re-invent the wheel.  I confused the 12F675 (which has
A/D) with the 12F629 (which does not).  Thanks for catching my mistake:)

Chip



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2004\09\07@105110 by Philip Pemberton

face picon face
In message <009c01c494dd$148e7460$7c01a8c0@y2k>
         Russell McMahon <spamBeGoneapptechspamBeGonespamparadise.net.nz> wrote:

> > Nothing to stop you using a homebrew single slope ADC with an R and a C
> and
> > a port pin.  Granted the 12F628 is something of a rarity :)
>
> Is that the one they are bringing out in TO92 ?
> (Power version in TO220 with 2 amp I/O)

Sounds nice, but how are you supposed to program it if it only has one I/O
pin? :)
No, wait, how about a PIC in a "Pentawatt" (5-pin TO220) package, similar to
the L200CV voltage regulator? Three usable I/Os, INTRC-only, PIC16 core,
FLASH program memory, 64 bytes of E2PROM, all outputs capable of 1A or more
:)

Later.
--
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2004\09\07@105110 by Philip Pemberton

face picon face
In message <RemoveME200409071027.i87ARiO10498spamTakeThisOuTd1o270.telia.com>>          "Jan-Erik Soderholm" <jan-erik.soderholmEraseMEspam.....telia.com> wrote:

> Maybe some of the 0.05 mm double sided copper laminate I have
> would work ? It's a 0.05 mm fiber glass core with one 18 mu (1/2 oz)
> copper layer on each side. One could etch the "keys" directly from
> the copper and put a plastic sheet in between. The "wiring" would be
> etched directly from the laminat at the same time, of course.

Right - I've got the designs sorted. The PCBs are etching now - one
0.05mm-thick PCB with the "coins" on it and one 0.8mm-thick PCB with the
fingers.
I hope 3M PhotoMount spray adhesive isn't electrically conductive :-/

Later.
--
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EraseMEphilpemspamdsl.pipex.com              | ViewFinder, 10BaseT Ethernet, 2-slice,
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2004\09\07@121137 by Engineering Info

picon face


Philip Pemberton wrote:

{Quote hidden}

Please refer to the detailed discussion on I-Buttons from about 2 weeks
ago.  Dallas has been doing this for YEARS!!!!!!

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2004\09\07@135127 by Dwayne Reid

flavicon
face
At 03:35 PM 9/6/2004, Philip Pemberton wrote:
>Hi,
>   I've just been looking at <http://www.bytecraft.com/touchsw.html> - has
>anyone tried out the circuit on that page? Does it work?
>   Even better - does anyone know if there's a way to reduce the number of I/O
>lines in a similar way to a matrix of pushbuttons? I want something small,
>with at least 12 keys, hooked up to a 16F628A (eventually). The whole
>device - including the 3x4 keypad array - is going to be around the size of a
>credit card.

Have you looked at Motorola's "e-field" sensing chips?  They might do what
you need.  In addition, Qprox has touch sensitive matrix keypad chips that
work well.  I haven't used the Motorola stuff yet but I think highly of Qprox.

I've been using a couple of different approaches for most of my touch stuff
- one is loosely based upon Don Lancaster's touch switch as described in
his "CMOS Cookbook", the other uses a CMOS 555 timer in free-running
astable mode.  Both techniques have worked very well in many public
displays (museums and interpretive centres) that we've built controls for
over the past 20 years.

The 555 timer version works particularly well - there is no direct contact
to the circuit, allowing for pretty darned good ESD protection.  Our
standard demo involves getting the client to try destroying the unit with
anything he can think of - we give him the largest piezo sparker I've been
able to find - it easily generates 1" sparks.  The unit glitches when hit
with that nasty fellow but recovers all by itself.  No units have ever
failed in the field.

The downside of the '555 version is the relatively large footprint required
- the touch switches I've been building have pads that are almost 0.75"
square.  I'd think the smallest size that would work is about 0.5" square.

dwayne

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2004\09\07@140641 by hilip Stortz

picon face
i see a couple of problems with all those circuits for your'
application.  first, ESD!  it would be terribly easy to fry the pic with
static electricity even with a thin insulator.  a nice demonstration
idea, but bringing pic pins out where they can be touched, even with a
thin piece of insulation is begging for problems.  second, those
circuits rely on the other plate of the capacitor as well, i.e. the
power supply etc. that the pic is connected to are the other plate when
you touch the switch, if it's all on a credit card, that could be a
problem unless the bottom of it is mostly solid metal protected by
insulation so that your other hand and the circuit metal form part of
the capacitor.  
if you really want touch switches, motorola makes a very nice chip that
reads 10 or 12 keys and can easily be expanded, they call it an "e
field" sensor and it uses a fairly high frequency oscillator so very
little capacitance is required but it will work over a wide range. circuit cellar ran a design contest based on them and there were a
couple of clever ways to scan more keys, and it can be done through as
much as 1/2" of insulating material or more.  the ic normally uses 2
pads to compensate the sensitivity (which you set, but this way it
tracks changes due to temperature etc. ).  it's very slick and very
flexible, it's also great for sensing fluid level, up to 10 fixed levels
with no contact, you put the sensors in a piece of pvc or other plastic
pipe.  if you want it really thin, touch switches are probably the way
to go, or a membrane keyboard if you can find one surplus that's the
right size.  you said about the size of a credit card, but didn't
mention if it had to be thin like a credit card (or a "smart" card) or
how rigid that size spec was.  obviously to keep it thin you'll need
surface mount parts, and either external power of one of those flat
batteries like in polaroid film packs etc. (i forget who makes them, but
they are available).

Philip Pemberton wrote:
>
> Hi,
>   I've just been looking at <http://www.bytecraft.com/touchsw.html> - has
> anyone tried out the circuit on that page? Does it work?
>   Even better - does anyone know if there's a way to reduce the number of I/O
> lines in a similar way to a matrix of pushbuttons? I want something small,
> with at least 12 keys, hooked up to a 16F628A (eventually). The whole
> device - including the 3x4 keypad array - is going to be around the size of a
> credit card.
>   Am I stuck with normal mechanical switches?
------

-- Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist. Then they came for the Jews, and I didn't speak up because I wasn't a
Jew. Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist. Then they came for the Catholics, and I didn't speak up because I was a
Protestant. Then they came for me, and by that time no one was left to speak up."
-- Martin Niemöller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.

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2004\09\07@143916 by Harold Hallikainen

face picon face
The circuit at the provided URL seems like it'd have ESD sensitivity
problems. I imagine a big resistor could be put between the sensing plate
and the input pin to help reduce problems.

Many years ago, I got samples of a chip from someone that did touch
sensitive switches. It was a clever technique. On the "outside" of the box
was the touch plate. On the "inside" of the box were two plates behind the
outside plate, each taking about half the area (with a small gap between
them). This could easily be done with a double sided circuit board. A
square wave drove one of the inside touch plates. The square wave would be
sensed on the other touch plate. The circuit appears to be two capacitors
in series. Touching the outside touch plate grounds the junction of the
two capacitors, preventing the signal from getting to the inside sense
plate.

I never got to use it for anything, but I thought it was pretty clever.

Harold


{Quote hidden}

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2004\09\07@153442 by Dave Tweed

face
flavicon
face
Philip Pemberton <RemoveMEphilpemTakeThisOuTspamspamdsl.pipex.com> wrote:
> [A 3-pin PIC] Sounds nice, but how are you supposed to program it if it
> only has one I/O pin? :)

The same way you program any other "1wire" (Dallas/Maxim) device.

In fact, there's no reason that a 1wire device couldn't be a master on
the 1wire bus, as long as it's operating power can be supplied by the bus
pullup resistor.

Perhaps the pullup voltage could be raised to force it into slave mode
for programming, then reduced to 5V for normal operation in master mode.

Or it could poll a particular slave address at power-up and reprogram
itself from that slave if it contains a valid image (something like a
serial bootloader).

So there's really no reason you couldn't have a true 2-pin PIC!

-- Dave Tweed
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2004\09\07@154246 by Philip Pemberton

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In message <EraseME413DF8E5.88AF111EspamspamspamBeGoneearthlink.net>
         Philip Stortz <RemoveMEmadscientist.at.largeKILLspamspamearthlink.net> wrote:

> you said about the size of a credit card, but didn't
> mention if it had to be thin like a credit card (or a "smart" card) or
> how rigid that size spec was.

Thickness can be up to around 5mm, but thinner is better (for obvious
reasons).

> obviously to keep it thin you'll need surface mount parts,

Not a problem :)

> and either external power of one of those flat
> batteries like in polaroid film packs etc. (i forget who makes them, but
> they are available).

I was going to use a pair of tagged CR2032s in parallel.

Later.
--
Phil.                              | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB,
philpemSTOPspamspamspam_OUTdsl.pipex.com              | ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.dsl.pipex.com/  | 48xCD, ARCINv6c IDE, SCSI
.... Documentation is for people who can't read.
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2004\09\07@155825 by Walter Banks

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There are some potential ESD problems. I don't know of any failures due to ESD.
The key plates in most of our implementations were isolated by Mylar film. It
provides some isolation as does input protection diodes and the resistor.

Some of our applications would have been ESD prone. One was a security keypad
on a chain of secure storage of storage lockers where the entry was done from
arriving cars.

When we first started using this approach to switching I built a QWERTY
keyboard that would work with the glass TTY's we were using. The sensors
were CMOS gates. It had a keypress clicker inside. I used it for a few
months including over a Canadian winter where the relative humidity is
low and the static sparks are long.



w..



Harold Hallikainen wrote:
{Quote hidden}

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2004\09\07@173510 by Bob Japundza

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On Mon, 06 Sep 2004 15:59:11 -0700, Bob Axtell <spamBeGoneengineerSTOPspamspamEraseMEcotse.net> wrote:
> One of the best ways to connect switches to minimize PIC pins is to wire
> the switches so that when pressed, they present a different voltage
> level on a
> PIC analog input pin. I've sucessfully used 8 switches on a single PIC
> analog input. Just hook 'em up so that a each different resistor value
> causes the
> voltage at the analog pin to be 1/8FS, 1/4FS, 3/8FS, etc etc. A reading
> of FS means NO switch is pressed. If I had 12 switches, I'd connect them to
> 2 PIC A/D pins, 6 on each pin.

Bob, can you explain how you handle simultaneous keypresses?

Regards,
another Bob
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2004\09\07@191600 by Bob Axtell

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Hi, Bob!

Ever been to the "Bob Convention" in Iowa every summer? Gotta be a "Bob"
to get in...

Multiple keypresses are a problem on any multiplex scheme. If I catch a
voltage value between proper
values, I ignore it. If you have a "shift" function (can be down while
others are pressed)  I'd put that on
another pin. Ideally, you should make read the pin several times before
deciding to act on the voltage
value, too.

Of course you have to allow tolerance, but if you use 1% resistors of
high enough value, the
limits are very close (2% or so). If you use low ohm resistors, they
might self-heat (and change value slightly)
from the current through the switch; also some switches measure 100 ohms
when pressed!

BTW, this scheme is used by high-end surveillance cameras to control the
camera settings by the PIC. For that,
I used a  74HC4051 for the 8 different settings, with a precise resistor
on each leg. The digital signals from the
PIC control the camera.

--Bob

Bob Japundza wrote:

{Quote hidden}

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