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'[PIC]: Touch sensitive switches? cat skins'
2004\09\07@122349 by Robert Rolf

picon face
Bob Axtell 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.

Detecting 12 different levels is not THAT hard for an RC PIC A/D.
(e.g. digital input measuring time to cross threshold).
The trick is deciding what to do about simultaneous presses.

Using a series R chain, where each switch bypasses all resistors
farther away from the pin, is one way to avoid the ambiguity of
the suggested 1/N*R method. e.g.

             Vcc
              R
+R+R+R+R+R+R+R++-PIC
S S S S S S S  C
+-+-+-+-+-+-+--+ Gnd

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

Temperature???
More likely moisture as that more directly affects capacitance.
And one can also have a 'reference' touch pad with a known
fixed capacitance to calibrate the varying pin leakage currents.

The interdigitated (comb) finger method works quite well,
as long as the users' hand's are not so cold that they have gloves
on. Use wide fingers with minimal separation to maximize the
capacitance change seen. Look at an old PC capacitance keyboard
to see how it's laid out.

If pin count is the big issue, I'd suggest the resistor ladder with
Snaptron domes as most 'user proof'.

R

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2004\09\07@175501 by Support - KF4HAZ

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face
Or an R2R network (if you can get 250 ohm 500 ohm 1K 2K 4K 8K 16K 32K 64K
128K 256K 512K) you could theoretically read any number of buttons down at
the same time.
For example All pressed would yield Zero none=4095 or vise versa.
(this is assuming ADC of at least 12bit resolution)
But in practice the cheapest way is to use a matrix if you have the pin
space.
Next best solution is a pic coprocessor that reads keys and feeds serial
codes, there are serial keypads outhere, or it is not that complex to use an
8 I/O pic to do the keypad polling.
KF4HAZ - Lonnie

----- From: "Robert Rolf" <Robert.Rolf@
> Bob Axtell 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
{Quote hidden}

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