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PICList Thread
'Touch sensitive code'
1998\07\15@133401 by John Haggins

picon face
Hi,

Anyone got code to make a PIC input touch sensitive with no external
components?

i.e. if you connect a wire to an input, the PIC "knows" when you touch the
wire.

Thanks!

1998\07\15@144406 by Michael S. Hagberg

flavicon
face
i've set one port to an output and sent a pulse train out then looked at the
inputs for the signal.
the output pin is used for all inputs.

michael

-----Original Message-----
From: John Haggins <spam_OUTjawgTakeThisOuTspamEARTHLINK.NET>
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU <PICLISTspamKILLspamMITVMA.MIT.EDU>
Date: Wednesday, July 15, 1998 12:44 PM
Subject: Touch sensitive code


{Quote hidden}

1998\07\15@151147 by Peter L. Peres

picon face
On Wed, 15 Jul 1998, John Haggins wrote:

> Hi,
>
> Anyone got code to make a PIC input touch sensitive with no external
> components?
>
> i.e. if you connect a wire to an input, the PIC "knows" when you touch the
> wire.
>
> Thanks!

                           R2
                        +-/\/\/\- +Vcc
                 R1     |               |
Touch here O----/\/\/\---*---------------O Pic pin
                                        |

The code simply counts pulses as long as the signal goe LOW. If it gets
over 25 pulses in a second (we have 50Hz here) then it assumes that
something is touching the sensor. The sensor can be painted or under a
paper sticker, however it MUST have low C vs everything else. R1 is 1 Meg
and R2 is as high as you can get (I used a 22 Meg one I think).

The circuit is sensitive to EVERYTHING, including neighboring pins, which
must be set as outputs, and driven LOW. Use a guard ring. Also reacts to
walkie-talkies and cellular phones. Probably also to cats that brush
against one's trouser leg and thunderstorms ANYWHERE.

The circuit is unreliable for production systems. The one where I checked
it out had a 32768 kHz crystal and almost nothing else on it (timer app).
The sensor was made by gluing small pieces of thin copper to a glass board
(bare). Wires were very short (5 cm) and power was 9Volts + 78L05. The
guard ring was a piece of wire-wrap wire that surrounded the outline of
the sensor. The shape does not seem to matter but it won't work at all
outdoors in a quiet spot.

There is a version of this circuit that uses the oscillator output to
drive a capacitive bridge, with the user's finger being the short to
ground over it. It involves etching a certain 3-wire pattern into a
circuit board, that relies on the copper thickness / pattern area ratio to
achieve its effect. This one is reliable but forget about passing any FCC
or CE EMI/RFI test with it. It can be used only for very limited things.
It can be used for less limited things if it is shielded by a conductive
membrane that contains elastic 'cups' to keep the membrane away from the
pattern when not pushed. The cups used in remote controls and membrane
keyboards are just perfect for this. The solution is water-tight and I
suppose that it should last about as long as the polymers in the membrane.

Peter

1998\07\15@154619 by andre

flavicon
face
John:
I haven't try it with PIC but this is
what I would do first.
16c71 or any 7X family pic use analog input.
you need at list 1 resistor like 100 k or 1mohm  to ground.


do this

1. make input analog
2. start conversion
3. check for end of conversion
4. load result in w
5. xorlw b'00000001'
6. check z flag
7. if set then party time

Andre Abelian

John Haggins wrote:

> Hi,
>
> Anyone got code to make a PIC input touch sensitive with no external
> components?
>
> i.e. if you connect a wire to an input, the PIC "knows" when you touch the
> wire.
>
> Thanks!

1998\07\15@155600 by Sylvain Bilanger

picon face
I think if you just put the wire on the A/D input of the 16C71 there is
a big enought change of voltage on the wire for the PIC to know that you
touched it... just make a condition in the PIC that says if there is a X
amount of variation something touched it... I dont know if it helps but
i hope so.

Just try it you will see!
Cya! Sylvain


>Hi,
>
>Anyone got code to make a PIC input touch sensitive with no external
>components?
>
>i.e. if you connect a wire to an input, the PIC "knows" when you touch
the
>wire.
>
>Thanks!
>


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Get Your Private, Free Email at http://www.hotmail.com

1998\07\15@161643 by Harold Hallikainen

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On Wed, 15 Jul 1998 22:01:15 +0000 "Peter L. Peres" <.....plpKILLspamspam.....ACTCOM.CO.IL>
writes:
>On Wed, 15 Jul 1998, John Haggins wrote:
>
>> Hi,
>>
>> Anyone got code to make a PIC input touch sensitive with no external
>> components?
>>


       Probably 20 years ago someone made a chip for this.  I probably
still have the sample buried somewhere.  I imagine it could be simulated
with a PIC.
       Ascii art follows...  Change to monospace type now!



      ||-------- square wave from PIC pin
      ||
      |
      ||
      ||-------- PIC input


       The touch plate is on the left and is insulated from the right
plates by a suitable dielectric (perhaps fiberglass circuit board
material, or a glass panel like on a microwave oven).  The circuit is two
capacitors in series, which passes the square wave from the PIC output
back to the PIC input.  When the left plate is touched, the junction of
the two capacitors is grounded, preventing the square wave from getting
back to the PIC input. Your circuit watches for loss of the input square
wave (perhaps by having an edge reset a timer and having it overflow on
loss of signal).
       I'd probably add a pull-up or pull-down resistor on the PIC input
side (maybe even a port b weak pull-up), though that may not be
necessary.
       Anyway, something to play with.

Harold

[back to normal typeface...]

Harold Hallikainen
EraseMEharoldspam_OUTspamTakeThisOuThallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm



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1998\07\15@162300 by Ohtsji, Randie

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face
Watch out for ESD!  Semtech (http://www.semtech.com) makes some pretty good
protection devices (TVS).

Randie
randie.ohtsjispamspam_OUTglenayre.com


> {Original Message removed}

1998\07\15@171409 by John Haggins

picon face
Thanks to everyone for your suggestions!

At 12:54 PM 7/15/98 PDT, you wrote:
{Quote hidden}

1998\07\15@171409 by John Haggins

picon face
Thanks to everyone for your suggestions!

At 12:54 PM 7/15/98 PDT, you wrote:
{Quote hidden}

1998\07\15@175206 by Sean Breheny

face picon face
It would seem to me that this would be very difficult to achieve reliably
with NO external components, but with about 2 or 3 components, it could
be done properly. If you have a pull up resistor of about 2 Meg from
the PIc pin to +5v, then toughing a finger between this input and GND
should pull it low enough to change the state. THe only remaining problem
is that of ESD protection and keeping the circuit from being thrown off
by picking up stray signals. A .01 uF cap from the pin to ground should
help with the stray signals. The input protection diodes might be good
enough for some applications, but you might also add a 6 or 7 volt zener
diode, reverse biased, to ground.At this point, the same software
debouncing routines which work for bushbuttons would probably work on
this, too.

Sean


On Wed, 15 Jul 1998, John Haggins wrote:

{Quote hidden}

1998\07\15@181038 by Sean Breheny

face picon face
In my previous reply, I forgot to mention one thing: It would also be a
good idea to include two 100k resistors, in series with the grounded
touch terminal, and in series with the touch terminal going to the pic
pin. I have not tested the idea, but with these resistors, the pull up
resistor, the zener, and the cap, I think that this would be very robust,
and still cheaper than a pushbutton.

Sean


On Wed, 15 Jul 1998, John Haggins wrote:

{Quote hidden}

1998\07\17@180846 by Walter Banks

picon face
The following  the touch sensitive switch is

                                      -----------
                500K              |
        ----- \/\/\/\/------------|   Pa
       |                             |
       |                             |
       |                             |
        ------------------------|  Pb
       |                             |
    ------                          |
    ------
       |
   Finger separated by an insulator


A port Pa  puts out a step from 0 to Vcc through a 0.5M
resistor (470K or 560K) 5 microseconds later the port art Pb
samples the value on the touch plate (Which is insulated
from the finger).  A sampled 1 indicates nothing touching the
plate. A sampled 0 indicates that the capacitor has not charged
up to the switching threshold of Pb.

The way it works is the number of free electrons in the finger
acts like a virtual ground and the RC time constant of 0.5 M
and the C establishes the 5 microsecond sample time.

I have used these in a variety of ways usually etching the plate
directly on a PC board and using a heat shrink plastic as an
insulator

In simple applications where you need only one or two
switches then the following circuit will also work.
        |
        |
       /
       \  500K                    ------
       /                             |
       |                             |
       |                             |
        ------------------------|  Pb
       |                             |
    ------                          |
    ------
       |
   Finger separated by an insulator

This requires a single port bit that outputs a 0 and discharges the
capacitive switch. The port data direction is changed and sampled
5 microseconds later. A 0 indicates the presence of a finger
and a 1 the absence of a finger.

Walter Banks
http://www.bytecraft.com


----------
{Quote hidden}

the
> wire.
>
> Thanks!

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