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'[EE]: touch switch?'
2001\10\11@151250 by Alan Shinn

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Does anyone have any good clues as to how to impliment touch switches? I
rigged something with the output of a 555 on one side of a grid and a
CMOS flipflop on the other grid, worked OK but it still worked when I
turned off the 555  (in some locations but not others) - it was just
reacting to the 60Hz stuff. By the way, I am trying to make it work
through a layer of plastic (scotch tape) for some immunity from ESD destruction.
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Alan Shinn


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2001\10\11@153011 by Chris Carr

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www.qprox.com

Regards
Chris Carr

----- Original Message -----
From: "Alan Shinn" <spam_OUTalshinnTakeThisOuTspamMINDSPRING.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, October 11, 2001 8:17 PM
Subject: [EE]: touch switch?


> Does anyone have any good clues as to how to impliment touch switches? I
> rigged something with the output of a 555 on one side of a grid and a
> CMOS flipflop on the other grid, worked OK but it still worked when I
> turned off the 555  (in some locations but not others) - it was just
> reacting to the 60Hz stuff. By the way, I am trying to make it work
> through a layer of plastic (scotch tape) for some immunity from ESD
destruction.
{Quote hidden}

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2001\10\11@155535 by Brian Aase

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In the past I had pretty good success by building a sinewave
oscillator, and bringing one of its "sensitive" nodes out to the
touchplate.  A nearby finger will de-tune the oscillator, or even
make it stop completely, which worked okay for me.
BTW the reason I used a sinewave oscillator was to keep EMI
emissions low.

Brian Aase

> Does anyone have any good clues as to how to impliment touch switches? I
> rigged something with the output of a 555 on one side of a grid and a
> CMOS flipflop on the other grid, worked OK but it still worked when I
> turned off the 555  (in some locations but not others) - it was just
> reacting to the 60Hz stuff. By the way, I am trying to make it work
> through a layer of plastic (scotch tape) for some immunity from ESD destruction.
> --
> Looking forward:
> Alan Shinn

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2001\10\11@155940 by Dwayne Reid

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At 12:17 PM 10/11/01 -0700, Alan Shinn wrote:
>Does anyone have any good clues as to how to impliment touch switches? I
>rigged something with the output of a 555 on one side of a grid and a
>CMOS flipflop on the other grid, worked OK but it still worked when I
>turned off the 555  (in some locations but not others) - it was just
>reacting to the 60Hz stuff. By the way, I am trying to make it work
>through a layer of plastic (scotch tape) for some immunity from ESD
>destruction.
>--
>Looking forward:
>Alan Shinn

I have one that I use for museum and interpretive centres - it seems
bulletproof (so far).

Its based on free-running 555 oscillators with pins 2&6 tied together and
fed with a 470K resistor from pin 3.  Each sense pad is two 0.75" square
pads on the PCB, one on each side.  There are no vias between the 2 pads.

The frequency is measured and averaged over a fairly long period.  Touch is
detected when the instantaneous frequency changes dramatically from the
averaged period.

A single 12c508 handles 3 touch pads and 6 programming jumpers easily.

The following notes are taken from the source:
;* Touch switch based upon frequency change in a 555 oscillator caused by
;* human body intrinsic capacitance when touch sensor is contacted.  Touch
;* pad is 0.75" square on both sides of a 0.062" FR4 circuit board - acts
;* as a low value coupling capacitor and provides ESD protection.  Top side
;* ground plane has 0.05" clearance from touch pad and acts as spark gap.
;* Bottom side of touch pad capacitor is isolated from 555 pins via 10k series
;* resistors and the feedback resistor.
;*
;* Touch pad capacitor is about 2 pF, stray capacitance estimated at 2 pF,
;* human body intrinsic capacitance is 200 to 400 pF (depending upon total
;* area).  Because the human body intrinsic capacitance is so much larger than
;* the touch sensor coupling capacitance, the net effect of touching the sensor
;* is to increase the oscillator timing capacitor by the value of the touch
;* sensor capacitance.  This results in a dramatic change in oscillator
;* frequency.  Touch is sensed by measuring the change in frequency.
;*
;* Shortest possible 2 instruction delay loop takes 767 cycles to execute
;* and is used as the gate timer to allow TMR0 to measure frequency.  555 osc
;* feedback resistor is picked so that TMR0 does not wrap even with 30%
;* component variation.  Resistor value of 470k chosen so that free running
;* period is about 6 uS, increasing to 12 uS when pad is touched.  Counter
;* wraps if period drops below 3 uS with gate time of 770 uS - good headroom.
;*
;* Switch TMR0 to ext input and clr TMR0.  Enable one oscillator for 770 uS.
;* Read TMR0, average with old reading (256 samples).  Repeat for remaining
;* oscillators.
;*
;* Each Osc section accommodates reading 2 programmable jumpers (midgies).
;* 1st jumper pulls a 10k resistor feeding the osc enable pin either HI or LO
;* and is read by turning that pin on the PIC into an input.  2nd jumper feeds
;* the osc enable to pin GP3 via a diode or resistor and is read by enabling
;* that oscillator.
;*

Hope this helps!

dwayne



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

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2001\10\11@161106 by Thomas McGahee

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Touch switches come in several different forms.

OPTICAL
These consist of an IR transmit and receive section. When
a finger is placed over the pad area the IR reflects and
the receiver picks up the reflected IR. This is useful
where only a few keys are needed, but expensive for a large
number of keys.

Good points: immune to ESD and electrical noise. Plastic
pad covering may be colored as long as it passes IR. Reacts
to fingers, and also to anything else that will reflect IR.
The IR can be a steady beam or one that is turned on and
off at a fixed frequency. The receiver can be tuned to the
transmitter frequency so it is immune to surrounding light
but sensitive to the tuned IR. Has a wide operating range.

Bad points: for large arrays it becomes expensive.

CAPACITIVE
These normally consist of a set of interlocking conductor
patterns with a plastic or glass covering. One set of
interlocking fingers connects to a square wave source.
The other set of interlocking fingers goes to the receiver
circuitry. In its simplest form the receiver consists of a
biased CMOS gate. A resistor of 1 megohm or more is connected
to 1/2 Vdd and from there to the input of the CMOS gate.
When a finger is applied to the plastic or glass covering
the pad area, that changes the coupling between the inter-
locking fingers. Sometimes the biasing is adjusted for
normally on, and sometimes for normally off. Either way, when
a finger is applied, the output of the gate should become
a square wave. A single square wave source can drive an entire
panel of switches.

Good points: fairly inexpensive.

Bad points: may initially require some twiddling to get the
circuit running solid on all switches.

Fr. Tom McGahee



{Original Message removed}

2001\10\11@173648 by Eben Olson

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www.bytecraft.com/touch.html
this site has a nice simple method for making a capacitive touch sensor with
only one resistor and a PIC input pin. I've tried it and it worked pretty
well.

{Original Message removed}

2001\10\11@180217 by Mike Hardwick

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>Does anyone have any good clues as to how to impliment touch switches?

Alan,

Drive a sensor pad with a square wave through a series resistor. Connect
the sensor to one input of a CMOS XOR gate. Connect the other XOR input to
the same square wave source through an equal resistance. The XOR output is
a series of narrow spikes that get wider due to capacitive loading when you
touch (or come near) the sensor. Adjust frequency and R values for best
results. If the sensor is remote, add balancing capacitance at the second
XOR input to minimize pulse width when the sensor is unloaded.

You could integrate this signal with a simple R/C network and, if
necessary, use a comparator to insure perfect logic output. Or, you could
measure pulse width with a PIC timer. The XOR gate could even be
eliminated, at lower operating frequencies, if you're willing to use
another PIC I/O pin and do that function in software.

RF emissions are minimized because the sensor is fed through enough
resistance that most high-frequency harmonics from the square wave source
are removed by stray capacitance.

You could do something similar using the PIC for a signal source, much like
the canonical R/C measuring technique.

Unfortunately, with any of these methods, the sensor can still bring
destructive ESD jolts into your system. It's difficult to balance
sensitivity against multi-KV insulation. You may be compelled to add
protection circuitry...

Mike Hardwick, for Decade Engineering -- <http://www.decadenet.com>
Manufacturer of the famous BOB-II Serial Video Text Display Module!

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2001\10\11@214701 by Robert Shanks

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There is an old book by Don Lancaster, The CMOS Cookbook that had some
grea, easy touch switches.  I have used that design many times in the past.
I think it used a 4049 and a NOR gate.  -rs

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