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'[PIC]: Touch sensor using a PIC?'
2002\07\17@071410 by GB

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Hi,

Has anyone made a successful sensor that can detect a finger either touching a contact point or even better though  glass or plastic?


Gary Baker

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2002\07\17@124402 by Richard Mellina

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I remember reading somewhere about a circuit that that has two metal
contacts so when you touch them it closes a circuit. The circuit detects the
very slight current that flows through you finger. Unfortunately I don't
remember where I read this.

{Original Message removed}

2002\07\17@125910 by Harold M Hallikainen

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       Somewhere buried here I have a 20 or 30 year old sample chip from
someone (AMD seems to come to mind) that is a touch sensor chip. This one
worked by having a metal plate on one side of an insulator (glass,
plastic, fiberglass, etc.), and another pair of plates behind it. The
pair of plates could be thought of as a single plate the same size as the
one on the other side of the insulator, but split in half. Looking at it
from the inside of the box (the side with two plates), we have two
capacitors in series: one small plate to the big plate, then the big
plate to the other small plate. A signal is put on one small plate and
detected on the other small plate. A person touching the large plate
grounds the junction of the two capacitors and the signal detected at the
second small plate gets a lot smaller.
       Doing this with a PIC, I'd try driving one small plate with a square
wave. The other small plate goes to an input with a large pull-up
resistor. The input should follow the output unless someone touches the
large plate.


Harold



On Wed, 17 Jul 2002 11:39:16 -0500 Richard Mellina <spam_OUTrsillemTakeThisOuTspamFLASH.NET>
writes:
> I remember reading somewhere about a circuit that that has two metal
> contacts so when you touch them it closes a circuit. The circuit
> detects the
> very slight current that flows through you finger. Unfortunately I
> don't
> remember where I read this.
>

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2002\07\17@131543 by John Walker

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I'm sure you'll find what your looking for here.

http://www.qprox.com/products/qtouch.php


Good luck!

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2002\07\17@131754 by Eben Olson

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www.bytecraft.com/touch.html
simple and fairly reliable.

> -----Original Message-----
> From: pic microcontroller discussion list
> [.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU]On Behalf Of GB
> Sent: Wednesday, July 17, 2002 6:02 AM
> To: PICLISTspamKILLspamMITVMA.MIT.EDU
> Subject: [PIC]: Touch sensor using a PIC?
>
>
> Hi,
>
> Has anyone made a successful sensor that can detect a finger either
touching
{Quote hidden}

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2002\07\18@135805 by Peter L. Peres

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On Wed, 17 Jul 2002, GB wrote:

>Hi,
>
>Has anyone made a successful sensor that can detect a finger either
>touching a contact point or even better though glass or plastic?

Yes.

Peter

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2002\07\18@144202 by A.J. Tufgar

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Good memory about the LM10 Peter.  :) Thanks for your response.  The
LM10 wasn't sensitive enough when the sensor ran on only 1.5V.  If you
power a sensor with 5V, their typical output is about .1mV-.5mV so going
to 1.1V was just to low.  Oh well it was worth a shot.  :)

Now that I know about Vcm I actually just ran the sensor off a lower
supply then LM324N.  Seems to work ok.

I'd still like to know more about CMRR and Vcm, I've got a bit of
knowledge on them but not alot, any good tutorials out there?  I looked
around the net but I couldn't find much.

Thanks,
Aaron

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2002\07\18@163144 by Peter L. Peres

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On Thu, 18 Jul 2002, A.J. Tufgar wrote:

>Good memory about the LM10 Peter.  :) Thanks for your response.  The
>LM10 wasn't sensitive enough when the sensor ran on only 1.5V.  If you
>power a sensor with 5V, their typical output is about .1mV-.5mV so going
>to 1.1V was just to low.  Oh well it was worth a shot.  :)
>
>Now that I know about Vcm I actually just ran the sensor off a lower
>supply then LM324N.  Seems to work ok.
>
>I'd still like to know more about CMRR and Vcm, I've got a bit of
>knowledge on them but not alot, any good tutorials out there?  I looked
>around the net but I couldn't find much.

Short version:

Vcm (?) = common mode range. The voltage applied on either input of an
opamp must be within this range at all times. If/when it is outside the
behaviour of the amplifier is undefined. It could oscillate, drive output
hard high (LM324), influence neighboring amplifiers in the same package in
strange ways, etc. A common design mistake involves using some combination
of coupling/decoupling capacitors which briefly place the inputs outside
Vcm at start-up time. Since there is no guarantee that once back inside
the Vcm the amplifier will function right again (esp. true if using
feedback) this is a recipe for disaster. Rail to rail input amplifiers
allow the input voltage to reach either power rail without trouble. I
think that this can only be done with MOSFET input amplifiers.

cmmr = common mode rejection (ratio). The amount of output vs. input
applied to a unity gain differential amplifier, when the input signal is
applied in phase to both inputs. The ideal is infinite (zero output).
Usually given in dB.

There are also other issues wrt. slew rate, propagation time and large
differential signal inputs.

Peter

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2002\07\18@193423 by Jinx

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> Has anyone made a successful sensor that can detect a
> finger either touching a contact point or even better though
> glass or plastic?

I've made many of these

http://home.clear.net.nz/pages/joecolquitt/proximity.html

It's basically a capacitive divider and comparator

The R and C on the output will depend on your sensor size
and the amount of eg mains hum around. 10k and 100n is
usually enough. They should be there as some oscillator
frequency will be super-imposed on the op amp's output. It's
not a lot but it could confuse following electronics. The output
DC voltage increases as the hand is brought near the sensor
so for micro applications you could add a Schmitt trigger to
square it up. Any inverter gates can be used for the oscillator.
For single sensors you could use the 4th gate on the output

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2002\07\18@223114 by Russell McMahon

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**** OFFLIST ****

> >Has anyone made a successful sensor that can detect a finger either
> >touching a contact point or even better though glass or plastic?

> Yes.
>
> Peter

Careful, if the wind changes you may turn into Olin :-)


       Russell

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2002\07\18@224558 by Patrick J

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> > >Has anyone made a successful sensor that can detect a finger either
> > >touching a contact point or even better though glass or plastic?
>
> > Yes.
> > Peter
>
> Careful, if the wind changes you may turn into Olin :-)
>         Russell

Here we scare small children with tales of the big beast in the woods,
on the piclist we have something even better...is that what ure saying? ;-)

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