Re: Touch switch

On Wed, 31 Jan 2007 03:01:57 +1300, Nicholas Sherlock
<N.sherlock@xxxxxxxxx> wrote:

Hey all,

I need some advice on a basic touch-switch.

I'm hoping to make a "Dance mat" for a computer game I play: This is
four 20x20cm touch sensitive pads arranged in a cross formation,
attached to a flat board or foam mat and laid on the ground. You play
the game by putting your feet on the right pads at the right time. I'm
disappointed by the life and reliability of commercial soft
(fabric-like) pads, and cannot obtain a commercial hard pad, so I want
to make my own.

The capacitive touch sensors I've seen on the 'net look unsuitable, as
the electronics are too complex (I'm just a beginner! :)), and I'm
worried that they may be more like proximity switches than contact
switches (In the game, it is just as important to be not standing on a
pad at the right time as standing on it, so if it detects a hovering
foot, it would be a disaster).

The design at:

http://www.imagineeringezine.com/PDF-FILES/fingersw.pdf

is attractive as it is so bloody simple :). However, I have some
questions. My pads' sensing areas will be prototyping circuit boards,
with every second row forming one contact and every other row forming
the other. I want my touch pads to replace the buttons on a butchered
USB hand-held joystick - I suppose that the joystick will expect a
circuit to be completed between two contacts when the button is
depressed, with low (practically no) resistance. So is it reasonable to
arrange my circuit like this:

http://www.sherlocksoftware.org/pad.png

How do I choose my MOSFET? Can I adjust the sensitivity of my touch pad?

Cheers,
Nicholas Sherlock

Everything is a compromise . . .

Both of the contact sensors you show are simple and should require an
actual contact unless the environment is very noisy (electrically).
You have to have conductive shoes or bare feet. But you'd have to
build it to find out. Perhaps static electricity will trigger them.
Eventually the things would stay on because of dirt ground into the
sensor grids or a spilled drink.

You could lower the sensitivity by decreasing the value of the
resistor from the gate to the source.

Capacitive sensors are nice. If you plan on jumping up and down on
these things it might be more comfortable to include a resilient pad -
and that's where I think the contact sensors would work against you.

If you want a resilient long lasting surface something like closed
cell polyethylene foam would be ideal - and it has the additional
advantage of being hard to compress. Capacitance increases as the
plates get closer together - use a foam pad and it should be possible
to make it only trigger when the foam is compressed.

If it has to last a long time and be relatively bullet proof a
capacitive switch with a foam dielectric pad and perhaps a harder thin
plastic like vinyl on top of that for water proofing and wear
resistance, would be the best idea.

If you are just interested in trying something out and it isn't for a
commercial application the touch sensor is simple cheap and easy.

Them's the choices . . .

Or a photo electric light curtain close to the surface.

Or retro reflective proximity switches.

Or how about just include a photo sensor in the mat - your foot blocks
all the ambient light and the thing triggers? Relatively simple to
implement for a beginner. The sensitivity could be high so that you'd
really have to occlude all ambient light to get it to trigger. It
could be used with hard or resilient surfaces. Easy to incorporate
sensitivity. Down side might be dirt changing the sensitivity over
time.

Or an air table type surface with a pressure switch. You block the
flow from a small air pump (aquarium type pump?) and the pressure
rises and switches a switch.

Mull it over for a time; there's probably lots of ways to do what you
want.
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