Re: DIY Touch Screen

[Keith Doxey <keith.doxey@xxxxxxx>]

Hi Steve,

For years I have been meaning to do the same thing, by years I mean circa
1985 when I designed a control panel screen on my BBC B with 64
"buttons"
on screen.

You have seen someting similar to what you describe......exactly the same
infact !!!
It was what inspired my non completed BBC project.

The thing you more than likely saw was BT's "City Business
System" (CBS)
which was used in the dealing houses (stocks and shares....not drugs) in
the city of London. I believe the monitors for thiose were made by
Microvitec in Yorkshire but I could be wrong.

As I understand it they used infra red beams in exactly the way you
describe.
My thoughts on it are this....
Multiplex through the X and Y IR emitters so that only 1 of 32 (16x16
display) is active at anyone time. Dont worry about other receivers picking
up the signal (probably only the adjacent ones will pickup anyway) because
you multiplex them as well. ie you only look at the receiver for the
emitter you are driving. Beam getting through = no finger on screen, no
beam -someones sticky finger is plonked on the face of the monitor.
register that information and look for a broken beam on the other axis to
find the finger.

CONSTRUCTION

I suggest a frame made of some form of black perspex, preferably matt
black, approx 12x25mm. drill holes for 3mm IR leds at accurately measured
intervals and mount the LEDs at the outside edge of the frame so the light
is firing down a barrel to minimise beam spread....

LED at inside edge....

/
/
/
O      wide beam of light  :-((
\
\
\

LED at outside edge firing through a barrel
-----____________
O    ____________ narrow beam of light....hopefully :-))
-----

It could be made quite neat if the perspex had a groove cut around the
outside edge with a router to use as a wiring channel and then filled with
another small piece of perspex. Paint the bezel of the monitor matt black
as well to minimise reflections and keep the cureent as low as possible for
the IR LED. If the screen is 14" across you want a beam that will
reliable
travel 15", not 20 feet !!!!

As I said, this can be done, BT's City Business System was marketed around
1985 and was pretty successful although capacitive/resistive touch screens
have taken over these days. Thats fine if you want to resolve exactly which
pixel (or pixels) of your 1280x1024 display the finger is on but too
expensive for what I want/need. As long as I can tell which 256th of the
screen is being touched thats fine by me.

In fact 8x8 would probably be enough for my application and I think the
capability of 64 buttons on a screen at once would be more than enough.
CRESTRON and AMX touch panels are really user friendly IF programmed
correctly because there usually no more than 16 buttons on screen at any
one time. As you select an option more buttons pop up on screen to suit the
purpose. Thats what City Business System did as well.

When I went on my CRESTRON course last year, they told me not to put
transport controls on the screen if you havent selected the source, once
you press CD then you get most of the transport buttons, if you press VCR
you get a record button as well.

Keith

Keith Doxey
http://www.btinternet.com/~krazy.keith
Krazy Keith's World of DIY Home Automation

-----Original Message-----
From:	Steve Morgan [SMTP:steve@xxxxxxx]
Sent:	24 July 1999 23:07
To:	UKHA_D (E-mail)
Subject:	[ukha_d] DIY Touch Screen

Folks,

Continuing the monitor theme, has anyone got any thoughts on
constructing a touch screen to retrofit to a standard (CRT or LCD)
monitor.

I'm sure I've seen before units that use an array of infra-red
emitter/receiver pairs but I've got no idea where.

So the question is, what's the scope for coming up with our own?

The idea is this... a bezel that fits on the front of an existing
monitor with a row of IR LED's along X & Y axes with corresponding IR
photodiodes opposite (say, 16x16 giving 256 crosspoints). If you can
minimise the spread from the emitters, I would have thought that a
fairly simple microcontroller-based module could handle the scanning.
The thing I'm not sure about is that issue of minimising the spread. An
IR LED has a spread of about 60 degrees, if I remember rightly. Thus I
would expect to several photodiodes to detect the signal from each of
the emitters. How about simply recessing the emitters? If a different
modulation where used for each emitter/receiver pair, would that help?
What about reflections?

Any ideas, suggestions, other issues?

I quite like the idea of bits of 2x2" timber glued to the front of all
my monitors!!

Steve




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