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How to Build Pressure Pads on a Budget
Two of my CHI2001 projects use a pressure pad
setup to communicate with the computer. This was all designed and
built on a shoe string. The computer was running software that I
had wriiten in Director and the pad presses were simply mapped to
key presses. This made the development of the software very easy.
Also I was happier using an approach based on system level events
rather that one that had to poll for IO. The following designs were
fine for a three day show but I would use more expensive alternatives
for permanent exhibitions. However I was pleased to find that over
three days of pounding by 3-6 year olds there were no failures.
The Pad Itself
The pad design is very simple. It consists of 2 squares of masonite
with separators on the corners (a foam and masonite combination).
Each square has a steel wire running across the center.
The squares are then put together so the
wires run at right angles on the inside. When the pad is stepped
on the two wires touch and if the ends are in an electrical circuit,
a connection is made. Thus the pad is just a simple switch - it's
a circuit or not a circuit. It is also very easy to assemble and
disassenble - I finished mine with printed, colored paper and a
plastic wrapping.
The pads were then mounted on larger boards in groups of four -
I needed 26 pads. This greatly simplified the wiring and helped
in transporting them. They were designed to be extremely simple
so that they could be swapped out with new one were there to be
any breakages. Ideally I would have liked to have used a piece of
carpet or flooring for the whole setup so no wires would be visible.
Communicating with the Computer
The cheapest solution to the IO was to hack a second hand computer
keyboard. It is essential to find an older model as the newer ones
use a very different technology which, although it is possible, is
much harder to hack. I made setups using a Mac and a PC keyboard.
As keyboard connecting technologies have changed on both platforms
I needed adapters to connect them to the machines (ADB to USB and
PS/2 to whatever to the PC uses). I must say the Mac adapter was surprisingly
expensive).
I chose to use ethernet cables to run the connections from the pads
to the PCB inside the keyboard. This was because ethernet cables consist
of eight very small wires that are in 4 color coded pairs. This meant
that each pad group needed just one cable running to it. The color
coded wires meant that it was much easier to get the wiring right.
On the older types of keyboard it is very easy to see where the two
points are that are connected when a key is pressed. Each pad was
wired up to a different letter key position so that every time someone
walked on a pad it was equivalent to pressing a particular key on
a keyboard. For my purposes this meant that is was very easy to test
the software - you could use the keyboard instead of having to set
up the pads. Also it would help isolate the source of any problems.
The keyboard end of the wiring was done by snipping a complete ethernet
cable into a 1 foot length and a length long enough to run out to
the pads. The short length was then wired up to the keys with the
end just poking out of a hole drilled in the back of the keyboard.
This was glued in place.
The bare ends of the long end were connected
to the pads (just using insulating tape to make swapping out components
that died easier). The connection was made by using a male to male
ethernet adapter. The color coding inherent in ethernet cables meant
that it was much easier to have an idea of what wire went where. It's
a good idea to use as many different colored ethernet wires as possible,
use the same one to connect to itself (at the male to male adapter)
and be consistent about the order of the internal wires.
Alternative Approaches.
Various other alternatives were examined for this design. The main
problem was that 26 triggers is rather more than most off the shelf
solutions allow.. One interesting technology was circuit boards for
MAME controllers, (they help you take an old arcade game and put a
modern computer inside it but still use the original controls). This
would have cut out the need for the keyboard.
MAME controller links:
http://www.mameworld.net/
http://www.imhz.net/mame/
The I-PAC board looked particularly useful: awls14521@blueyonder.co.uk
or andy@spaceinvaders.uk.com
http://www.andywarne.pwp.blueyonder.co.uk/ipacprog.htm
The gold plated midi route would be the iCube: www.infusionsystems.com
You could also use a trigger-to-MIDI device such as a MIDIKat.
Another approach that I would have liked to have researched properly
have been to use MIDI and hack a musical keyboard and wire the switches
into the keys.
A pad alternative might be to attach a piezo disc to a metal surface, the one
the kids jump on, and use this signal as the trigger.
As this was being done on a budget I took the cheapest
option which worked absolutely fine.
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