There's a couple of multitouch system methods, FTIR and Diffuse Illumination. I'll be building the FTIR system.
FTIR (frustrated total internal reflection)
is a very simple principle that basically says that if you light up a
certain material with some light, the light will stay inside the
material itself, boucing on it's external surfaces until the external surfaces is "frustrated",
for example, with a finger touch. In this case the light is bounced
back from the finger and exit the material. So, with FTIR multitouch
display, this material is plexiglas and we use the usual IR-pass
modified camera to track the finger's position. This principle works
also with infrared light so that the tracking light will be not
disturbed by visible light.
I wanted to start off small given that this is only a "dip your toe in the water" type of project. After a bit of research I decided on the following shopping list:
- 500mm x 500mm x 6mm Acrylic Sheet
- 500mm x 500mm x 2mm Acrylic Sheet
- Safety mirror (A3 size)
- A0 Tracing Paper
- Two 2m x 10mm x 10mm C/U channel extruded aluminum (8mm internal dimension)
- InfraRed LEDs
- Resistors
- 350W ATX PSU (I didn't have any old ones lying around)
- Wet & Dry paper (300, 600, 800, & 1000 grade)
- Brasso Metal Polish
- 2m x 50cm strip of cord material
- Webcam
Most of this stuff can be found in local shops or purchased off the internet.
InfraRed LEDs
The first decision was regarding the IR LEDs and the circuit design. Near infrared leds (the type we want) are in the 810nm-1400nm range. IR leds around 840/850nm are used in remote controls. They provide lots of IR light however, because they are close to the red end of the spectrum the naked eye can see them glowing red. I didn't want this so I moved up the scale towards the 940/950nm range which is outside what the eye can detect. The final decision was made purely on a cost basis. Farnell (www.farnell.co.uk) had some wide angle 940nm 5mm IR Emitting LEDs for £0.10 each if you bought 100 or more. I opted for Avago Technologies HSDL-4271. The data sheet can be found on this page. The datasheet is very important when designing the circuit and calculating the resistor values.
There's no hard and fast rules about how many LEDs you'll need so I decided I would try and experiment a little. Some people have leds along one side, two sides, or along all four. I chose the latter to give maximum experimental potential. The maths suggested that if I spaced my LEDs at 2cm intervals I would need 24 (3 groups of 8) for each side making 96 LEDs in total. The experimentation would come later as I proposed to build the tablet such that I could connect & disconnect each bank of 8 LEDs to see what impact it had.
Using this great online LED Resistor Calculator I had my circuit and the required resistors (27R 1/2W). Here's what the wizard suggested:
Solution 2: 8 x 12 array uses 96 LEDs exactly
| +12V |
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- each 27 ohm resistor dissipates 270 mW
- the wizard thinks 1/2W resistors are needed for your application
- together, all resistors dissipate 3240 mW
- together, the diodes dissipate 11520 mW
- total power dissipated by the array is 14760 mW
- the array draws current of 1200 mA from the source.
That's quite a high current draw so the cheapest power supply is a computer AT/ATX PSU. These can be purchased very cheaply now and even a 350W PSU will be more than enough.
Webcam
OpenSolaris hasn't had webcam support for very long but any UVC supported webcam should work. These are the ones known to work with OpenSolaris:
Device Name Vendor ID, Product ID
Logitech Quickcam Pro 5000 046d,08c5
Logitech Quickcam Pro 5000 046d,08ce
Logitech Quickcam Fusion 046d,08ca
Logitech Quickcam Orbit MP 046d,08cc
Logitech Quickcam Pro for Notebooks 046d,08cb
Logitech Quickcam Pro for Notebooks 046d,0991
Logitech Quickcam Ultra Vision 046d,08c9
Logitech Quickcam Deluxe for notebooks 046d,09c1
Logitech Quickcam Communicate Deluxe 046d,0992
Logitech Quickcam Pro 9000 046d,0990
I managed to find the 'Logitech Quickcam Pro 5000' on eBay for £25 so I ordered it as it looked to be one of the easier models to modify and convert it to an InfraRed camera. Normally cameras have an Infrared filter built in such that they only detect visible light. We want the opposite, we need to block visible light and allow IR light in. I'll cover how to accomplish this in a later blog but essentially I'll be taking the camera apart to remove the IR filter and replacing it with some 35mm film which is an excellent and cheap filter for my needs.
