Let there be light…

Friday, 30 May, 2008 § Leave a comment

When it came time to choose the leds there were only a limited number of choices, and since the most popular on NUIgroup forums is the OSRAM SHF485, I went with that.

A bit of background when picking IR emitters…

For everyone else out there when picking leds dont look at that their power for a measure of their luminosity. But rather look at their radiant intensity, which is measured in mW/sr. Typically radient intensity of 100+ is sufficient for for FTIR, on the other hand for DI you want both luminosity as well as dispersion angle, Osram SHF485’s have a radiant intensity for 160mW/sr.

On a side note, you also need to keep in mind the lens on top of the led it self and the angle IR emmits from the led it self. For FTIR its an narrower beam is prefered of around ±20° and for DI a wider angle beam is prefered ±40°, so if you are creating a DI setup then Osram SHF485P would be the better choice.

When determining quantity/number of leds, it solely depends on the size of your setup and the amount of force you want to exert of the screen for the blob to be detected. from various test performed over at NUI its determined for an FTIR setup to get decent input detection you need one led per inch on both of the longer edges of your screen. For DI its solely dependent on if you are able to diffuse your IR effectively over or under your screen. Generally DI setups require a lot more emitters.

Since my setup is an FTIR and i really dont plan to use a compliant surface (unless i don’t have to) i went with 1 led per inch, going around all 4 sides. I know this seems to be an over kill but since i plan to use my screen in with a lot of ambient light this seemed to be the best option. By doing this my 24″x32″ = 60cm x 80cm screen required around 112 leds, but since i ordered 150 i’m pretty much going to use all of them.

Below are the steps i performed to make my circuit.

To create my led array shematic i used basic ohms law to calculate the resistors required for my arrays. For novices out there you could use led array wizard to create schematic.

In general key about the SHF485 needed to create your own array is as follows:

Forward Voltage: 1.5V
Forward Current: 100mA

I’ll be using a 12V DC voltage for my source. As such after 8 leds i need one 1ohm resistor.

After creating the schematic i tested one array out on a bread board to see if the array was functioning as it should.

Breadboard Test

Breadboard Test

Knowing that the circuit worked i created my PCB schematic using Express PCB. Below are some designs i created




Using the toner transfer method (on projection transparencies), i transfered these PCB8_Rev.1 layout on a copper clad board and etched the circuit.

Etched PCB

After the circuit was etched; using the dremel rotary tool and 1/32″ drill bit, drilled the pads and soldered the components on.

Soldered Components

More information regarding Toner Transfer Method and Etching can be found here at NUIGroup.

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