I’m making a multi touch table using the FTIR technique. The LEDs I have readily available are VSMS3700-GS08.
My acrylic sheet is 30" x 24". If I space the LEDs at 3/4" apart, I’ll need 144. I don’t know how to wire them together. Should I do it in one long string? 4 strings (2 x 40 and 2 x 32)? What voltage/amperage should I use to power it? Do I need resistors?
Can anyone help fight my electronics ignorance?
At 100mA, the forward voltage on these LEDs is 1.7 volts. You could power all 144 of them in one long string, but that would require a 1.7v x 144 = 245v power supply. Also, if one LED died, the whole string would go out. You are better off running something like 12 strings of 12, at 24 volts, with a 36 Ω resistor on each string:
V = IR
V = 24 - (1.7 x 12) = 3.6v
I = 100mA (from data sheet)
R = V/I = 3.6/.1 = 36Ω
You will need a power supply capable of supplying 24v at 12 x .1A = 1.2A, so make it 2A.
Thanks for the fast reply.
Because this will be used in conjunction with a computer, the most convenient power supply at my disposal is the computer itself. I know the voltages are either 5V or 12V but I don’t know the amps? (I’ll search, but if anyone knows…)
If I use the 12V does that mean 24 strings of 6? If I follow the math…
V = (12 - 1.7 x 6) = 1.8v
R = 1.8 / .1 = 18 Ω - so I need one of these resistors on each string?
And with sets of six the amps must be at least 0.6A (how high is too high?)
How physically do I wire this up? I need the LEDs to go around the perimeter of the acrylic.
If it’s one string it’s easy, of course:
(-) -LED+ -LED+ … -LED+ (+) in one big circle.
If I have 24 strings, does that mean 24 wires coming off of the (-), staggering around the acrylic to connect to each of the sets of six, followed by 24 wires all connecting to the (+) with the resister inline? Is there an easier way to daisy chain that many sets of six?
Does it matter if the resistor is before or after the LEDs?
OK, your resistor math is correct - 18Ω for 6 LEDs in series.
However, each string takes .1A, so 24 will take 2.4A from the 12v supply.
That’s probably OK - check the side of the power supply, and give yourself plenty of margin to account for the 12v power your PC consumes.
The resistor can go on either side of the LED string.
Wiring:
Think of it as a ladder - one side is 12v, the other is ground. The rungs are the individual strings of 6 LEDs + resistor.
Remember that LEDs are polarity-sensitive - they need to be wired (+)-Resistor-AK-AK-AK-AK-AK-AK(-).
You will need a way to visualize IR - a video camera will work (or course - you will be using it in your table).
You really should consult with someone knowledgeable as there are many more issues than first meet the eye.
To begin with 100 mA is WAY too much. It is the absolute max. In other words, you will probably damage the LEDs. And there are other issues like pairing LEDs, heat dissipation, etc.
Did you even LOOK at the data sheet for these LEDs?
Okay, I will be using a conductive (aluminum) frame. So if I connect this frame to (-), then run a wire around the perimeter from (+), then string together six LEDs plus the resistor, I can connect one end to the frame and the other end to a bare spot in the wire from (+)… will that work?
That’s why I’m here 
Heat dissipation I understand (although I’d not considered what to do about it - thanks for the thought-nudge). What do you mean by “pairing LEDs”?
Yep. The one I am looking at says :
Am I missing something?
Sure seems like it, when you state that 100mA is WAY too much current.
100mA is the rated current - one could certainly reduce it to reduce the output and power dissipation, but there is nothing wrong with running these devices at 100mA indefinitely.
Now, these would not be my first choice, because they are surface mount. I would go with something like this.
This is a good point. I’m using the LEDs I mentioned only because I happen to have a mess of them on a reel. A popular choice for others building this type of thing is this one. I may end up buying these or something like it - esp. given how many of the ones I’ve got I’ll have to solder. But if I can make these work it’ll be far cheaper for me.
Note that at rated current (100 mA), you may need to epoxy these to a heatspreader with thermally conductive epoxy to avoid overheating them. Any metal-filled epoxy will do. You may also find that 100mA is too bright - I have no idea how much IR you need, but IR LEDs are very efficient, and 100mA may be overkill.
I have been looking at doing something like this for a while.
I have a couple of questions hope its okay to ask them here.
What do you think is the total cost of everything involved?
And how difficult would you consider it?
The data sheet says forward voltage is 1.3V typical, 1.7V max. If you design for 1.7V and it turns out to be 1.3V, you end up with 8.4V across the resistor, which would pass 233 mA of current. That would burn out the LEDs for sure. Probably better to design around the typical value, and accept somewhat lower brightness if forward voltage turns out to be closer to 1.7V. And with a crude circuit like this, it’s probably best to add some safety margin (e.g. design it for 70 mA current).
If you need the LEDs to operate at maximum brightness, you need a constant-current source that outputs 100 mA exactly regardless of varying LED voltage or supply voltage. Adjustable voltage regulators can easily be wired to work as a constant-current source, or you can just use a transistor.
Agreed.
I am surprised by the LED power requirements re amperage. I’m just a laymen re this stuff, but I thought LEDs drew almost nothing in the way of power, and it seems to drive just 10 of them would take a full amp or so of power?
As far as costs I’ve been lucky. My wife bought me a new projector for our home theatre, so I have a spare just collecting dust. This would otherwise be the most expensive part of the project. (more on this in a bit)
Further, a buddy of mine had this reel of LEDs he gave me when I told him I was thinking about doing this (otherwise I’d probably spend $40-$100 for LEDs, or maybe spring $200 for LED ribbon to avoid soldering).
A friend of my wife works at a company that carries acrylic. Through this connection I was able to get the acrylic I mentioned for <$50 where other shops quoted me $115 + tax.
Because I’m going FTIR, and because I’m only doing this to play with the software, the actual enclosure is no big constraint. I plan on fashioning a minimalist frame to hold the acrylic using materials I already have on hand. I’ll place the old projector halfway down the hallway onto the acrylic held at an angle.
For an IR camera, I followed some online tips to convert a PSP Eye, which I bought for $30. It was really easy to do (open it up, remove the IR filter, replace it with a piece of old floppy disc material). I installed PSP Eye drivers for windows, tested it at 100fps and it worked like a charm - the image was completely black except for my TV remote control’s flashing when I pressed the buttons.
I did have trouble finding the right sized diffusion material - something to put on the acrylic to stop the projector light from simply passing through. I ended up buying 36x22" vellum from a local art supply company (two inches smaller than the height of the acrylic - but it’ll do for now).
For FTIR to work, you need what’s called a compliant surface - the theory behind this is facinating dealing with refractive indicies, evanescent waves, and quantum tunnelling - but I digress. More importantly, others have found techinques that work and I plan to follow “Tinkerman’s method” I bought a tube of GE Silicone I for a few dollars, and after some searching found Xylol from a local building supply company for $23 a gallon (the smallest qty they’d sell me)
(note the prices are Canadian)
On projectors: My buddy’s interested in this too: He wants to build it into a coffee table. Because of this, he’ll need a short throw projector - likely to cost $700-$1200.
Further: I’m using FTIR mainly because it’s the most flexible way for me to get to playing with the software. Assuming all goes well, I’ll likely convert to DI - which means building an enclosed container and upgrading my projector to a short throw too.
I hope to start construction this weekend. I’ll post results once I’m all done (successfully or otherwise 
)
Well, they’re not supernatural!
They may be 5x as efficient as an incandescent lamp, but that still means that it takes around 10W to equal a 60W incandescent lamp.
Also Amp != Power.
Amperes are units of current, Power is current x voltage.
(BTW one can buy 50W (!) LED arrays - around the size of a postage stamp, and very, very, very bright - picture a 250W light bulb that size).
It is not the rated current. It is the Absolute max. as is clearly stated. The device is not intended to be driven at this current. You seem very sure of your interpretation of the data sheet. I think you are mistaken.
It’s only 1.3 V. 100 mA across 1.3 V is only 0.13 W. A single rechargeable AA battery can run one of these LEDs for 20 hours. Ten of these LEDs would consume 1.3 watt of power.
But some LEDs are quite powerful, and consume lots of power. Some modern white LEDs are rated for 1400 mA or higher, and consume over 4 W. My bicycle headlight uses one of these and it’s so bright that I normally don’t run it at full brightness.
Correction: “PSPEye” should be “PS3 Eye”