I was wondering if there was a limit to the ammount of LEDs you can use. I plan on making a display no bigger than 36 by 27 inches, could I use 80-100 of the OSRAM SFH485, or would that make the blobs way to bright?

How many LEDs do you guys use in your panel?

Thanks.

i use 10 on each side, 40 total for my 1m x 0.75m panel. although i’m going to add more because my led’s are not high power.

1m by 0.75m is about 39.4 by 29.5 inches. I bet that is a really nice display. I will probably be ordering 100 or more SFH485s because of the price break at 100 (38 cents each). Considering that most low power IR LEDs seem to put out about 20-40mW of light, 40 LEDs is not bad for such a large display.

Thanks for the input!

18x24 inch display, im using 10 leds (SFH485P flat-head version) down one side.  I will probably add another 10 down the opposite side though as the blobs get a bit weak in poor light conditions.

I think most poeple are using too many leds

Wow, only 10? That’s a lot less than I would have thought. So I am correct in thinking that having too many LEDs makes the blobs brighter and less distinguishable, no? You also mentioned the flat head LED, do you think that they work better than the normal round head?

Thanks for the input, Joobs.

if i could get my hands on high power led’s i would have needed far less. i thought that more led’s would require less pressure?

EDIT: i’ve contacted osram south africa to find out if i can get SFH485 led’s over here.

Yes, I imagine that you wouldn’t have to press down as hard, but it should also make brighter blobs.

As for the SFH485 LEDs, if you aren’t able to get ahold of any, I could send you some when I order them, although that won’t be until June or July.

My SFH485’s arrived, and I’ve got three wired up so far, driven at about 40 mA.  They are the regular LED type, not flat-headed, and are just held up against the edge of the acrylic.  With the lights off, they provide enough illumination to pick up touches on my entire 24x18 surface (well, camera can only see 16x12 or so, but it doesn’t matter which 16x12 section I look at).

If I turn the overhead light on, my camera is nearly pegged white, so I’m not sure how I’m going to solve that one… I’m still planning on wiring them up every 2 inches, and I’ll adjust the current if they’re too bright.

So, I’ll second Joob’s comment that you really don’t need a bunch of LEDs, at least I haven’t yet.  If I can get the camera to work in higher brightness situations, I may need more LEDs to overpower the ambient illumination.

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As an aside, I’m a little concerned about running these above 20 mA, even though the graph in the datasheet seems to indicate that it’s OK to run with duty=1.0 at 100 mA, as all the other figures for I = 100mA specify a pulse duration of 20 ms as well.  They’re getting slightly warm, but they’ve been on for an hour with no noticable loss of brightness yet.

Going to have to contradict myself.  A few leds works fine when there is no ambient IR light in the room, but unless you have some really nice camera, you’re going to need many more LEDs to get any sort of discernable blobbing in regular lighting.

I’m trying to get my surface to work well with normal room lighting, and I had to add a considerable number of LEDs to overcome ambient illumination.  I don’t have any compliant surface on top of it, cutting out some of the light, so that may help reduce the need for extra LEDs.

I’m using 6 strings of 6 right now, 4 on one side, and two on the middle of the other side, with the plan of adding another 2x6 on the second side when I get around to it.  I’ve also got some duct tape (metal foil duct tape, not gummy cloth duck tape) on the two shorter sides to keep reflected light in a bit more.  This works fantastically in low to moderate light (think lamps not pointed at the ceiling), but I’m having dynamic range problems with full lighting (halogen or incandescent torch-stand).  Moderate-pressure touches are full-brightness, but the ceiling reflection of the light forms a large hot-spot of full-brightness too, so I can’t detect touches in that area.

Hey joat,

Thanks for the info. 36 LEDs, eh? Well, I think I’ll use 60-80. May be overkill, but I’d like it to work well. I think I’m going to order 100 OSRAMs anyway.

joat did u benchmark the difference of the optical performance after applying the aluminum tape/foil?

Not rigorously.  It’s hard to do comparative measurements at all with my setup, as the camera still does some form of adaptation even after disabling auto-exposure, auto-brightness, and auto-contrast :(

I’d say they helped a little bit, but it could just be a placebo effect since I expected it to help a bit before application.  I wasn’t planning on putting LEDs on the short sides anyways.  The foil is removable as the adhesive isn’t that strong, but on the other hand, it tears sooo easily, so it would still take a bit of time to remove.  I’m tempted to remove it and see if it reduces another problem: with all these LEDs, the screen is pretty bright even without a touch.

The roll of foil was about $7 and is enough for dozens of screens and a few air conditioning ducts too, so if you’ve got a decent way of doing measurements, I’d be interested in your results

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What I’d really like to do to reduce the number of leds and avoid much of the issue with ambient lighting is to get a camera that has external trigger and real exposure control, and then hammer on the LEDs at 1A for T us while the camera is exposing for T us.  Alternatively, with a high-framerate camera that still has an external trigger, take two pictures for every one you want to track, one with the LEDs on, and one with them off.  Tada: nearly perfect background subtraction (fast motion will still screw things up, but the lighting I want to subtract out isn’t often moving fast, or at all).  Or do both.

Now, I just need to find a camera like a pointgrey for less than a grand…

Kyle, deff. order more than you expect to need (I thought I was when I only ordered 50, and I’ve soldered 8 strings of 6, lost one, and now have only one spare...).  I’ll probably be ordering more just to have them around, they’re nice.

I think the number you need depends on your piece of acrylic, method of coupling the LEDs to the screen, ambient light, camera, etc… (not in that order, I’d place ambient light and camera first).  If you plan on putting LEDs on multiple sides, try one side first, and if that does well enough for the rest of your setup, then you’ve saved the work of doing the other sides :)

Thanks for the advice, joat. I will probably wind up ordering 100-150. I am also wanting to light up as many sides as I can; hopefully I can light all 4.

And BTW, WhiteNoiz said the Point Grey Firefly MV was $300. (60fps at 640x480 or 120fps at 320x240)

If I have the money and pick up one I’ll report on how it does.

My understanding is that you have to buy a full kit first, and then subsequent cameras are cheaper ("NOTE: One full KIT must be purchased prior to ordering individual cameras, unless otherwise noted.").  *edit*, removed bit about charging a ton for stuff most people don’t need, as the kit seems to only be $100 or so.  However, for the FireFly MV, you have to buy one plus the kit, and after the first one, you have to buy them in packs of 5!  C’mon, it’s not like they’re Sam’s Club or BJ’s…

There’s got to be some other camera that exposes these controls; the hardware on a pointgrey camera isn’t really anything special, it’s nearly identical to Sony firewire cameras, and you can use Sony drivers to capture video without the SDK.

I’ve got another high-speed camera I’d like to try out, but it has no drivers and my USB fu is weak.  I’ve tried the simple things like finding a driver for the same chipset and changing the VID/PID to match with no luck.  If I get around to it, I might try setting up a spare box with linux and playing around with the drivers there, which gives a lot more flexiblity (or at least I might get *something* besides a code 10, device cannot start).

If you guys want to continue to talk on camera’s, you can create a thread in the camera section.