FreeTrack Forum

Hello everybody,

first I would like to say a big thank you for all people involved with the development, releasing, documentation of Freetrack. You have done a simply wonderful work.

I just built my first system (4DOF). I spent the night assembling the headset, I had a try, and was really happy to see that everything was working with the electronic part (I have only used a soldering iron once in my life before yesterday!), LEDs were working, and Freetrack was capturing the light from them.

My systems is composed of:

- 4 IR White LEDs (1.7V 20-100mA, the vendor coult not precise the exat current!). I sanded them as recommended in the manuals.
- 1 30Ohms 0.5W resistor
- 1 9V battery in a battery box with on/off switch
- System is connected in series.

I followed the LED documentation from Tristan68 to assemble my systems, and as I understood from there, I went for this 9V serial assembly as that would be the best (did I get it wrong?). But on the Freetrack 1.33 manual, it says parallel connection with four resistors is the best way. So here are the question:

1. What is the best choice (related to power of light)? Serial x Parallel? Is my setup (see above) a good one?

2. When configuring Freetrack in the dark, my Webcam (Creative NX Pro) tried to compensate the low light with longer exposure times (can´t find a way to disable that), and then I got 3FPS only. If I turn on the light (or doint this during the day) I cannot get only the 4 LEDs shown, no matter how low threshold I put, there is always stray light sources included. Is that saying my LEDs are too weak? Should the LED really standout from any stray lights?

3. If the conclusion is that my LEDs are weak, what should I do? Just replace the resistor for a lower resistance one (like 15 Ohms)?

4. In the 1.33 manual there are mentions for red and orange LEDs from conrad.fr. They look like normal LEDs, not IR. Is that right? Shouldn´t we use only IR?

5. Should I bother getting the OSRAM SFH485P? Are they so much better than any other standard IR LED?


Sorry for the long message, that is the entushiasm with all this nice thing.

Thanks a lot for your answers!

1)  9V doesn't last as long with parallel circuits compared with series circuits.
Series is a few cents cheaper than parallel because you only need one resistor. :P

2) Turn off your webcam's automatic exposure and manually turn down its exposure (it may reset to default so be sure to do this in Freetrack _after_ you've started a video stream (use the camera properties button). Use a visible light filter (developed photographic film as described in manual).

3) Using less resistance than the specs will fry your leds. They might burn a bit brighter but their lifespan will go from years to minutes. Don't do it.

4) Yes, they're normal leds, some people have had some success with them but IR leds are still preferred.

5) The SFH485Ps have a super-wide 80 degree viewing angle, normal IR leds usually only have a 20 degree viewing angle which is almost useless for head tracking. SFH485Ps aren't normally sold at consumer electronic stores, you'll need to get them from a more trade-oriented electronics company, preferably a big one that lets you order online. In Australia, RS Components and Farnell stock them.

Good luck!

Kestrel,

thanks for the answers. I just ordered some SFH485P at farnell.com, and will install them in place of the standard LEDs. Some more questions

1. Should I sand the LEDs as mentioned in the manual? Does it really improve the detection? I am just a little affraid of actually damaging the LEDs.

2. After checking some other posts, I suppose that I can keep using the 30Ohms 0.5 W resistance in a serial 9V setup, correct?

Again, thanks a lot for your answers.

1)  Standard leds have a large lens which can be lightly sanded to widen the angle of emission (but this will also reduce brightness), SFH485P leds are wide angle with only a tiny lens on top of the semiconductor junction and _probably_ wouldn't benefit from sanding. I haven't tried this because I don't have a problem with tracking.

2) The 30 Ohm resistor will satisfy 4x SFH485P leds connected in series operating at max brightness. If you want the battery to last longer (not a such a problem if you're using rechargeable) you might want to consider a slightly larger resistance, however this will sacrifice led brightness, so only do it if you don't have a problem with background light.