FreeTrack Forum

Hi folks,

I am planning on making myself a FreeTrack system using 4 AA batteries and 3 SFH485P IR LEDs.

Firstly, can someone please confirm these values are correct for the LED Setup Wizard:

Power supply voltage: 6V
LED:
Forward voltage: 1.5V
Forward current: 100mA

Assuming this is correct, it tells me I need a '15 Ohms 5%' resistor, current 100mA and dissipated power 0.15W.

I have been looking around various UK online stores and have come across this one: [1]. (Which is the same as this one from RadioShack: [2].)

The 15 ohms and 5% are there, but it gives a wattage (which I assume is the dissipated power (?)) of 0.5W. I also see no sign of a current listed anywhere. Does this matter? If so can someone direct me to a more suitable one?

Cheers.

/electronics novice

Hello lorian, and welcome to freetrack  



First of all , you shouldn't power your leds at 100mA, its a max values when they are PCB mounted, and that's usually not the case in freetrack.

So i rather suggest you use 70mA or even 50mA only, thats already far than enough as an operating current.

According to the led wizard , and using you other values :
Powered by a 6v battery pack (4 * 1.5v)
led forward voltage : 1.5V
3points clip or cap setup

You get :

for 50 mA :
1 x res 30ohms value and 0.08W admissible power at least

Since they dont have 30ohm, but only 33ohms at radioshack, you can use this ref , it's still ok.


for 70 mA :
1 x res 22ohms value and 0.1W admissible power at least

use this ref at radioshack.



cya.

Thanks for the reply Tristan, I think I will go with 50mA and get the resistor you suggested.

Cheers.

tristan68 @ 12/01/2008 - 18h36 a dit:

First of all , you shouldn't power your leds at 100mA, its a max values when they are PCB mounted, and that's usually not the case in freetrack.

Can you explain why a PCB mounting allows for a higher LED max current than a normal wire connection?

So i rather suggest you use 70mA or even 50mA only, thats already far than enough as an operating current.

I have to use 100mA to cut through a Logitech Quickcam Express IR filter enough that all head orientations can be tracked.

It should be noted the LED heat output increases significantly when going from 20mA to 100mA with only a moderate increase in brightness.

Can you explain why a PCB mounting allows for a higher LED max current than a normal wire connection?

If the led is soldered to a pcb, and the lead lenght is short enough, the PCB acts like a kind of little heatsink and helps cooling the led.

This is shown rather well in the max current vs lead length diagram, in the OSRAM or SIEMENS datasheet (last page).

The max current (100mA) is suggested only if lead length is less than 10mm, then decreases to reach about 70mA if lead length is more than 30mm.

Since a classic wire can be considered like a lead, that diagram applies in the case of freetrack assemblies, unlike you fix a pcb on your cap.

I have to use 100mA to cut through a Logitech Quickcam Express IR filter enough that all head orientations can be tracked.

I don't need that much.

I've tested with all webcams i've found in my group of friends, and in almost every cases, a 50 mA SHF485 cuts through the inbuilt IR filter.
The only case i've noticed not working was a noname webcam.

But it sure works far better with an IR modded webcam.

Did you test with a SFH485 (or another 880nm IR led) , or with a 940nm IR led ???
I ask this because 940 nm IR leds are known to have difficulties to cut through the IR filter.


I've (so far) built 6 freetrack setups for me and my friends :
- 1 x 4dot cap , operating with 940nm flat filed noname IR leds.
- 1 x 3dot cap using SFH485P
- 4 x 3dot clip using SFH485P

I've powered all my setups at 20mA.

- The 4dot cap,  the 3dot cap and 2 of the 3dot clip work with IR modded VX1000 s.
- 1  3dot clip work with an IR modded logitech quickcam messenger.
- The remaining 3dot clip works with an unmodded noname (i presume labtec) webcam.

The only setup definitely NOT working with unmodded webcams is the 940nm led equipped one. Every other work with unmodded webcams, although not very efficiently (except the last one) because I choose to work with really low current, and usually modded cams.

It should be noted the LED heat output increases significantly when going from 20mA to 100mA with only a moderate increase in brightness.

Right.
But this serves my point of view  


cya