FreeTrack Forum

Hey everyone,

It's Sunday and the local electronic stores is closed. I have a fast-build challenge for you.  I want to use an IR illuminated 3-point reflector system because I don't feel like carrying batteries on my cap and i'd rather build an IR flashlight around my webcam pointing at me.  I've salvaged some reflector tape, 10x Osram SFH 485P leds, a variable power supply (3V to 7V, 3-2 amp max), some cable and...

...rolling drums...

4x 100 ohms and 4x 330 ohms resistors (rated 1W 5%).

Now you can guess the challenge... with this limited number of resistors, what mix of parallel and series circuits can I use to light up 9 or 10 leds. Idealy, they should run at max 90 mA when supplying 7V (90% of max stable).

Should I go for something like :
       ---- led ----- led ---- led ----         -- 100 ohm  --
      /                                         \      /                       \
POS ----- led ----- led ---- led ------------- 100 ohm  -----  Neg
     \                                          /     \                       /
       ---- led ----- led ---- led ----         -- 100 ohm  --

Chalenge failed ;-) Well, I've played around with some circuit simulator applet and I couldn't find a way to do it with these 100ohms + resistors. So I opted in the car and tried to find an open electro shop.  Here's the way I want to power the leds using 7V :


      /----|>|----|>|----|>|----|>|---/\/\/----\   R = 10 ohms
7v+ -----|>|----|>|----|>|----|>|---/\/\/----- - R = 10 ohms
      \----|>|----|>|-----------------/\/\/-----/   R = 47 ohms

Before soldering everything, I decided to do a test on the 2-diode parallel line using 2 of my 100 ohms resistances.

7v +----|>|----|>|-----------------/\/\/--- -    R = 100 ohms+100 ohms in parallel

Any reason why this is not lighting up ?  I'm sure that my diodes have the same orientation (I soldered the short leg of the first to the long leg of the second, then soldered the short leg of the second to the resistances in parallel).  I tried reversing the polarity, but  still no luck.  The diodes do not look fried.  Are-they emitting 100% in the infra-red spectrum ? Should I be able to see anything ?

ok, they do emit a tiny spot of red light, guess my wiring was right.

Now, I just need something better then my salvaged bike-safety reflective ankle-strap to make a good 3-point cap. ;-)