FreeTrack Forum

Hi,

I need to buy IR LEDs in order to build a 3-point clip.
I do not know what the recommended specifications of the IR LEDs are. Note that I cannot purchase Osram SFH485P LEDs, so disregard those.

NM: Does it matter if the IR LEDs are 850, 880, or 940 or a similar number of NM?

Viewing angle: Most of the IR LEDs I found on eBay are 15°, and 25°-30°. The recommended SFH485P are 40°, but I read that flattening the LEDs helps increase the viewing angle of the narrow ones.

Current and voltage: do these matter, or do they merely affect the type of batteries and resistors I will need to power the LEDs with?

Other: Anything else I forgot, please mention it.

Generally, any of the LEDs with the wavelengths you listed will work.  However, if you want to use infrared LEDs without removing the infrared blocking filter, the longer the wavelength (higher number) the poorer performance you will get.  So in that case, you are better off with 850 or 88nm LEDs.  If you will be removing the infrared blocking filter and replacing it with a visual blocking filter then any of those wavelengths will work just fine.

For this application, the wider the viewing angle the better.

Pretty much all IR LEDs have about the same forward voltage value.  Use 1.3v as a guide.  No matter how you think you are driving LEDs, they are ALWAYS current devices, meaning they get brighter with current, not voltage.

If you are using three LEDs, I strongly recommend that you connect all three in series with one resistor.  Also, brighter isn't always better.  Too bright and your image may "bloom" in the camera which gives you poorer performance.  I use 485P LEDs that they work the best for me at 24mA which is 1/4 their max rating.  I would think that any of the LEDs you use would work just fine between 20mA and 60mA.

I use 4 AAA batteries and  100ohm resistor.  It all works great.  That should work just fine for you and you will have plenty of battery life.

Good luck.

I found a website which ships SFH485Ps to Israel. I assume they also sell resistors, so should I buy 100ohm ones?

Sure.  You can try one 100ohm resistor which will give you a current of 25mA.  If instead you put 2 100ohm resistors in parallel, that is the same as a 50ohm resistor and will give you a drive current of 50mA for a 4 AAA battery pack. Try them both and see which one works best for you.

Why use a 4 AAA battery pack and not a 2 AA battery pack?
The battery holder should be significantly smaller.

Actually, four AAAs are not very different in size or weight than two AAs.

The size of the battery relates to the amount of capacity the batteries have.  AAA batteries have a capacity of 1200mAh.  AA batteries have a capacity of 2800mAh.  So a AA battery would have more than twice the life of a AAA battery in the same application.  (Divide the capacity of the battery in mAh by the current in your application in mA and that will get you the lifetime in hours.  So 1200mA divided by 50mA gets you 24 hours.)

For this application, 20-50mA is plenty to light three LEDs if all connected in series.  With 1200mAh for AAA batteries, they should last 24 hours of continuous operation at 50mA and 60 hours at 20mA.  So your question really should have been, "Why not use 2 AAA batteries instead of 2 AA?"

(At least in my opinion) AAA batteries are the way to go.  Light weight and pretty good battery life.  The question now remains, how many.

Serial connection is the way to go.  It is the lowest current use and therefore the longest battery life.  (There are other reasons but current use is the best reason.)  Given that, the number of batteries is then determined by the total forward voltage needed to turn the LEDs on.

LEDs are a current device but they need a minimum voltage to turn on.  Once the voltage across the LEDs is high enough to get them to light, their brightness is determined by how much current is provide.  (That is determined by the resistor.)

So, if you have three LEDs, each with 1.3V of forward voltage required, and all connected in series, you need a total of 3.9 volts across the combination of the three LEDs to get them to light.  2 AAA batteries (or 2 AAs) is only 3 volts.  Not enough.  3 batteries is 4.5 volts.  That would work but three battery holders are tough to come by.  That leaves 4.

I assure you that four AAA batteries connected into a 49 to 100 ohm resistor into three LEDs will give you very acceptable performance.

Hope that helps.

(Just for the record, I am a 57 year old electrical engineer that has spent the last 35 years designing for aerospace and medical systems.)

Well, I appreciate the information.
However, I have already ordered a TrackClip Pro. I just couldn't find any of the parts for building a clip myself in my country. I also wanted it to be aesthetic and not have exposed wires and such, and the TrackClip Pro looks pretty sleek.