Building a 3 LED cap for freetrack?

[humble]

I'm curious about building a cap to check it out. They're are some nice prebuilts out there but I thought it would be a fun project to show my son a bit about building "gadgets". It's been awhile since i've messed with this stuff so I'm a bit rusty on calculating stuff. If I was super serious I'd run it off a USB port but dont want to risk the MB. Guessing wither a 1.2V AA or a 9V as a power source and what ever 5mm LED's I can get at radioshack. I ran a search but couldnt ferret out a thread with actual nuts and bolts. I'm going to cruise by the local RS at lunch and see what I can find available for LCD's...once I have that then I can work back to the PS.

[Fulmar]

If I were to do mine again, I'd probably change the power source.  I'd go with the AA since they offer high mah ratings than a 9v battery does.  I haven't used my FreeTrack enough that I haven't worn out a 9V yet (maybe 3 hours on current battery).  But my 9v is down to 8.5volts (alkaline).  9v's run about 9.5-9.6 when they are new.  I do not know how low of a voltage the battery will go before I lose function in the LEDs.

Rechargeables may be the way to go for a battery source.  I do not like a power pack/USB cord idea since I don't want the wires.

These are the IR's I got at Radio Shack:
http://www.radioshack.com/product/index.jsp?productId=2062565&cp=&sr=1&kw=infrared+led&origkw=infrared+led&parentPage=search

I helped a friend build a 1 point FreeTrack setup which works great and was easier to construct, you just don't get the 6 degrees of movement (you get 2).  I.e. you can't roll your head nor can you move in the cockpit side to side/front back/up down.  He didn't play AH but used it in games like ARMA.

[humble]

yup I just bought 3 of those LED's...they are 1.2v 29mA. Technically you could run them in parallel on a 1.2V AA rechargable but I'm going to run a 1/8w 10 ohm resistor on each leg so they will work with 1.5V AA as well.

[633DH98]

How did you figure the 10ohm resistor?  I don't see the LED resistance listed on the Radio Shack web page.   

[Fulmar]

I can't recall what the resistance is, but it is listed on the packaging of the LEDs.

[humble]

The resistor's job is to modify the power that reaches the LED's. Technically a 1.2/1.2 circuit wont need one in theory but in practice you dont want a 100% max load. So if your using a 1.2V rechargable then a 1/8w 2.2 ohm resistor would be plenty. The 10 ohm allows use of a 1.5v AA battery as well. Now if you used a 9V then you'd need a 200 ohm resistor and run the 3 leds in serial vs parallel. I used to be able to do it in my head but a lot of little utlities that do it if you know the variables. I used the one on the freetrack site.

[potsNpans]

http://www.free-track.net/english/ Freetracker has a led assembly calculator on their site which can assist in selecting power source and resistors to be used

[715]

The important parameter for an LED is current.  The resistor is chosen to set that current.  Here's an example calculation for a 5V supply.

V = voltage across the resistor = 5V - 1.2V(the voltage across the LED) = 3.8V.  Now V=IR so R=V/I.  I = 29mA = 0.029A.  So R = 3.8V/0.029A = 131 ohms.  Nearest standard size is 130 ohm.  The required resistor power P = I^2*R = 0.029*0.029*130 = 0.109 Watt, so a 1/8 watt resistor would be OK. 

You can see from this formula that using a 1.2V supply without some kind of constant current source would be problematic.  You can't just hook up a 1.2V LED to a 1.2V battery without some current limiting resistor; it could fry.