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hi guys, im making my own wii-sensorbar, and i bought 10 leds from dx.com wich have a rating of 1.4 ~ 1.5 volt and 20 mA, i want to run them without using resistors (wich i think is possible right?) so how would i need to line them up?
legodude wrote:
i want to run them without using resistors (wich i think is possible right?)
No.

Examine the I-V curve of a diode (LEDs included):

The specified voltage (Vd) is usually assumed to be constant. That is, you can typically just model the diode as off when the voltage across it is less than Vd (ignoring the breakdown region and assuming the reverse current is negligible), and as on (lit) at all higher values of Vd with the voltage drop clamped at Vd (since current increases extremely rapidly when you reach that point).

You might think you could carefully specify the voltage of your supply to not need resistors, but that 1) requires an unusually precise power supply, and 2) requires that you individually characterize every LED (since the actual value of Vd varies between devices due to manufacturing tolerances).

By putting a series resistor on the diode(s), you limit the current. As a simple example, we'll assume you put two 1.3V diodes in series across a 5V supply:

Code:
   |\ |    |\ |
   | \|  B | \|   A
+--|  |----|  |-------+
|  | /|    | /|     --+--
|  |/ |    |/ |      ---
|                     -
| +5                0
The overall Vd is less than 5V, so the diodes will be on. Point B is at 3.7V, and point A is at 2.4V. Using Ohm's law on this ideal model (assuming ideal diodes), the current through the circuit is infinite because we're dissipating 2.4V across 0 Ohms of resistance (2.4 / 0 = ???). In practice of course the resistance is nonzero (wires have some resistance), so the current is simply very high. The LEDs light up very bright for a moment then die because they got too hot.

Using the rated current on the LEDs (we'll assume 20 mA here), we can spec a current limiting resistor: 2.4V / R = 20 mA. Solve that, and we get R = 120 Ohms.

As for practical layout, it requires the fewest components if you put as many LEDs in series as possible (given your supply voltage), since each string then only requires one resistor. If your application calls for more light, then just put multiple strings in parallel.
You should tell us more about the power supply you're using. If you only have access to 5v, then you'll probably end up with 5 series pairs of LEDs in parallel with each other (that is, 2 LEDs in series with each other and a resistor, and five of those pairs in parallel with each other). If you have something like a 12V supply, then you need only 2 chains of 5 LEDs in series along with 2, rather than 5, resistors. Note that you might then need a higher-wattage resistor, depending on how bright these LEDs are and therefore how much current they need.
When I made a wireless Wii sensor bar a few years ago, I had a 9-volt battery, 4 IR LEDs in 2 groups of 2, an extra-bright green indicator LED, and a switch. I didn't use any resistors, and it works. Now, whether or not I should have used them is a completely different story. I just thought I'd share my two cents.
Maybe you got lucky and the internal resistance of the 9v battery was enough.
thanks for the help everyone! i ordered some resistors on dx.com wich should be coming in in a few days Smile
legodude wrote:
thanks for the help everyone! i ordered some resistors on dx.com wich should be coming in in a few days Smile
Glad to hear it! Be sure to let us know when the project is done or if you run into any issues along the way.
will sure do Smile in the meantime ive been fiddling around with my arduino too haha
http://www.youtube.com/watch?v=s0UsD2PuwNc
  
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