# Is it possible to approximate specs of a LED?

I am thinking about gutting a few infrared LED's from some old remote controllers and using them as part of a battery powered wii sensor bar. Obviously, I do not have the spec sheets for these LEDs, so I don't know the maximum and minimum voltage inputs, max currents, etc. Calculating the needed resistance required may not be possible.

Is there some kind of test that would allow me approximate said specs without destroying the LEDs? Or is reusing parts (without available specs) generally considered bad practice? I don't want LEDs to explode or catch on fire.

• Infrared LEDs use a slightly lower forward voltage than red LEDs. For similar physical shape, size, and wires, they probably can withstand similar currents. You could certainly look up "likely candidates" and see what the specs say. But I'd not be too worried. If the IR LED is about the size of a typical RED, and you have a $9\:\textrm{V}$ battery handy, just use that along with a $1\:\textrm{k}\Omega$ resistor to start, and test it out: measure the voltage across the resistor and across the IR LED. That should give you an approximate forward voltage for the current you can then calculate. – jonk Jun 2 '17 at 1:24
• cdn-shop.adafruit.com/datasheets/IR333_A_datasheet.pdf – jonk Jun 2 '17 at 1:24
• it's not critical for the wii, they just need to turn on and stand out against the bg. – dandavis Jun 2 '17 at 2:05
• An IR LED probably drops about 1.5V. Without a datasheet, I would assume a current of only about 20mA. But if you're pulsing the output, then the peak current can be higher. – Simon B Jun 2 '17 at 8:05
• How much do you value your time? Buy new ones from a recognized supplier that have a decent data sheet. – Andy aka Jun 2 '17 at 10:42

As indicated in the comments it is easy to make some basic assumptions and be fairly safe. Just about any 3 mm or 5 mm LED will have a max current of 20 mA with very little benefit in terms of brightness once you get to around 10 mA.

Do you have a multimeter handy? If so follow this process:

Based on your chosen battery voltage pick a resistor that will give you a current of 5 mA. e.g. for 3 V use 3/0.005 = 600 ohms. Any value within 50% of the calculated value will be OK.

Connect the LED into the circuit and measure the voltage across it. Subtract that from your battery voltage and recalculate the resistor values needed 5 mA ( (3 - x)/0.005). Aim for a resistor between the calculated value and half of that number to put you in the 5-10 mA region and it should work fine.

Alternatively if you don't have a meter but do have the Wii console and controller on hand the put the wii into the sensor bar calibration mode which shows you the controllers sensor reading. Start with a large resistor (e.g. the first value calculated above) and slowly decrease the resistor until the LED is clearly shown in the sensor readings.

Or is reusing parts (without available specs) generally considered bad practice? I don't want LEDs to explode or catch on fire.

It's bad practice for anything requiring any level of accuracy or control. Or anything that will be resold and so needs a reasonable level of quality control. But for something like driving an LED of unknown type for a home project it's perfectly reasonable to do. LEDs have a huge operating region so you can afford to be a little sloppy in how you handle them.

There's no danger of fire or explosion, worst case they make a tiny pop sound and a small puff of smoke.