I am new to Arduino, electronic circuit design, and assembly. Please excuse my crude methods. Perhaps my experience will help other newbies.
I harvested some components from a PCB pulled from an optical mouse. The mouse wheel has a bunch of slots cut in it. Mounted on each side are what I assume are an IR LED and IR sensor.
I want to see if it is an IR LED and if it works. My desoldering skills are horrible and it's possible that I overheated it.
I cannot normally see IR unassisted so I took my cell phone camera and pointed it at the TV remote and pressed the volume up button. I could see flashing violet light. My cell phone camera can detect IR.
I took the LED and hooked one lead to pin 13 and the other to ground with a 270 ohm resistor in the circuit on the ground side. I loaded the 'blink' example and executed it.
I pointed the cell phone camera it the LED. No joy. Then it occurred to me that I don't know which lead is anode and which is cathode. I flipped the LED around and can see it flashing.
Woohoo!