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I have a LED i cant light, no matter what. It has a little black dot in it. What kind of LED is that? Answered

It is a common 5mm, transparent case, LED, being different from all others only by having this very tiny square dot over its anvil, kind of covering the reflexive cavity where the semiconductor (i don't know which) gets lit up.

I could not light this LED whit voltages from 1.5V to 3.5V. =O
It came out of a photocopying machine, and was paired with an IR LED in some kind of emitter-receiver set. Other components of this set were a pair of smd resistors and a smd trim pot.

Would it be and IR receiver or photodiode?

I've took some photos of the mysterious LED. BUT, as no macro-enabled camera was available, i had to do the ghetto-way of photography. Soo: i have photos of it compared to the IR LED, and some photos of both anvils (IR, unknown), at 200x magnifying. Here they are.
BTW, i hadn't tested the suggested circuits yet. =(



Best Answer 8 years ago

Paired with an IR-emitter, it will be a photo-transistor.

It works like a normal transistor, except the 3rd leg is replaced by the light shining on it.

Connect it in series in a simple circuit, say an LED, and the LED won't light.  Shine a light on the component and it will switch and allow current to flow.

I've done the circuit. Works perfectly!

You're welcome - thanks for the "best".

*Hastily scribbles note*
Shine a light on the component...

That LED is Infra Red IR you can check it with phone camera
if it doesnt work it is a phototransistor or Photodiode .

It'll be and IR paper / mechanism detector like you suggest. Look at the LED with a video camera (assuming it's not been fried already), the photodiode won't emit light.


I did that. Actually that was the method i chose to verify if my IR wasn't really a BLED (in the words of robotguy4).
Obviously, the IR shone on my camera, but the other one, nope.

Well, as we've said (e.g. Kiteman) the other part won't emit light.


Photo posted. Take a look!

Did you check your polarity?

Hmmm...  It could be a BLED. (note: the B stands for "Broken")