SOLVED Chinese IR emittor/phototransistor pair doesn't seem to work

Hey guys,

I recently bought a pack of 10 3mm IR LEDs + 10 3mm phototransistors on Ebay from a chinese seller and have had no luck getting them to work. In the past I’ve used various 5mm IR LEDS and phototransistors with no problems. I’ve bought plenty of components from china and have never had issues, so it’s hard for me to believe that these straight up dont work, but I haven’t even been able to get a peep out of the IR emmitter, which is bizarre as it’s simply an LED. None of my working 5mm phototransitor setups are registering it, even with a 50 ohm resistor on it, which should be really bright. (I tried every LED in my pack, none of em work).

As for the 3mm phototransistors, I’ve dropped em into a circuit to replace a 5mm phototransistor, but they don’t respond to light at all. I posted the circuit as an attachment; with the 5mm phototran it reads around 1012 at rest and about 300 when shone on. If I put the 3mm in there, it just stays at 1012 and doesn’t respond to any light, visible or otherwise. The only possible clue I have here is that if I put the 3mm chinese phototran in backward, instead of reading an unresponsive 1012 like the 5mm will, it instead read somewhere around 400. However this doesn’t seem to be a response to light, cause it’ll read the same in pitch black darkness.

Does it sound like these are duds or am I just missing something here? I don’t want to buy more and wait for them to arrive and have them not work either. I don’t have enough space in the device I’ve built to accomodate 5mm LEDs, so I’ve got to get a working 3mm IR pair. Also, does anybody have experience buying 3mm IR pairs that work correctly and if so, from where did you buy em?

Thanks alot for any advice!


With regard to your 'emitters', get out the voltmeter - there should be approx. 1.5V across them when forward biased.
Maybe your 'phototransistors' are mislabeled photodiodes. Does a variable voltage develop across their pins (just the device alone is sufficient) when exposed to light, like a 'solar cell'? If so, that's a photodiode. Again, Mr. Voltmeter is your friend.

So one is white, and one is deep blue. Which one do you think is the emitter?

Use a digital camera (try it on your TV remote first) to see if the LEDs are working.

They're not phototransistors mislabeled as photodiodes, they are are sold as photodiodes. Read the Ebay listing you posted. It says diode.


So one is white, and one is deep blue. Which one do you think is the emitter?

They look black to me, so they would be the IR Transistors, DarkRed IR Filter.
Apply power to the emitter LED with current limit resistor and see if you can see it in your digital camera.

Tom... :slight_smile:

Hey guys,

thanks for the replies, you helped me realize my error: I had the emmitter/reciever mixed up! They came with no info and I assumed that the tinted one was the IR Emitter, since any IR LED I've ever used has been tinted either blue or dark purple.


IR LEDs may be tinted or clear - it makes no difference to their function.

Photo-diodes/ phototransistors specifically made for IR must be tinted in order to reject visible light.