I've got some IR diodes and I have forgotten which are the Tx and which the Rx. They are in the standard plastic LED 10 mm package with no identification. Some are purple transparent and the others clear. I don't know anything else about them.

Anybody know how to tell which is which in a simple manner such as ohm check or Vd ?
Cheers

The forward drop of an IR photodetector diode is about the same as for a silicon diode: up to about 1V at up to around 50mA:

For example:

http://docs-europe.electrocomp.../0900766b80dea1a5.pdf


The forward drop of an IR LED is a little lower than for a low brightness (old tech) red LED: up to about 1.6V at up to around 50mA.

For example:

http://sales.hamamatsu.com/ass...pdf/parts_L/L8013.pdf

An easy way to check is to connect one to a low current source (or a voltage source with a series resistor of 1k or so) and the other to an ammeter and point the two diodes at each other. The connection that gives a current of a few uA or so in the ammeter will be that of the LED connected to current source and the photodiode to the ammeter. Photodiodes don't normally emit light!

Be careful of the current source polarity and don't push too much current through either diode.

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Andy Fierman

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http://signality.co.uk
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You can see photodiodes light up if you look at them through a mobile phone camera.
Try it with a TV remote.

David

Thanks guys. Helpful comments.