Quote Originally Posted by flotulopex View Post
A.- As IR-emitter, one use IR-LEDs. As IR-receiver, one can use IR-LEDs, phototransistors or IR-Modules. What makes the difference between all these receivers?
B.- Some IR-LED's datasheet mention pulse frequencies (mostly 38kHz and 40kHz), other don't. Why, what's the difference between those LEDs?
If this is what you wanted to know:
TX:
IR emitter - could be just an IR LED, could be a premodulated IR LED with a built in oscillator running at 38khz (give or take, it varies)
IR LED - just that, an infrared LED, generally, if you want to use it in an IR comm scheme, you have to modulate it.
RX:
IR LED - not really a receiver, but you could use it in the way that I used them in my touchless sensor project.
IR phototransistor - = IR detector, does just that, detects IR, turns on a transistor, generally doesn't to any 'detecting' of a 38khz carrier
IR modules - have a built in IR phototransistor/diode and a filter to detect a 38khz carrier wave, generally pulls an output to ground when a carrier is detected.

The datasheets that mention 38khz/40khz/whatever mean that the specific module is set up to either TX or RX an IR beam modulated at that frequency. If you're just dealing with a 'beam breaking' application (like a garage door sensor), you don't need to modulate anything. If you want to send some data, you want to modulate the signal.
Think AC vs. DC. You can't send any useful information over a DC line. Once you start changing the DC line, it becomes AC and you can send info. (and I'm using AC kinda wrong in this instance, I should be calling it pulsing DC).

Don't know if it helps much. I've done a little bit with IR Tx and Rx, modulated a carrier, detected a carrier here and there. I used the old Sony IR protocol to handle the data transfer, works for me... I guess I need more info from your end on what you want to accomplish...