IR frequency difference
Can I use 37.9 or 38 kHz IR-receivers for 37.8 khz communication?


Yes you can, it should be close enough. (I am assuming you are talking about some kind of prebuilt receiver that just receives the IR data and transmits it via RS232 or something like that)



I'd suggest you'd use a phototransistor and a IR diode for the communication. You will have a lot more control over what frequencies you are operating on. Altho it might take a bit more work to program it, the end result will be exactly how you wish.