> I'm nit-picking, but I suspect it is more likely that the 
> receiver was sensitive enough at the red LED's wavelength.  
> LEDs tend to have a pretty narrow band output, whereas 
> photodiodes have a much broader optical bandwidth which is 
> why IR filters are used on the receivers.

could very well be. AFAIK LED emission (at least for RED LEDs) is a
state change (electron orbit step?), which would result in an almost
single emission ferquency. The IR-filtering of an IR receiver is an
optical filter, which is by its nature more wideband. I don't think I
have the means to distinguish between those two causes.

Wouter van Ooijen

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