--- Tobias Wieler wrote: > I want to do a bidirectional half duplex communication over a > distance of about 10cm (4") which seems quite trivial at first=20 > sight. The difficulty is the fact that I have just narrow tunnel=20 > with 2,5mm diameter (1/10") for the 4" distance so the LED (TX) and > phototransistor (RX) for each end need to be very close together to=20 > be able to 'see' each other thru this tunnel. You could use an LED on both ends instead. The IR LED would be matched automatically, and act as a photodiode. It's not a perfect solution but might be passible for your need. > I found a IR-LED and Phototransistor in 0603 packages which could be=20 > arranged close enough - just the beam pattern is about 120=B0 so the=20 > efficiency is about the worst you can get. > Using an optical fiber unfortunately not possible - the tunnel needs > to be necessarily empty. Hmmm you have 100mm x 2.5mm empty tube? and it absolutely must be empty or the thing absolutely won't work in the situation? :) Just curious as the reason for it to be void may be quite important, and would likely affect your component selection. In fact it might just making a workable design impossible. So the reason might be prudent to include perhaps? > I was thinking of using a 45=B0 semi-reflective mirror on each end to > have a beam splitter so the LED could look thru straight and the=20 > phototransistor sit at 90=B0 for example - but the mechanical assembly=20 > probably is a mess and how do you get the 45=B0 mirror well aligned on > the pcb? Does anybody know of a device with LED and phototransistor=20 > in a single package looking thru the same lens? So far I couldn't=20 > dig up anything truely useful on Google. See above, you can use an LED as a photodiode, though it won't be easy to use it's doable. A pair of T1 devices should fit your dimensional requirements however you will need to do experimentaton. Obviously it will only work half duplex this way. I don't know about speed either. It was however a project in popular electronics back eons ago.=20 > The wavelength is not bound to IR - visible light would be acceptable > also. For the receiver both phototransistor or photodiode is ok - but > just the plain part without any preamplifier or other fancy stuff. >=20 Hmmm this seems on odd addition. Are you sure you aren't unnecessarily constraining what you have to use? Stephen R. Phillips was here Please be advised what was said may be absolutely wrong, and hereby this = disclaimer follows. I reserve the right to be wrong and admit it in fron= t of the entire world. =09 __________________________________=20 Do you Yahoo!?=20 Yahoo! Small Business - Try our new Resources site http://smallbusiness.yahoo.com/resources/ --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist