Okay, that is like the design I first intended to build. But if anybody would provide a more complex and more accurate tester I would be very happy.
And again, which one is the better diode or transistor? I've read that diodes give a clearer rise and fall in voltage? And also, why is IR better than any visible light?
Originally Posted by olleorama
I used a photo-transistor (part no. SFH300-3) for my simple tester - This was chosen as a: cheap & in stock, b: fairly fast rise time.
The vast majority of diodes/transistors are most sensitive to IR, and even the visable light versions tend to have peak response at around 8-900nm.
As BetterSense pointed out, people are making complicated - For my part, I plugged the SFH300 in to the parallel port and wrote a 200 line C++ program to record the on time. The second version may well use a fast DAC card and quite a bit more code.
Diode vs. transistor really doesn't matter here. Photodiodes are faster, but phototransistors are plenty fast for this application. Phototransistors seem to be a lot more common; you can get them at radioshack in a pair with an IR emitter, and also at Fry's. When I was looking I couldn't find photodiodes anywhere, until I discovered a bag of 5000 in the electronics lab here at the university.
As paul_c5x4 said, it's not that IR is better...it's just that silicon PN junctions are sensitive to IR, because of their band-gap. They have a peak in close IR, even the ones used for camera meters.