[paul_c5x4]

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?

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.