Quote:
So you're saying the observed behavior is the opposite of what we would expect. Wow, very puzzling indeed.


Yeah, it's actually slightly worse than I stated - 2V is a typical forward bias ('turn on') voltage used when calculating for a regular (redyellow/green) LED. The new 'exotic' LEDs (blue, purple, UV, white) have a higher bias voltage. White is typically 3V. That means that the signal would have to get below 2V before the button LEDs turned on, whilst the transistor only needs it to be below 4.3V.

Both the LEDs and transistor work on the same principle with regards to turning on - a doped semiconductor junction that allows current to flow one way once a bias voltage is reached. The bias voltage is dependant on the semiconductor material used. In the case of the transistor it is nearly always silicon, and that gives a Vf of 0.7V. LEDs use different chemical compounds to achieve different colours, and each of these compounds will have a different bias voltage.
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