June 3, 2010
about 5 years ago
I am puzzled by the design too,
as taking a look at the schematic, there is (R5) a 300 ohm resistor for +Ve. This sounds a bit heavy for connecting a white led to 3.3V:
- assuming about 3.2..3.6V drop for the white led + some 0.2..0.6V for the transistor we have something like 3.4..4.2V drop in components. And on top of this here is this the resistor that limits with 1.5V drop the current to manufacturer-recommended 5mA. To me this would seem that it as intended to be used with +5V power source, now way it should be workable with 3.3V.
With that 300ohm resistor it gets nicely about 5mA for the led. (assuming close-to-zero drop in the transistor)
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