[sdiy] driving blue LED

[ASSI]

On Donnerstag, 19. Oktober 2006 12:02, Bert Schiettecatte wrote:
> I have been playing around with blue LEDs here and I have one that
> has a forward voltage of about 3.4V max. I'd like to drive it from a
> microcontroller, which is powered by three AAA batteries (1.2V
> nominal so varies between 3 and 3.6 V? ). Would it be possible to use
> one of the high current drive I/O pins (16mA) to add the necessary
> voltage/current on top of the battery voltage to create a higher
> voltage to drive the LED with?

There's several ways to do that, but in this case I think a capacitive 
charge pump (voltage doubler) would work just fine.  If you've got two 
outputs and at least one of them can go tristate and is overvoltage 
tolerant, then you need just an external capacitor connected to two 
outputs.  Create non-overlapping pulses on these two outputs, one 
should switch between Vdd and Gnd and the other Vdd and tristate.  On 
the latter side you'll find a signal between Vdd and roughly two times 
Vdd.  If 2xVdd is too much for the tristate output you need to add a 
protection diode, preferrably a Schottky.


Achim.
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