In article ,
Steve writes:
Dave wrote:
I'm not so sure. Once you start driving LEDs hard as a light source

rather

than just an indicator, their life span reduces dramatically.

My curiosity knows no bounds, it appears. When I bought one of
those LED cycle lights, I hooked up a sillyscope to it and
discovered that on the "steady" setting, it actually spent 3/4 of
the time switched off. 100 Hz square wave mark to space 1:3. A
clever way of stretching battery life. If you own one, you simply
have to move it rapidly across your field of vision to
demonstrate this.


driving them like this also means you can pump more current through them
thus making them brighter (maybe this is the reason rather than saving
battery life?)


The mean brightness is still the same, since the mean current must be
the same,

Actually LEDs get slightly more efficient at higher current, so
the mean brightness is higher.

Furthermore, it appears that eyes don't perceive mean brightness,
but perceive something which is between the mean and peak brightness,
so a flashing light will appear brighter than a steady light of the
same mean brightness (over some frequency range).

Neither of these effects is very large though. I've seen conflicting
reports of which of these effects first resulted in pulsed operation
of visible LEDs, although the high pulse current techneque was used
to drive infra-red LEDs before visible LEDs were invented.

because you can't leave the LED on at the current used when on
or it will burn out. However driving them switch mode allows the circuit
to run more efficiently which IS why they are pulsed.

--
Andrew Gabriel