GENERAL
INTEREST
58
Elektor Electronics
11/2002
If you made a wish-list for the ideal torch it
would probably start with the need for it to
be small enough to fit comfortably in your
pocket or rucksack but at the same time be
powerful enough to provide enough light at a
couple of metres distance to work by for
many hours. The environmentally aware
would also vote for a rechargeable power
source that would hold its charge even if the
torch had been lying in a drawer, unused for a
couple of years. An indicator showing the
amount of charge left in the battery would
also be nice together with an automatic cut-off
to guard against accidentally draining the
battery. It would also be useful if the light
level could be efficiently regulated to
allow continuous operation for sev-
eral nights and when the battery
gets really low (to 1% of its capacity)
still provide a useable emergency
light level for a few hours.
To fulfil these criteria we have
chosen to use white LEDs instead of
a halogen lamp. Contrary to popular
belief, the efficiency of a white light
LED is not much better than a low
power tungsten bulb and in fact
worse than higher wattage (> 3 W)
bulbs. The LED however offers a sig-
nificant advantage over filament
lamps; they are can be dimmed
down to 1/1000th of their maximum
power without any loss of efficiency.
A tungsten filament, by contrast will
glow less brightly and energy will be
lost in producing heat but little light
so that at half power its efficiency
drops to zero. A further advantage is
the improved life expectancy of an
LED. You can expect it to last more
than 1000 times longer than a tung-
sten lamp so spare bulbs are unnec-
essary. The LED package also incor-
porates some optics to direct the
light beam so that the torch head
LED Torch
flashlight goes high-tech
Design by H. Reisinger
You would be forgiven for thinking that a torch is probably the ultimate
minimalist circuit: all you need is a battery, switch and bulb and you’re in
business but add a microcontroller and you can build in some interesting
features that make the humble torch rather more useful…
Summary of Contents for EPROM
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