On Wed, 12 Nov 2003 00:19:46 -0500, No1 wrote:
Nice project. A circuit diagram would be nice.

Use fixed pitch for the crappy ascii art.

minus battery ------resistor-------led-------switch------plus battery
------resistor-------led-----

All LED's are in parallel with a dropping resistor in series.
I used 100 ohm for the 6 volt lights and 330 ohm for the nine volt lights.

Rdroppin = Vsupply - Vled / Iled

Vsupply = 6 or 9 volts

Vled = 3.6 volts ( I use the lower voltage rating)

Iled = 20ma. or .02 amps

6 - 3.6 = 2.4 / .02 = 120 ohms so I am a bit low on the 6 volt dropping
resistors. In the 10 led flashlight the led's run a little bit warm, probably
because new batteries are a higher voltage.

9 - 3.6 = 5.4 / .02 = 270 ohms. I used 330 ohms because it was what I had on
hand at the time and that give .016 amps rather than .02 amps, not much
difference.

There are a couple of problems with these designs. Number one is that
the resistors are dissipating more battery power than the LEDs, in other
words, system efficiency is less than 50%. Number two is that light output
diminishes as battery terminal voltage falls. Both problems can be solved
by using an LED controller IC instead of the brute force current limiting
resistors.

An example would be the LTC3205 charge pump LED controller chip. This
chip is a switch mode constant current source with 92% efficiency. It will
maintain the programmed LED current down to an input voltage of 2.7 volts.
(Max input voltage is only 6 volts, so it is only suitable for a smaller light,
but it will double the battery life, and maintain full brilliance until the batteries
are completely exhausted.)

Other choices would be the Maxim MAX1553 or MAX1554. The latter can
drive 10 LEDs at a maximum (pulsed) current of 970 mA with an efficiency
of 82%. The former can drive 6 LEDs with a maximum current of 470 mA,
and an efficiency of 88%. Either of these would be suitable for a larger
light.

There are a number of other driver chips available. I only picked these
as representative. Do some searching of the various manufacturers'
product literature to find other devices and application circuits.

Gary