Subject:      DIY red LED flashlight
From: (Greg Granville)
Date:         1995/10/19
Message-ID:   <>
Newsgroups:   sci.astro.amateur

Everybody needs one or two red flashlights, right? Well, If you're to
cheap to just buy one, here's how to make one for about $3. (assuming that
you've already got an old 2-cell flashlight lying around somewhere) 

Parts needed:

Jumbo (super-bright) LED - Radio Shack part # 276-086 - Price $2.49

150 ohm resistor - 1/8watt (1/4 watt OK. but it's larger) - Price .10cents?
[Note: Jim Large, <>, recommends using a 47 ohm resistor for maximum brightness]
PR series lamp - the PR-4 out of the flashlight is OK, but any PR lamp is 
OK, it doesn't even have to work (you're gonna smash it anyway :-))

- --- Notes on the LED: Get the Radio Shack LED. Standard red LEDs that are
used for indicator lamps usually only have about 4mcd of light output (and
still need 20ma of current) This LED from Radio Shack puts out 5000mcd of
660nm (deep red) light and still only needs 20ma of current. This is
plenty of light to read your charts with and your batteries will last a
LONG time! 

This may be excessively detailed for many people, but it might help those 
who don't play with a soldering iron very often...

Step 1: Take the lamp (the PR-4 or whatever) and CAREFULLY break off the
glass. Remove everything inside the lamp - the filament and wires - even
the hardened adhesive inside the metal base. 

Step 2: Using a soldering iron, remove enough solder from the base of the 
lamp so that you can stick a wire thru the bottom hole. Be careful not to 
overheat the insulator around the solder - it can crack.

Step 3: Trim the anode lead on the LED to about 1/4" long. (this is the
lead that is NOT next to the flat spot on the side of the LED - the lead
next to the flat spot is the cathode) Trim the lead on ONE side of the
resistor to about the same length. 

Step 4: Tin (apply some solder) to the short leads of the resistor 
and the LED. Then overlap them and solder them together.

Step 5: Insert the LED/resistor into the lamp base so that the long
resistor lead goes thru the hole in the bottom of the lamp. Bend the long
lead on the LED to the side slightly and slide it down the slot in the
side of the lamp. If the lead won't slide into the slot, flatten it out a
bit with a pair of smooth jawed pliers until it is thin enough to slide
down into the slot.  IMPORTANT! : be sure the resistor lead doesn't touch
the metal part of the lamp base - it should be centered and go straight
into the hole in the bottom of the lamp. 

Step 6: When you are sure everything fits and the wires are not going to
touch, remove the LED/resistor far enough (not completely) so that you can
put some glue (hot glue works well) into the cavity where the resistor
goes.  Then shove it back together. 

Step 7: When the glue has set, solder both of the protruding wires. Try to
make the solder button at the bottom of the lamp look like it did
originally.  Trim the lead on the side of the lamp very close after
soldering - don't leave a big blob of solder on the side of the lamp or it
may not fit back into the lamp holder. 

Step 8: Put your "new" lamp into your two-cell flashlight and turn it on. 
Hopefully, you'll be basking in red light.

- ---

The nice thing about this approach is that it doesn't require that you
alter the flashlight in any way. You can stick this LED-lamp into any two
cell flashlight and it should work fine. AA- powered flashlights have
enough power to run the LED for a very long time (at least 20 times longer
than a normal flashlight bulb) AA- flashlights are so small and light that
they are perfect for this application.