Blue is cool. Blue is also the preferred color for us theatrical types and other people who have to work in the dark without shining annoying lights around. It's either blue or red, and there's a trick about blue:
The trick is, that "White" LEDs are mostly blue LEDs (or UV) in a phosphor doped case. The case glows yellow, the LED glows blue, and the combination fools the eye into thinking it is "White." (Look at something green under a white LED some time.)
Which means, to within an acceptable margin of error, a bright blue LED will have the same electrical characteristics as a white one.
(For some projects, like my blue night light, there was a slight difference in forward voltage that was enough to keep the photocell from turning the light off during the day. Fortunately, a common IN4001 diode in series with the LED added enough of a voltage drop enough to fix that.)
For this Instructable, we will be modifying a very cute miniature generator flashlight from RadioShack.
Step 1: Parts and Tools
For a replacement, we need 3mm bright blue LED's. The ones I used are from the local electronics store; they were apparently 900 mcd (the marking on the bin was a bit hard to read) with a forward voltage of 3.2 to 3.4V.
In a previous adventure (I rewired my night light) I had to add a series diode to drop the voltage just slightly (the photosensor switch wasn't activating properly with the new LED).
You will also need the basic tools; at least a small phillip's screwdriver, a soldering iron, and solder. A de-soldering pump and a third-hand jig are also very handy for this project.
Step 2: Disassembly
2) Gently pry the case apart, being careful not to dislodge the gear train (the gears can be put back together but why make more work for yourself?
3) Gently disengage the generator from the gear train (note the small white gear stays on the generator shaft) and pry the circuit board, reflector, generator, and switch from the case. Put the case in your parts jar as well, being careful not to spill the gears.
Step 3: Remove the old LEDs
It is much easier to remove parts if you remove most of the solder first. Hence the desoldering pump. Hold the iron to the joint until the solder becomes liquid, and quickly press the button on the desoldering pump. Repeat a time or two until most of the solder has been sucked away. If you are lucky, you will then be able to pry out the part with minimum force (you can also lightly push at the leads with the soldering iron to help free them).
Sometimes a part doesn't play nice, and then it is working a thin-bladed screwdriver under it, and pushing on it while with your third hand you apply the soldering iron...
In the cast of this device, one of the LEDs is soldered under the battery. To get to it, carefully de-solder one leg of the battery and gently bend it aside. Do NOT apply the iron too long or accidentally touch the battery itself. The NIMH battery should vent if overheated, but it is a safety risk. And besides that, you'll ruin a perfectly good battery.
Step 4: Install the new LEDS
I probably should have told you to mark the polarity before removing anything. Never mind; in the case of this circuit, the polarity of the LEDs is marked on the circuit board for you already. Just match up the flat side of the case to the flat side of the silkscreen.
In most cases, you poke the leads through, then bend them outwards to keep the part secure. In this case, it is very important the LEDs sit flush and straight on the circuit board, or their light will not converge properly. My usual trick for cases like this is to use a little bit of masking tape to hold the part tight. You can also use a small vice, or if you are dexterous enough, hold the board flat against your work table while soldering.
If the part is only a little off, you can usually squeeze it between thumb and forefinger and touch the leads for a moment with the soldering iron.
Once the leads are soldered and the LEDs seated, use a small pair of snips to trim the leads.
Step 5: Re-assemble the flashlight
2) Now slip the board and reflector into the front of the flashlight. The wires will be on top if you are doing it the way I am.
3) Push the generator gently into the slot and in engagement with the gear train.
4) Check the position on the switch; you want the flashlight to switch ON when the switch is FORWARD.
5) Gently press the case together. You may have to use a small screwdriver to gently persuade the reflector to sit just inside the case edges. Re-install the screws.
Step 6: Testing
...Or, rather, admiring. The first one I did, I managed to create a solder bridge between the legs of the LED under the battery. I also didn't line up the LEDs correctly. The one I shot for this Instructable was better. I'll be making more of these...I have friends in theater who think they are very cool!