Here's an easy hack - turn part of a flashlight into:

BLACKLIGHT (UV) - Forensic fun, raves and festivals, rockhounding and even antique shopping.
REDLIGHT - Keep your night-vison safe with a red light - great for camping and night photography.
IR (INFRARED) - You can't see it, but your phone shows IR illumination - cheap night-vision tech.

This is a very simple project, only about ten minutes of actual work per light. Just replace the original LEDs with the appropriate type of new LEDs and you have a multi-function illuminator.

I have a bunch of those pillbox style flashlights that are so popular right now. They're everywhere and you can get ones with dead batteries for a buck each. They have one big 20+ array of LED lights on the flat side and another, smaller 3-4 LED light on the skinny end. They use a single button to toggle between on/off and the bright/dim settings. These rounded box lights are super handy and I already have at least one of them for every work station, car and backpack.

I thought having two levels of lighting to choose from would be very handy. But I've noticed that I never use the small light, only the big light. I started to think about how I could change the small light array to make it more useful.

Since the LEDs are standard sizes, I knew I could replace them with many types of LEDs. So I tried it, and it worked very well. The blacklight version makes objects glow from several feet away, and the red LED version is bright enough to see the ground well on a moonless night, but dim enough not to night-blind me.

Step 1: Hack-n-Mod: the Movie

The movie explains the entire process quickly. And it might be easier to understand than the written instructions. Sometimes the simplest process is difficult and tedious to describe with text.

Step 2: Tools & Materials

Flashlight:

I used inexpensive, boxy flashlights. But you can use any LED flashlight with a few changes to the technique. I chose this style of flashlight because they have two separate light arrays, so I could modify the small light and still have the bright light for normal use. (And because I have 20 of them)

LEDs:

Since these flashlights use 3xAAA batteries, the maximum voltage would be 4.5v. The flashlights also have inline resisters, so the voltage and amps would be even lower, well within the specs for my LEDs. I chose LEDs designed to be easily and safely used with 5v Arduinos with minimal protection. But if you use a lower 3v or higher 9-12v flashlight you might want to use LEDs with different ratings.

The original LEDs were 5mm in size, so I replaced them with other 5mm LEDs. If your flashlight uses another size, like 3mm or 10mm, then use the correct size LEDs.

I have a lot of scavenged LEDs, but I'm no expert on calculating loads and voltages. So I used new ones from two of my favorite hobby electronics suppliers. Both suppliers have good info on their LEDs so I knew how much voltage they could handle. I used the diffused 5mm red LEDs from Sparkfun for the night-vision hack. And I used the 5mm UV-blacklight LEDs from Adafruit for the other hack.

None of my scavenged IR LEDs were bright enough to be useful, but Adafruit and Sparkfun both have good versions that should be bright enough (be careful with the current on these).

Soldering Iron and Solder:

Any standard soldering-iron and electronic solder will work. This isn't fine-pitch work. I used the most basic technique and there is plenty of room to work, so even a crafter's style iron will work. My solder joints turned out kind of blobby, but they still worked fine.

Other Supplies:

A pair of clippers to trim the leads, maybe some extra wire, and a "third-hand" stand is always handy.

Step 3: Your Light Might Be Different

Don't freak out if your flashlight looks different than mine. I have several versions of these boxy flashlights and I expected there to be variations between the different versions. However, I was surprised to see that there were also differences in what appeared to be the exact same model.

Don't worry, just use the basic technique and you can make it work.

Some versions had the toggle button, the large LED array and the small array of LEDs all soldered onto a single PCB. Other versions had the main light array on a PCB along with the button, while the small light array was put onto a separate, tiny PCB. In an identical looking model, the main light and small light were on PCBs but the button was free-standing.

Even if the LEDs are soldered onto the main board, you can de-solder them and replace them with new ones. Or you can clip the original leads and tie into the circuit with free-standing LEDs.

Step 4: Basic Technique

Disassemble the flashlight case. The versions I used have three layers in the case, front cover, back cover and the center section. The PCBs and main lights are seated on the center section.

These flashlights have three or four LEDs soldered onto a small PCB that fits in the nose of the case. The LEDs stick through holes in a plastic holder. The holder slips out of the case along with the mini-PCB.

Snip the wires leading to the mini PCB and remove it from the holder. Snip the wires as close to the mini-PCB as possible (not close to the larger, main PCB.)

Put the new replacement LEDs into the holder so that stay spaced correctly.

When you arrange the LEDs be sure to use a parallel circuit, not a serial circuit. Make sure all the tall leads are on the same side and all the short leads are on the other side. That way, when you start soldering, all the tall positive leads (anodes) are connected to each other, and all of the short negative leads (cathodes) are connected to each other. None of the cathodes connect to any of the anodes. This forms two parallel lines of connections with all the positive on one side and all the negative on one side.

(The positive anode leads will usually be longer than the negative cathode leads. The head of the LED will also have a flat side next to the negative anodes (Sparkfun Tutorial))

I used the "dead bug" or free-wire style of soldering. It's simple and doesn't require any extra parts.

Bend the leads from one of the end LEDs so they touch the leads of the other LEDs. If the single lead doesn't span all the other LEDs, then bend the leads from the other end LED.

The bent cathode lead(s) should be touching all of the other cathodes. Solder the leads together where they cross. Just put a blob of solder at the crossing point. It doesn't take much, just be sure it makes a good electrical connection.

Now repeat the process for the anodes. When you're finished, you should have two rows of solder connections.

When all the LEDs have been connected, clip off the leads that are still standing up.

Check to see how the wires are run into the case from the mini-PCB. Test fit the new LEDs and holder in the case. Be sure you have enough wire left to put the new LEDs back into the case. If the wire is too short, splice more wire into the circuit.

Now, solder one of the LEDs into the power and ground wires. (positive wire to one of the anodes and the negative (ground) wire to one of the cathodes.) Usually, it's best to use one of the end LEDs, but you can solder to an LED in the middle of the array if it's more convenient for your wiring harness.

(I wanted to keep this project simple, but yes, you could use perfboard instead of free-wiring. Of course, if you already own perfboard you already know this.)

Test the new lights, fix any errors, re-assemble the case, then go have some fun with your new multi-function flashlight.

Step 5: Conclusion

For 10 minutes of work, this turned out to be a great project. It's not what I originally intended to do with these flashlights. But with some improvising and ingenuity, I got something even better - two or three somethings in fact.

I had forgotten how much fun blacklights are. Now, I am not only ready for Halloween and Burning Man, I'm also equipped to investigate crime scenes, detect florescent minerals, spot uranium glass at the local thrift store, and find secret messages left by other agents.

The red LED version is exactly what I needed for star-gazing, meteor showers, night photography, camping --- or basically any place I need to save my night vision. I put this one in my camera kit right away when I saw how well it worked. It's not as showy as the blacklight version, but I will actually use it a lot more frequently.

I have several more of these boxy flashlights and I've been thinking of other ways to hack and mod them. Since there is plenty of room in the case to include a small Arduino microcontroller - how about a variable speed strobe light, or maybe some RGB LEDs for making a custom light show, or a motion activated night-light, or maybe a Bluetooth remote controlled light?

I think I've found a great, inexpensive platform for experimenting with portable light sources, so stay tuned for more projects with these handy little flashlights.