- Education and Science
How to Build Your Own Ultraviolet UV LED (Black Light) Flashlight
Work in a well ventilated area. Solder fumes are no fun.
Wear safety glasses.
UV light can be dangerous. DO NOT look directly into the lit LEDs. Don't shine the light in the eyes of others.
Very Simple Electronics
In Middle School I took some kind of shop class that consisted of 6 weeks of electronics. All I remember from the class is that I built some sort of "any idiot can do this" radio that didn't work. Obviously I was the exception to the rule.
Looking back, I'm pretty sure I know what I did wrong. It was something so simple that the teacher should have been able to spot it and the project would have been salvaged in a minute or less. I'm pretty sure I had a diode backwards.
Diodes and LEDs
The simplest description of a diode is that it acts as a 1-way gate for current. While there are many sizes and types of diodes, they all perform this basic function. If current is sent from the positive to negative poles of the diode, it flows. If it's sent from negative to positive, current doesn't flow. A diode's negative end is marked with a stripe around its barrel.
LED stands for Light Emitting Diode. LEDs are simply diodes that light up when current flows through them. Since they aren't cylindrical like most diodes, the positive and negative poles are marked differently. The positive pole of an LED has a longer leg than the negative.
Resistors simply resist current flow. In actual application, they drop the amount of current flowing through a circuit. In our simple circuit they are merely there to drop the power down so that the 9 volt DC battery we're powering the flashlight with won't burn up the LEDs. The stripes on the resistor tell what its resistance is. We won't bother with learning to read them for this simple project.
Tools you'll need
Solder (resin core)
Side Cutters (dikes)
Small Needlenose Pliers
Phillips Screwdriver #1 bit size
Hot Glue Gun with Glue
Lots of bad, blurry pics. Sorry.
Parts You'll Need
I built some of these flashlights as a project for work. We got all of our parts from Allied Electronics. Any electronics warehouse will have similar parts. While Radio Shack doesn't stock Ultraviolet LEDs, they do have a range of normal colors if you want to build a different color..
LEDs, Qty 6, Mfr. Part: VAOL-5GUVOT4, Allied Stock: 398-0973. These are 5mm, "through hole," clear lens, Ultraviolet LEDs. Through hole means they can be mounted flat against a circuit board. Most LEDs are this design.
RESISTORS, Qty 3, Mfr. Part: CF-25-151-JTW, Allied Stock: 840-0078. These are 150 Ohm carbon film resistors with radial leads. Radial leads means that the resistor has wires coming out of each end of its cylindrical body.
PUSHBUTTON, Qty 1, Mfr. Part: 30-101, Allied Stock: 948-0321. This is a "normally open" switch. When you push the button down, it makes a complete circuit. If it's released, it's open and no current flows.
BOX ENCLOSURE, Qty 1, Mfg. Part: 40-12-9V-R-GL, Allied Stock: 278-0033. This is simply a black box with a battery compartment for a 9 Volt DC battery. It includes the connector and wiring to hook the battery to the simple circuit you're going to build.
WIRE. While I didn't use any wire other than that which came with the box enclosure, some 18 or 20 gauge wire is useful. It's also great to practice your soldering on before moving up to components.
How to Solder
The simplest thing to solder is scrap wire. First practice stripping the plastic coating off of the wire. You only want about 1/4" of the wire exposed. If you strip too much, it can always be trimmed later.
The first step in soldering is to "tin" the parts you are going to solder. Tinning is simply applying solder to each part before you try to put them together. Let your soldering iron warm up. Test it with the solder to see if it's warm. If the solder quickly melts when it touches the tip or the iron, it's time to solder. I keep my soldering station set to about 650 degrees F.
Touch the tip of the iron to the wire. Hold for about 3 seconds. Now apply the solder to the wire (not to the tip of the soldering iron). If the wire is warm enough the solder will be drawn into it. Once you've tinned a few pieces of wire, try to solder them together. Hold two tinned ends together and apply the tip of the soldering iron. The solder from the two ends should remelt and flow together. You need to apply a bit more solder with larger wires. With the components we're working with, you'll need very little extra solder.
While it sounds like you need 4 hands, the guy who trained me would hold the two ends of the wire in one hand and the soldering iron in the other. He'd have the spool of wire on a table with an end sticking up. Put the iron to the wires, one they're warm move down to the end of solder sticking up. Voila, you've got it. I'm not that nimble, so you'll often find me with a soldering iron sticking out of my mouth. I don't recommend that, though. It's just what works for me.
Get Ready, Go.
I find it easiest to do everything assembly-line fashion. Lay out your LEDs, push button, resistors, and wiring harness for the battery. Tin all legs of the components and the ends of the harness as well as the legs of the button. The wires on the harness are pre-tinned, but you want to be sure that the solder you're using sticks well to the components, so I always resolder them.
Clip the leads on the resistors so they have one long lead and one lead about 1/4" long. Take three of the LEDs and clip the longer lead so that they're about 1/4" long. Solder the short lead of the resistor to the short lead of the LED. You should now have three LEDs with a resistor for one leg. The longer lead is the positive side of the diode, so the resistance is on the side where the power has to flow for the diode to activate.
Now take the other three LEDs and clip the long leads so that they're about 1/4" long. Set all of the electronic components aside and grab the box and your drill.
You'll want to figure out how you want to mount the LEDs. I put one pair in the middle of the box front and the other two pair spaced out about 1/8" away on either side. The spacing really doesn't matter except they do need to be in pairs. Once you think you have it measured out, mark and drill. Drill with small drill bits and go up slowly in size until the LED sticks through, but won't fall through.
I used either a roll of tape or the solder roll as a work stand at this stage. Put the front that you've drilled over your roll of solder. Bend the six 1/4" legs on the LEDs so they stick straight out. Basically it should be about 90 degrees. Put one pair of the LEDs (one with a resistor, one without) in a pair of the holes so that the two bent legs point toward each other. Solder the two bent legs together. You can pull them out and do the next two pair. Be sure and put each pair in their correct holes. If your drilling was a bit off, each pair will have slightly different spacing.
Put all three pair back in with the resistors all on the same side. Bend the leg of one of the end resistors 90 degrees so that it crosses the other two resistors. Solder the bent leg to the two other resistor's legs. Clip the excess legs to where they're about even with the crossing leg.
On the other side, bend one of the legs off of the LED across the other two legs. Solder them together and clip the ends like you just did on the resistor side.
Solder the black battery lead to the non-resistor side. Clip a battery to the end and touch the red wire to the resistor-side wire. It doesn't matter where. Since this is a parallel circuit, the current flow is the same everywhere. All 6 LEDs should light. If they don't, check your solder joints and be sure that everything's still connected.
If nothing lights, flip the battery around and touch it to the connector (yes, backwards). Do they light? If so, you've got the LEDs in backwards. I built about 40 of these this week and I did end up with one pair backwards and one bad solder joint on the battery connector (so not mine, thank goodness).
Ok, remove the battery and set it aside if everything's working ok. Put the end of the box with the LED assembly you just made into the box. What you want to do is mark a spot to drill the hole for the button. What I did was simply solder one of the button contacts directly to the wire running across the resistors. If you prefer, you can solder a wire from one terminal to the resistor-side wire.
Drill the hole, pop the button in (and no, it won't be that simple), solder one contact to the resistor side (or a wire between the two) and then solder the red wire from the battery connector to the other lead of the button. Hook the battery back up and test again. Everything ok? You're done. If not, check your solder joints again.
Soldering takes time and practice. "The bigger the blob, the better the job" is not the truth. It shouldn't take a whole lot of solder to get the job done. At the beginning, you'll use more. As you get more confident and experienced, you'll use less. Also, a tip to test your soldering is to solder two wires together. Pull hard. The joint should not give. If you're using really light gauge wire, it should break, but not at the solder joint. The solder joint will be the strongest part of the wire if done correctly.
The final thing I did was glob a ton of hot glue over everything. This will hold all of your LEDs and the button in place as well as holding the solder joints together if the flashlight gets dropped. Don't glue yourself. It's hot glue.
Position the battery and wires so you can finish assembling the box without crimping the wires. Screw the case together with the provided screws. You're done. You now have a flashlight you can play CSI: YOU with.