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Ultraviolet LED Flashlight Tutorial

Yellow highlighter and the UV light in a dark room.

This is a project that I completed around Valentine's Day of 2010. I remember shopping at the Shack, and I saw some ultraviolet LEDs. I just had to have them so I bought two bags of two (RS P/N 276-014 - $1.79). I didn't know how this project would turn out, but I just had a feeling that it was going to be cool. When you are done with this project, get you some UV yellow glasses like a criminal investigator. At night, things come alive in hotel rooms, bathrooms, and other crazy places. The possiblities are endless...

  1. Cut a 1-1/2" piece of PVC pipe to 1.5"-2" in length. Set it aside.
  2. I had to figure out how I wanted to lay them out. I decided on a square pattern on the end of a 2" flat PVC plug. The square shape had 11/16" sides. The LED holes were drilled 11/16" apart. Use a 1/4" drill bit (or the drill size that corresponds to your LED holder) and drill your holes. Really simple. I just placed the LEDs at the corners of the square.
  3. Connect the LED legs so that they are inline. The cathode lead is the one that is next to the flattened portion of the LED body. Leave the anode of LED #1 sticking out straight (don't mess with it). Connect the cathode of LED #1 to the anode of LED #2. Connect the cathode of LED #2 to the anode of LED #3. Cathode of LED #3 connects to anode of LED #4. Leave the cathode of LED #4 sticking up.
  4. We need to know what size resistor to use. The forward current of the UV LED is 20mA. We will use 3.2 volts as the forward voltage. Since we are connecting the LEDs in series, the total forward voltage is 4*3.2=12.8 volts. Our voltage source will be two 9-volt batteries, which gives us 18 volts. The total resistance (R=V/I) will be equal to the voltage source minus voltage drop divided by the total current that we want. R=(18v-12.8v)/.020A=260 ohms. The nearest standard value (what the Shack sells) is 270 ohms.
  5. Solder the resistor to the anode of the anode that we left alone in step 3. Solder a red 8" wire to the other end of the resistor. Solder the black wire of one battery snap to the cathode of LED #4 that we left alone in step 3. Solder the red end of that battery snap to the black end of the other battery snap.
  6. Drill the threaded end cap to accomodate the switch. The size of the hole depends on what switch you purchase. I had to do a little filing to get my switch to fit because I didn't have a drill bit big enough. Install the switch into the endcap.
  7. Take the 2" end cap and thread the wires into the 1-1/2" to 2" adapter. Lightly press the 2" end cap into the adapter. Take the 1.5"-2" length of pvc pipe that you cut in step 1 and press it into the other end of the adapter. Make sure the wires are still sticking out. Thread the wires and battery snaps through the 1-1/2" slip to threads adapter and lightly press it onto the other end of the PVC pipe.
  8. Solder the red wire from the LED and resistor to one tab of the SPST switch. Solder the red wire from the battery snap to the other tab of the switch. At this point, you are almost done. [Connect two 9-volt batteries and flip the switch. If the LEDs do not light, check your circuit.]
  9. If your lamp is working, we need to secure the LEDs and wires with some hot glue. you can skip this step if you are confident that your LEDs won't come loose. Higher-end LED holders may not require this. I used cheap, plastic LED holders so I needed to secure them. I simply shot some hot glue onto the LEDs and the connecting wires. Then I shot some hot glue onto the switch tabs so that the wires wouldn't pull off in case of rough handling.
  10. At this point you can press the pieces firmly together. Insert the batteries into the PVC pipe and press the rest together. Screw the end cap on. You're done.
uv flashlight end cap. Open view of uv flashlight. Finished UV light.

Parts List:

Schematic:

UV flashlight Schematic.

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