Electronics Fundamentals: Ultraviolet LEDs

Purple LEDs are ultraviolet and they can be used to create a blacklight effect. Though you can make a brighter purple by mixing red and blue on an RGB LED, it won't have the same effect as the ultraviolet lights used in counterfeit bill detectors and for revealing security holograms on credit cards and driver licenses. Let's see how much fun we can have with these ultraviolet LEDs.

Background Theory: Ultraviolet LEDs

LEDs are energy efficient, long lasting and don't produce a lot of excess heat. Ultraviolet spans the electromagnetic spectrum from 400 nm down to about 10 nm. Although an output of around 380 nm would be ideal, these LEDs peak at 400 nm and the range is between 390-440 nm. Serious ultraviolet light is more energetic and has a shorter wavelength. It is invisible to the human eye. Like sunrays, it can burn skin and damage eyes, so shorter wavelength ultraviolet LEDs are more expensive and not as readily available.

The Project - Ultraviolet Light Detector

Ultra Violet Detector Plugging together a pack of LEDs, a 9V battery and some 100Ω resistors on a breadboard shows the impact of the ultraviolet LEDs. They are not so ultra that the light can't be seen, but it does make white and fluorescent colors on clothing really pop. Using it against some credit and identification cards reveals holograms. Also the security thread in some bills can be detected. Now let's make something a little more permanent, portable and... purple? It's up to you to paint the small plastic enclosure the color of your choice.

Since the UV light will only be used for short durations, using a momentary push button switch will suffice. Wiring the LEDs in series as a pair saves from having to use a regulator or dumping unnecessary amounts of current into resistors. Normally one resister per LED is appropriate, but this is a low-use circuit and only uses a couple of LEDs so you can get away with using five, though the kit contains ten.

The Ultraviolet Light Detector Kit

You will need:

Soldering iron
• 7/32" drill or rotary tool

(10) Purple Water Clear T-1 3-4 LED
(10) Resistor Carbon Film 100Ω
(1) 9V Alkaline Battery
(1) Battery Snap
(1) Momentary Push Button Switch
(1) Protoboard
(1) ABS Plastic Project Case

Layout the LEDs at one end of the protoboard making sure you leave room for the battery and the switches body. Wiring can be tedious, so bending the leads and soldering them to the next component can simplify the process. You'll have to be clever with placement, but use the schematic and layout below (Fig. 1, Fig. 2) as reference.

Fig. 1
Fig. 1

Fig. 2
Fig. 2

Arrange the LEDs in a diamond pattern of three, four and three on one edge of the protoboard. The beam angle is pretty narrow at 20° and it will be used at close distances to the object. The protoboard is a pad-per-hole type without bussed traces, so you'll make your own traces for power and ground.

Figure 3 shows that the power comes in from the positive lead of the battery to the power trace connecting one lead of each resistor. Each resistor will connect to the anode of an LED. The cathode will connect to another LED's anode and that LED's cathode will go to a ground trace. The ground trace also runs up to the middle LED cathode in the top row via a bent cathode from the middle row; this LED cathode brings all ground points to one spot.

The black wire from the battery pack is connected just below the power trace, but given the tight space inside the enclosure, it's probably best to attach it somewhere near the top of the board. That would also put it near the grounding point for the LEDs. Connect a piece of wire from one leg of the switch to the grounding point for the LEDs and solder the negative lead from the battery snap to the other leg of the switch (Fig. 4).

Fig. 3
Fig. 3
Fig. 4
Fig. 4

Drill a 7/32" hole in the side of the enclosure for the switch (Fig. 5). Mount the switch with the provided hardware. The battery sits lengthwise in the case and even with the PCB and LEDs biased at the edges, it's a tight fit. Mark an area on the lid to cut for the LEDs. A simple square opening will work, but a diamond shape to profile the LEDs looks better. You can use a 7/32" drill bit to make holes for each LED. Connect the battery and hold the button down to confirm everything works. Close the lid and check your currency or credit cards or whatever else you find.

Fig. 5Fig. 5

Definitions and Concepts

Ultraviolet / UV: Beyond violet in the spectrum, corresponding to light having wavelengths shorter than 4000 angstrom (nm) units.

Fluorescent: strikingly bright, vivid, or glowing; possessing the properties of fluorescence.

Fluorescence: The emission of radiation, especially of visible light, by a substance during exposure to external radiation, as light or x-rays.

Electromagnetic spectrum: The entire spectrum, considered as a continuum, of all kinds of electric, magnetic, and visible radiation, from gamma rays having a wavelength of 0.001 nm to long waves having a wavelength of more than 1 million km.

Visible light spectrum: Visible light is the composition of light that we detect with our eyes as colors from about 400 to 700 nm.

Discussion Questions

In what parameters is ultraviolet light the most safe?

What are some other applications of ultraviolet LEDs?

What would happen if you used infrared (IR) LEDs instead of ultraviolet LEDs?

What are some applications for an IR light?

Where does ultraviolet light fit within the visible light spectrum?