If you've ever played with a black light, you've noticed that some substances glow brightly under these lights. In this case, we'll be using tonic water because it contains small amounts of quinine which glows a bright blue under ultraviolet.
Internet research shows that Quinine glows best under ultraviolet light at the wavelength of 350 nanometers (nm).
As far as I can tell, LEDs are not available at that wavelength, but quinine will still glow from nearby wavelengths. An ebay search turned up 355nm LEDs for $65 each (too expensive!) and 365nm LEDs for about $2 each, including shipping. The most time consuming part of this project for me was waiting the many weeks for ten 365nm LEDs to arrive from China. If you want to do this project, I suggest you go ahead and order a batch of these now.
- container with clear or lightly colored sides, and preferably a thin plastic lid
- Arduino to power your LEDs
- bottle of tonic water
- ultraviolet LEDs ( 365nm for choice of cost and quinine sensitivity)
- wires to connect LEDs to Arduino
Step 1: Punch Holes in the Lid of the Container
I used a hammer and a nail to punch pairs of holes in the plastic lid of my inexpensive container.
As long as the distance between the holes is somewhat close to the distance between the legs of the LEDs, it should work fine.
Step 2: Insert LEDs
Make sure to insert all of the LEDs inside the lid, with their negative legs facing in the same direction.
LEDs have a polarity, electricity can only flow through them in one direction. Wiring this together is much simpler when you can use a single alligator clip on either side to test your nightlight.
You can see here that I've used single alligator clips on each side to power these LEDs.
I'm also using a 100ohm resistor to limit the current flowing between the five volt pin of my Arduino and the positive legs of these LEDs.
The other alligator clip leads back to one of the ground pins on my Arduino ( GND ).
Step 3: Fill With Tonic Water, Connect Wires
My one bottle of tonic water did not completely fill my container, but it works just fine.
My LEDs did not all end up facing in the same direction, but that ended up spreading out the glow, so that worked out too.
The LEDs I purchased want between 3.6 and 3.8 volts, but according to my electronics friends, putting a 100 ohm resistor in the circuit will restrict the current so that it'll work just fine.
I first tested it by powering the positive pins of the LEDs from the five volt pin of my Arduino, but I later switched to using one of the pulse width modulation pins so I could vary the brightness.
While this first prototype uses a FreakLabs Freakduino for pulse width modulation, I'll be putting either an AdaFruit FLORA or a SparkFun LilyPad on top of the lid inside an enclosure.