Introduction: Glowing Elixir Using UV LEDs

About: There are some things you should just NEVER do.....

Flask with Glowing Liquid - Glows on Demand (turn it on & turn it off)... And you can Safely Drink It too!

This Erlenmeyer Flask (standard equipment for any mad scientist) contains an eerily glowing liquid that sloshes back and forth just begging to be sampled. Go ahead and take a swig... Be daring and see what happens!

This fun flask is not magic and the liquid is not radioactive but glows because the stopper conceals 2 Ultraviolet LEDs and 2 Lithium coin cell batteries. This Halloween prop makes use of the fact that Tonic Water contains quinine and quinine glows under ultraviolet light.

Shine UV LEDs down into the tonic water and voilà a fiendish glowing elixir ready to share with your next victim!


1 - 3D printed custom stopper

1- CR2032 Coin Cell Battery Holder

2 - UV LEDs; 395 nm T1 (3mm dia.; 3.2V forward V drop, 20mA max current)

2 - 150 ohm 1/4 W resistors

2 - CR2016 Lithium Coin Cells

1 - Miniature Push-on, Push-off switch (taken from a mini projector from dollar tree, orange thing in photo)

1 - Short length of small gauge hookup wire

1 - Erlenmeyer Flask, 1000ml (1L)

1 - 'Straw' 4.7mm OD, 270mm long, acrylic tubing (Optional - Only if you want to imbibe)

1 - Tonic water, 1 Liter (Available at your local grocery store. Boy this stuff tastes bitter; it's the quinine!)

1 - Black Electrical Tape

1 - Heat Shrink tubing (Short length; To go over the switch actuator and stick proud of the stopper)

1- Hot Glue Gun and glue stick

1 - Soldering Iron and Solder

Step 1: 3D Print Stopper

I designed the stopper in Fusion 360. The stopper is patterned after a rubber stopper I have for the flask. The stopper is designed for a flask with an opening diameter of 32.7mm.

I had just enough room to fit the LEDs, resistors, battery holder and batteries, and switch inside. I did lengthen the stopper just a bit compared to it's rubber counterpart so that it would hide the coin cells better.

Features I included in the design:

1) Compartments for location of switch and battery holder.

2) Cylinder for straw to pass through in order to keep wires out of the way of the straw.

3) Four standoffs in the switch compartment to space the switch away from the top of the stopper to allow room for the switch actuator to travel up and down.

4) Pillars for LED mounting in order to get LEDs located closer to lip of the stopper and allow the LEDs to have a wide beam angle of UV light and to get the LEDs closer to the tonic water.

5) Groves in the LED Pillars for the LED lead wires.

6) Pass through slots in the battery holder compartment walls to allow lead wires to get from one side of the stopper to the other.

7) Slots in the battery holder compartment wall for the + battery holder tabs to be assessible for connections.

The stopper was printed in PLA with no supports. I did use a brim, but probably didn't need to. I printed the stopper in black, but also printed a sample in gray so that the details could more easily be seen. This came in handy as it was very hard to see the slots for wires and all the components inside the black stopper during assembly. Using the gray sample as reference helped immensely.

I have attached the .stl file of the stopper.

Step 2: Coin Cell Batteries and Holder

I thought I could get away with a single CR2032 lithium cell (3V) but I was wrong. The UV LEDs needed more than 3V to operate at full brightness. I took advantage of the fact that my coin cell battery holder, in addition to holding a single CR2032 cell, could hold 2 CR2016 cells.

Coin cell numbering lets me know that a CR2016 cell is half as thick as a CR2032 cell, so two CR2016s will fit.

CR2016 coin cell nomenclature:

C - Lithium

R - Round

20 - Diameter in millimeters

16 - Thickness in tenths of millimeters: 16 tenths, or 1.6mm

So a CR2032 is 3.2mm thick and two CR2016 cells (1.6mm each) combined are the same thickness and will fit in the CR2032 holder. This gives me 3V x 2 in series = 6V.

Step 3: Pre-Assemble the Electronics

The space inside the stopper is very cramped. I chose to preassemble the electronics as it was really the only way to approach it. I used the gray sample to test fit components.

See schematic. The two 150 ohm current limiting resistors allow each LED to operate at just under 20 mA when powered by the 6V provided by the two lithium coin cells arranged in series.

I laid out the components and cut the resistor and LED lead wires to the appropriate lengths (except I misjudged the top right resistor and its lead needs to be longer than shown.)

I then tack soldered the parts together.

Step 4: Install Electronics Into Stopper

After having assembled the electronics I carefully lined up the electronics assembly with the corresponding openings and spaces in the stopper. While pushing wires and components into alignment with a small screwdriver, pushed all the components down into the stopper.

Apply some hot glue to the back of the switch to help hold it in place.

I installed two CR2016 coin cells. These cells are situated so that they can be grasped with the fingers for easy replacement.

Step 5: Wrap Stopper With Tape

To help the stopper feel like a rubber stopper I wrapped the bottom of the 3D printed stopper with two layers of black electrical tape so that there would be some give to the stopper and grip on the glass wall of the flask. It does not do much, but still feels better than hard plastic on glass.

I installed a short length of heat shrink tubing over the post of the switch actuator so that it just peeked above the surface of the stopper. I tested the switch. Push-on, Push-off. Worked great!

Step 6: Fill Flask With Tonic Water

Tonic water does not taste great but it glows very nicely. Fill flask with a generous amount.

You can drink it if you can stand the taste. ;-) And challenge your 'friends' to drink it!

With the pandemic perhaps it is better to pour it into a cup for them to try.

Step 7: Install Stopper in Flask and Switch On

Cap the flask with the stopper. Install the straw if desired.

Push down on the switch and light up that mad scientist elixir.

Enjoy! (Cue foreboding maniacal laugh!)

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