Hide in Plain Sight

Ultraviolet marker pens use ink which lacks color pigment, so it doesn’t leave a mark visible to the naked eye under normal lighting conditions. However, when illuminated with a UV light source (sometimes called a blacklight), it fluoresces brightly. This is easiest to see in dim lighting conditions.

A nice, compact source for UV light is a UV/Purple LED which emits light in the 400nm wavelength. This is right at the edge of the visible light spectrum, so you will see a purple glow cast from it, but it is a long enough wave to excite the fluorescent material and reveal the secret message.

Build the UV Light Decoder

With a forward voltage of 3.4V DC, you can power the UV light very well with the 3.3V output of your Gemma M0. No resistor required! We’ll connect the short, cathode leg of the LED to the GND pad and the long, anode leg to the D2 pad.

Coil each leg so that it will fit an M3 screw as shown here -- be sure to pay attention to the which is the long, anode leg so you can connect them properly in the next step.

  • Fit the LED leg coils over the respective pad holes on the Gemma M0 -- long, anode leg to D2, short, cathode leg to GND
  • Push the screws through the pads and LED coils as shown
  • Thread the nuts onto the screws

Next, we’ll code the Gemma M0 to light up the LED when we press the D0 capacitive touch pad!

This guide was first published on Mar 16, 2018. It was last updated on Mar 16, 2018.
This page (Building the UV Reader) was last updated on Oct 24, 2020.