Hallowing ships with a pre-loaded example of a human eye that looks around, blinks and reacts to light.
If you want to put the original spooky eye demo back on your Hallowing, enter bootloader mode by double-clicking the Reset button and then drag this UF2 file over onto HALLOWBOOT:
There’s a few customized variants as well, such as a fiery dragon eye:
A brown, animal-ish eye with no visible sclera:
A psychedelic eye from our Eye of Newt guide:
And a Terminator-inspired robotic eye:
The software controlling Hallowing’s eye is extensively customizable. This requires some familiarity with the Arduino IDE and, depending on the extent of customizations you have in mind, perhaps some image editing and using Python scripts on the command line.
This is all explained in the “Electronic Animated Eyes using Teensy 3.1/3.2” guide (despite the name, it also works on various Adafruit “M0” and “M4” boards as well, including the Hallowing).
Most of the code there will automatically work on the Hallowing hardware, such as the display and light sensor. Other Hallowing-specific features, such as the capacitive touch pads, are not handled by the code…but could be added if you’ve done some Arduino programming before.