What is it?
The NeonMatrix 8x8 matrix uses common neon indicator bulbs to display simple graphics.
They can be chained and controlled directly via SPI.
Operating voltage is about ~70V AC.
The circuit is pretty dangerous, all neon bulbs have high voltages on their legs, the 74HC595 shift registers are floating at mains/AC potential.
Hack: Audio amplifier inverter
To generate a (mostly) safe high voltage neon supply, a 50 Hz mains transformer can be connected (in reverse) to the output of an audio amplifier. By playing a 50 Hz sine tone, the transformer outputs a variable high voltage: