Chaos on Your Desktop

Build a Color-Pulsing Light Sphere Using a PIC or Raspberry Pi Pico MCU

- By Dev Gualtieri

Longtime readers of Circuit Cellar know that Dev enjoys creating desktop novelties. His current project is a glowing sphere that pulses with a random sequence of multicolored lights. He designed the circuit with a PIC 12F675 MCU, and provides additional circuitry and MicroPython code for construction using a Raspberry Pi Pico. It also should be easy to adapt the circuit for an Arduino.

Plasma globes were popular desktop novelty items in the 1980s. They were plasma discharge lamps created as clear glass spheres containing a mixture of the noble gases—typically neon, krypton, and xenon—excited by a central, high-voltage electrode. (In this context, “excite” means stripping an electron from the neutral gas atoms, creating charged particles called ions.) The high voltage is typically a 35kHz alternating current at about 2.5-5kV. While the voltage is quite high, safety is enhanced by the very low currents needed to excite the plasma. As for AC versus DC, alternating current is required because the ground return is through a capacitance.

The high voltage excites a plasma discharge from the central electrode to the surface of the sphere. This discharge changes path randomly, but it’s directed to a small area of the sphere when touched by your finger or the palm of your hand. Inexpensive versions of plasma globes are still available for purchase.

The term “chaos” is colloquially used to describe randomness, but there's a subtle difference between a chaotic number sequence and the pseudorandom number sequences obtained through computation. In principle, pseudorandom numbers given by a computer are inherently predictable, since they are derived from known algorithms. In contrast, chaotic number sequences are not predictable, and they are strongly influenced by the initial state of the process that created them.