Gallium Photonic Computer Series 74

Another one? I am flattered at having a waitlist, while wanting to keep things interesting. (NB pre-production pics here - this page will be updated)

The Series 74 is an evolution of the purity of the original BC-01, rather than the more feature-rich (er, relatively) MKII.

Quick timeline recap:

• Original BC-01 - initially a one-off; it's sort of a joke because it has no actual computing ability at all, it's just lots of self-flashing LEDs which rapidly drift out of sync. But they need hand-soldering, so I called it a day after doing approx 10,000 LEDs.
• MKII - new design with SMD LEDs, with a microcontroller emulating the original.
• Series 74 - an exercise to go back to the simplicity/purity of the original; use all SMD components but still not-actually-a-computer.

Various physical and virtual experiments ensued... the classic route seems to be a linear feedback shift register (LFSR), used by a number of similar props in old TV programmes etc. But this has a regularity to it - the whole matrix changes in sync, which isn't as pleasing as the more organic twinkly nature of the original GPC.

Finally settled using Schmitt trigger inverters - each LED is driven by an inverter/resistor/capacitor network; chained together in 9s to form a polyphase ring oscillator. This should in theory produce a very boring pattern - but like the original, component variance causes everything to drift out of sync almost immediately. To make it more interesting, I then added multiplexers to have switchable topology; changing each group of three loops between independent and linked together in a single 27-stage loop.

The last task, then, was to actually make viable PCBs - 81 LEDs requires a total of 18 ICs per board - my first time to design with tiny 0402 components.

The end result is working well - IMO I like the design, but a subjective exercise for the reader as to whether its ultimately an upgrade or a downgrade...