Hybrid Relay/Microcontroller Machine

The Hybrid Tic-Tac-Toe Machine

In addition to the Drof Computox and uComputox, the Museum of Computing exhibit at Retrofest 2026 included a hybrid relay / microcontroller noughts-and-crosses machine.  This employed the relay-based latching and lockout circuit described earlier to accept input from a keypad and to drive a bi-colour LED display.  The state of the game was monitored using an Blackpill STM32 development board which ran the same move selection logic as the Drof Computox.  As such, the hybrid can be used to play games of tic-tac-toe.  The STM32 is overkill for the task, but it has a ton of pins, a small form factor, is 5V tolerant, and most importantly was in my parts box.

The hybrid machine represents the next step towards constructing a full relay replica of the Computox for Museum visitors to use.  It builds on the experience of the earlier input latching circuit, expanding its scope and trialing new aspects.  Whereas the replica latching circuit only mimics two tic-tac-toe squares, the hybrid models all nine.  Wiring the large socketed relays of the latching circuit was a pain, so the decision was made to switch to smaller PCB compatible relays.  This allowed the machine to be put together on prototyping breadboard, with a view to the final design being implemented on a PCB.  The new relays are cheaper than those used earlier, but lack the clear shell, exposed workings and loud, satisfying click of the originals, so some further selection work is required.  Each relay was provided with an LED to indicate when it was active.  This was found to work well, creating a blinkenlight effect that will be carried forward.  Because some of the relays had to interact with the microcontroller, it was necessary to fit protection diodes to their coils and drive them with transistors.  It is anticipated that these will be removed in the final design, bringing it closer to the Computox.

Whereas the uComputox kit produced for Retrofest uses switch and LED matrices to minimise microcontroller pin count, the hybrid machine makes use of a common controller and display PCB that can either be populated and wired in the same way as the controller or display of the Computox.  These are simple and reliable, and will be used in future prototypes.

Overall, the hybrid machine was found to work reasonably well at the exhibition, with many visitors playing a couple of games against it.  However, the relay leads are not quite long enough for the prototyping boards to grip them well, and require periodic reseating.  Also, the machine was powered by USB through the Blackpill.  This, combined with the relatively long USB lead required, caused the 5V rail to drop as more relays and LEDs become active late in the game.  This would sometimes result in the player relay failing to latch the final move of a drawn game.  That said, the machine worked pretty reliably for something built on a breadboard, and the learning acquired is why we make prototypes!