For expediency, we’ll make an assumption that only one-way communication is needed. In tournament games like chess, the current state of the board is projected for all to see. An observer accomplice in the spectator gallery (or off-site if streamed) could do the work of feeding game state to an AI engine, then relaying moves to the player. Technically there’s nothing preventing input and two-way communication for solo use, but this muddies the waters for testing the core idea.

An Adafruit QT Py ESP32-S2 provides the brains. Inexpensive, incredibly tiny, and has built-in WiFi. This can communicate with a mobile hotspot (e.g. cell phone with “WiFi tethering” feature) carried by the accomplice theorized above.

How to communicate to the player? A graphical display is right out, as are visible LEDs and audible speakers. It must be silent, but deadly to one’s opponent. So we’ll use the same sort of tiny vibration motor that’s in your mobile phone. A small driver board accompanies this, as the motor requires more current than can be driven directly from a microcontroller pin.

Such a receiver needs to be discreet…watches or jewelry are too conspicuous (and might not be allowed by tournament rules). It must be concealable, perhaps inside a shoe or under one’s armpit. These body parts are naturally prone to sweat, suggesting some kind of moisture-proof enclosure.

Social Media Internet Cops keep DEMANDING that we warn people this doesn't have a flared base. We don't know what they are imagining people are going to do with this project???

These soda bottle preforms were left over from a prior project. They’re waterproof and practically indestructible…they’ve taken a pounding and we’ve never wrecked ’em. The smooth shape glides easily into…a back pocket. Similar capsules can be found on Amazon, eBay, etc.


Here’s a schematic view of the parts laid out for clarity. In physical reality, the microcontroller and battery charger boards are soldered back-to-back with headers to all pins. The motor controller has identical connectors on either end…it doesn’t matter which way you stick it in.

And the actual physical circuit. Battery wires are doubled back to fit all parts down the tube:

The interior of the tube is tapered slightly, and it was necessary to sand about 1/8" width from the motor driver to make it fit down in the narrow end. Best done on the edge with the motor connections, as the other edge sits close to a PCB trace.

The vibration motor is taped to the haptic controller board, and some craft foam is inserted alongside to keep these firmly pressed against the tube body to better conduct the vibration.

A 100 mm STEMMA cable gives enough slack that the motor and controller can stay put while other parts are removable to access the power switch or for charging and uploading code.

Once capped, the whole circuit is well protected from the elements!

If expanding on this project to add outside sensor or tactile inputs, one could incorporate a cable gland to maintain a tight seal.

This guide was first published on Oct 05, 2022. It was last updated on Sep 29, 2022.

This page (Hardware) was last updated on Sep 29, 2022.

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