The prompt-first pass
The original build below records how this idea first worked. The durable lesson now targets the M5Stack Stopwatch and lives in the prompt, behavior contract, generalized tests, and hand-review checklist. Generated firmware is evidence for those sources, not the source itself.
The internet arc opens with a deliberate softball: one GET, one JSON parse, one screen. After twenty-one days of timing grammars and DNS liars, the first networked lesson should be about the shape of a fetch-and-render app, not about fighting a vendor.
Open-Meteo is the whole reason this works as day one of the arc. No key, no account, no OAuth dance — the API-key lesson is day 27’s job, and weather gets to be pure.
One URL is the API lesson
Everything the screen shows rides a single request: current conditions, twelve hourly temperatures, the day’s range, all selected by query parameters.
Reading that URL is reading the app’s data model, which is the underrated virtue of well-designed REST — the contract fits in a snprintf.
The parse streams. ArduinoJson reads straight off the HTTP connection, so the board never buffers the full payload — a habit worth forming before the lessons where payloads stop being polite.
The board without a browser
Day 22’s captive portal assumed a touchscreen and a phone. The StopWatch has pushers and a serial port, and pretending otherwise would have meant porting a web portal to a device with no way to show one.
So this board’s portal is the serial line: wifi SSID PASS, persisted to NVS, same one-home-for-secrets rule, different door.
The rule survived the hardware change; the mechanism adapted.
That’s the sign the day-21 rule was the right abstraction — where secrets live, not how they arrive.
Rounds want radial data
The 12-hour forecast could have been a bar chart, awkwardly rectangular on a circle. Instead it’s a bezel: twelve dots sweeping 270°, radius riding the temperature. The round face keeps winning these design moments — data that has a cycle (hours, compass points, beats per bar) maps onto the rim like it was always meant to live there.
And the ten-minute refresh is a design position too: free APIs are a commons, and a weather station that polls every ten seconds is a tragedy-of-the-commons tutorial. B exists for impatience; the timer exists for citizenship.
What we learned
- Start the arc keyless. The first network lesson should teach fetch-parse-render, not account management.
- Stream the parse. Never buffer a payload you can read in flight.
- Abstractions survive hardware; mechanisms don’t. Secrets-in-NVS crossed boards; the web portal didn’t need to.
- Cyclic data belongs on the rim. Round displays turn hours into bezels for free.