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 pitch for this pair of lessons was made a month ago: build the walkie-talkie twice, changing only the transport, and let the diff teach. The diff came in smaller than promised.
What actually changed
On the transmitter: esp_now_send(BROADCAST, ...) became ws.sendBIN(...), plus a connect state machine.
On the receiver: the ESP-NOW callback became a WebSocket event handler.
The frame struct, the mic pacing, the jitter buffer, the squelch, the channel dial, the screens — byte-for-byte day 24.
That’s the lesson working as designed: when the frame is the contract, the carrier underneath is an implementation detail. Day 21 proved it across vendors; day 25 proves it across a planet.
The backend is sixty lines
The relay had candidates: public MQTT (higher latency, someone else’s broker), Mumble (real VoIP, someone else’s client), or a from-scratch WebSocket fan-out. The from-scratch option won because it fits in a screen of JavaScript: accept a socket, remember it, forward binary messages to the other sockets in the room.
Durable Objects make the room primitive free: idFromName("7") conjures a tiny stateful server for channel 7, living wherever Cloudflare finds convenient, holding nothing but a Set of sockets.
Twenty-two channels by convention.
Infinity by implementation.
Rooms are URLs, and changing channels is rejoining a different one.
The relay was verified before any firmware touched it — two host-side WebSocket clients, four bytes in, four bytes out the other side, half a second around the world and back. Test the new organ before transplanting it.
Jitter is jitter
The prediction was that internet transport would need a different audio strategy than radio. It didn’t. Internet jitter and RF jitter are the same disease — frames arrive unevenly, the speaker consumes evenly — and day 24’s 60 ms prebuffer treats both without modification. The only concession to distance is latency the buffer never sees: the relay adds real milliseconds, but a walkie-talkie conversation carries half-duplex rhythm anyway, and PTT culture absorbs delay that would kill a phone call.
Provisioning paid out
This is the first day that needed day 22 and day 23 simultaneously — two boards, two provisioning styles, one rule.
The RX lifts portal.c verbatim; the TX reads day 23’s NVS namespace so a board provisioned for weather is already provisioned for voice.
Secrets have one home, and the home has compound interest.
What we learned
- Transports are swappable when the frame is the contract. The audio pipeline crossed from RF to internet untouched.
- Rooms are URLs. Durable Objects turn channel numbers into servers with one
idFromName. - Jitter is jitter. One prebuffer treats radio and internet alike.
- Verify the organ before the transplant. The relay proved itself host-to-host before any firmware depended on it.