Apps meet the physical world: sensors, actuators and IoT

Until now a Reactive app worked on data typed in, imported, or fetched from the open-data service. From today it can listen to the physical world: a temperature sensor publishing over MQTT, a Bluetooth scale, an ESP32 plugged into USB, an NFC tag, the phone’s accelerometer. And it can command it too: a switch in the document that turns on a real light. As always, everything runs in the browser: no server to install, no account, data stays on the device.

The principle: readings are data like any other

Every IoT directive pours its readings into the reactive store — a key for the latest value, a collection for the history — and from there everything Reactive already does applies: tables, aggregations, conditionals, Python charts, even the AI directives that summarize or answer questions about the data. A thermometer becomes a chart in three lines of text:

::mqtt-sub[temp]{topic="home/living/temp" into="readings"}

Now: :value[temp] °C — average :avg[readings]{field="value"}

:::python{data="readings"}
import matplotlib.pyplot as plt
plt.plot([float(r["value"]) for r in data["readings"]])
:::

MQTT: the universal channel

MQTT is the lingua franca of IoT — Home Assistant, Tasmota, ESPHome and virtually every microcontroller speak it. Three directives bring it into documents, in every browser:

• ::mqtt-sub subscribes a topic: each message updates a key and/or is appended to a history collection (JSON payloads become row fields).
• ::mqtt-pub publishes on click, with a fixed value or one taken from a field of the document.
• ::mqtt-bind binds a key to a topic both ways: a ::toggle in the document drives the actuator, and the state published by the device realigns the toggle.

The broker is declared once in the frontmatter (mqttServer: wss://...) and only endpoints travel in the document — never credentials — the same principle as sync keys.

Bluetooth, serial, NFC: the device in front of you

For nearby objects you don’t even need a broker:

• ::ble connects to a Bluetooth LE device (battery, heart rate, environmental sensors…) and receives live notifications; ::ble-write sends commands — an LED on an ESP32 turns on from a line of Markdown.
• ::serial reads the USB port: the lines an Arduino prints with Serial.println are appended to a collection (JSON lines become fields), ::serial-send talks back. The perfect bridge between an electronics classroom and a living spreadsheet.
• ::nfc reads NFC tags with the phone: surveys, inventories, attendance by tapping the phone on a tag.
• ::motion and ::ambient-light use the phone’s sensors: accelerometer and ambient light, live in the store.

The browser acts as the guarantor: Bluetooth, serial and NFC start only from a user click, never automatically.

What about REST? ::iot-poll

For devices exposing an HTTP API (Shelly, Tasmota, or any JSON service) there’s ::iot-poll: it polls the endpoint at regular intervals and keeps the collection aligned with the current state. One practical caveat: the app is served over HTTPS, so local-network http:// endpoints are blocked by the browser — for local traffic the MQTT broker remains the main road.

What you can build

• Home dashboard: temperature and humidity over MQTT, toggles driving the lights, a Python chart of the daily trend — shared with the family through multi-user sync.
• Classroom lab: every desk with an ESP32 over USB, measurements appending into a table, live averages and charts; the document IS the lab report.
• Inventory with a phone: NFC tags on the shelves, ::nfc into a collection, ::ai-classify categorizing the items.
• Workshop monitoring: ::iot-poll on smart plugs, thresholds with :::show, an AI summary of the week’s consumption.

As with the rest of the platform, the editor helps: the new directives are in the autocomplete (with attributes and descriptions) and the AI assistant knows them and suggests them when you describe an app that talks to devices.

All the details are in the syntax guide and the tag reference.