ESP32 Reference

ESP32-family GPIO is 3.3 V class. Do not drive an ESP32 input from a 5 V Uno output or a 5 V sensor signal without level shifting. Many modules accept 5 V on USB or VIN while the chip I/O remains 3.3 V.

Design LED pin current around 12 mA or less. NeoPixel / WS2812 strips need an external 5 V supply and common GND for more than a few LEDs - do not power a long string from the ESP32 3.3 V pin.

Arduino-ESP32 often exposes 12-bit analogRead (0-4095). That is not the same scale as Uno 0-1023. The ADC is useful for pots and relative sensing; it is not a precision lab meter. Attenuation settings change the usable voltage range. Nonlinearity is worse near the rails.

Classic ESP32 DAC exists on GPIO 25 and 26. Many C-series chips have no DAC - check the datasheet.

Station (STA): the ESP32 joins your router. After WL_CONNECTED, WiFi.localIP() is the LAN address (usually from DHCP); gatewayIP() is typically the router; RSSI() is signal strength in dBm (more negative is weaker).

Access Point (AP / SoftAP): the ESP32 creates a hotspot for phones to join (handy for first-time setup). softAPIP() is often 192.168.4.1 - a different address from the STA LAN IP.

Classic ESP32 is 2.4 GHz only. Use Serial.begin(115200). Keep WiFi credentials as placeholders - never commit real passwords to shared repos.

{
  "room": "Lab room",
  "level": 2048,
  "alert": true,
  "sensor": {
    "temp_c": 21.5,
    "humid": 48
  },
  "tags": ["lab", "bench"]
}

JSON (JavaScript Object Notation) is structured text used by almost every web API and many MQTT payloads. If you can read the braces and quotes, you can invent your own documents for a room node, weather reply or heater command.

ArduinoJson by Benoit Blanchon (JsonDocument in v7) turns that text into values your sketch can test. Workflow: HTTP status 200 (or a known MQTT string) -> deserializeJson -> check DeserializationError -> read fields with optional | defaults -> then act. Print the raw body when a path looks wrong.

// Nested object
float t = doc["sensor"]["temp_c"] | -999.0;

// Array of objects
// { "slides": [ { "title": "Wake up" }, { "title": "..." } ] }
const char* title = doc["slides"][0]["title"] | "?";

// Hand-built (small)
String body = "{\"temp\":";
body += String(t, 1);
body += ",\"heater\":";
body += heaterOn ? "true" : "false";
body += "}";
// -> {"temp":21.5,"heater":false}

// ArduinoJson build (safer for growth)
JsonDocument out;
out["temp"] = t;
out["heater"] = heaterOn;
serializeJson(out, Serial);

Build it: ESP32 Lesson 10 · ESP32 Lesson 11 · ESP32 Lesson 17

WebServer on port 80 maps URL paths to handler functions (server.on). loop must call server.handleClient() often - a long delay blocks the page.

A 303 redirect after an action (for example /led/on) sends the browser back to / so the page refreshes. Raw string literals R"=====( ... )=====" make HTML easier to embed without escaping every quote.

BLE peripheral advertises; phone (central) connects and uses GATT. A service groups characteristics; each characteristic is a readable/writable value identified by a UUID. Properties include READ, WRITE and NOTIFY. Labs often use custom 128-bit UUIDs.

Prefer BLE for short-range phone control without a router. Prefer Wi-Fi for LAN browsers, MQTT and cloud HTTP. Classic Bluetooth is legacy for new ESP32 labs. ESP32-P4 needs a companion radio for BLE.

Deep sleep powers down most of the chip; normal SRAM is lost and wake typically restarts into setup(). Arm a timer with esp_sleep_enable_timer_wakeup(us) then call esp_deep_sleep_start(). Other wake sources include EXT0/EXT1 GPIOs and touch on supported pins.

RTC_DATA_ATTR keeps a variable across deep sleep but not across power loss - use Preferences/NVS for durable settings. DevKit USB-UART chips and LEDs can hide true sleep current on a USB meter.

Arduino-ESP32 runs on FreeRTOS. setup()/loop() usually run on core 1; WiFi/BT work often uses core 0. Create workers with xTaskCreatePinnedToCore; wait with vTaskDelay(pdMS_TO_TICKS(ms)). Start stack sizes around 2048 and raise if the board reboots under load.

Shared globals without protection cause race conditions - use queues or mutexes when tasks share data. Do not add a task for every blink; millis in one loop is often enough.

ArduinoOTA: after STA connect, call ArduinoOTA.begin() and ArduinoOTA.handle() often in loop. First flash is USB; later uploads can use Tools → Port → network hostname. Set a unique hostname and an OTA password outside trusted lab LANs. Keep USB recovery available.

HTTP OTA (device pulls an image URL) is the usual field-fleet pattern - different from IDE ArduinoOTA.

Addressable LEDs chain data (DIN to DOUT). Full white can draw about 60 mA per LED - budget the 5 V supply. Share GND with the ESP32. Put a series resistor on the data line (~330-470 ohm) and a bulk capacitor on 5 V near the strip. Call show() after you change pixel colours. Library for this course: Adafruit NeoPixel.