ESP32 Cheat Sheet · ESP32 Starter Kit

1 · ESP32 Quick Reference

30-pin DevKitC Power Pins

Pin	Label	Notes
1, 2	3V3	3.3 V regulated output, max 1200 mA total (board + peripherals)
3, 4, 5, 8, 9, 28, 29, 30	GND	Ground, all connected together
7	5V	5 V input (via USB or VIN) or output when USB powered
6	VIN	6–20 V raw input, feeds the onboard regulator to 3.3 V

Key GPIO Restrictions

GPIOs	Restriction
6, 7, 8, 9, 10, 11	Flash pins. Connected to the onboard SPI flash. Do not use as general I/O.
34, 35, 36, 39	Input-only. No `digitalWrite()`, no `INPUT_PULLUP` (external pull-up needed).
0, 2, 4, 12–15, 25–27	ADC2 channels. `analogRead()` fails when Wi-Fi is active. Use ADC1 (GPIO 32–39) with Wi-Fi.
2	Strapping pin. High = boot from flash. Bootloader prints to this pin.
12	Strapping pin. Affects flash voltage at boot. Avoid high at startup.
15	Strapping pin. Controls JTAG at boot.

I²C & SPI Bus Defaults

Bus	Default Pins	Notes
I²C SDA	GPIO 21	Changeable with `Wire.begin(SDA, SCL)`
I²C SCL	GPIO 22
SPI MOSI	GPIO 23	Used by SD card, displays, etc.
SPI MISO	GPIO 19
SPI SCK	GPIO 18
SPI SS	GPIO 5	Chip select, active low

2 · GPIO & Digital I/O

pinMode(pin, INPUT / OUTPUT / INPUT_PULLUP);
digitalRead(pin);     // returns HIGH or LOW
digitalWrite(pin, HIGH / LOW);

Function	What it does	Where to put it
`pinMode(pin, OUTPUT)`	Configures the pin to drive a load	`setup()` once
`pinMode(pin, INPUT_PULLUP)`	Enables the internal 45 kΩ pull-up	`setup()` once
`digitalRead(pin)`	Returns `HIGH` (≥1.7 V) or `LOW`	`loop()`
`digitalWrite(pin, HIGH)`	Drives the pin to 3.3 V	`loop()`

INPUT_PULLUP: The button reads LOW when pressed, HIGH when released. Wiring the switch between the GPIO and GND (no external resistor) is the standard pattern.

3 · Analog (ADC)

int value = analogRead(pin);        // 0–4095 (12-bit)
float voltage = value * 3.3 / 4095.0;
// Note: analogRead on ADC2 (GPIO 0,2,4,12-15,25-27) fails when Wi-Fi is active

ADC	GPIO Pins	Safe with Wi-Fi?
ADC1	32, 33, 34, 35, 36, 39	Yes
ADC2	0, 2, 4, 12, 13, 14, 15, 25, 26, 27	No

ADC2 + Wi-Fi: When the Wi-Fi radio is active, ADC2 returns 0 regardless of the actual voltage. Use ADC1 pins (GPIO 32–39) for any analog reading that will run alongside Wi-Fi.

4 · PWM (LED Control)

ledcAttach(pin, freq, resolution);  // ESP32 core 3.x API
ledcWrite(pin, duty);               // duty: 0 to 2^resolution - 1
// Example: ledcAttach(4, 5000, 8); ledcWrite(4, 128); // 50% duty

Parameter	What it controls	Typical value
`freq`	PWM frequency in Hz	5000 (LED), 50 (servo)
`resolution`	Bit resolution (1–16)	8 (0–255), 10 (0–1023), 12 (0–4095)
`duty`	Duty cycle value	0 = off, 2^res−1 = full on

Built-in tone(): ESP32 Core 3.x includes tone(pin, frequency, duration) and noTone(pin) natively. No external library needed.

5 · Serial Communication

Serial.begin(115200);
Serial.println("Hello");
Serial.printf("Value: %d\n", value);

Method	Output
`Serial.print("A")`	Prints without newline
`Serial.println("A")`	Prints with `\r\n`
`Serial.printf("x=%d\n", x)`	Formatted output (like C `printf`)
`Serial.println(value)`	Prints an integer in decimal

Baud rate: Always 115200. Set the Serial Monitor to match. USB CDC on the ESP32 auto-resets, so you may see a brief garbage burst at power-on.

6 · Wi-Fi (Station Mode)

#include <WiFi.h>

WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED) {
  delay(500);
  Serial.print(".");
}
Serial.println(WiFi.localIP());

Function	Returns
`WiFi.begin(ssid, pass)`	Starts connection (returns `wl_status_t`)
`WiFi.status()`	`WL_CONNECTED`, `WL_IDLE`, `WL_DISCONNECTED`, etc.
`WiFi.localIP()`	The assigned IP address (e.g. `192.168.1.42`)
`WiFi.SSID()`	Name of the connected network
`WiFi.RSSI()`	Signal strength in dBm (closer to 0 = stronger)

7 · Common Sensor APIs

// DHT11 Temperature & Humidity
#include <DHT.h>
DHT dht(pin, DHT11);
dht.begin();
float t = dht.readTemperature();  // Celsius
float h = dht.readHumidity();

// PIR Motion
int motion = digitalRead(pin);    // HIGH = motion detected

// Potentiometer
int pot = analogRead(pin);        // 0–4095

Sensor	Library	Key functions	Output
DHT11	DHT by Adafruit	`readTemperature()`, `readHumidity()`	Floats (°C, %RH). Returns `NaN` on read failure.
HC-SR501 PIR	None (digital)	`digitalRead()`	`HIGH` when motion, `LOW` otherwise
10 kΩ Pot	None (analog)	`analogRead()`	0–4095

8 · Web Server (AsyncWebServer)

#include <ESP Async WebServer.h>

AsyncWebServer server(80);

server.on("/", HTTP_GET, [](AsyncWebServerRequest *req){
  req->send(200, "text/html", "<h1>Hello</h1>");
});

server.begin();

Method	Use case
`server.on(path, method, handler)`	Register a route handler
`req->send(code, type, body)`	Send a response
`server.on("/led", HTTP_GET, ...)`	Handle GET requests with query params
`server.on("/led", HTTP_POST, ...)`	Handle POST from forms or AJAX

Libraries needed: Install both ESP Async WebServer and AsyncTCP via the Arduino Library Manager. See Project 5 for a complete example.

9 · MQTT (Ubidots)

#include <UbidotsEsp32Mqtt.h>

Ubidots ubidots(TOKEN);

ubidots.connectToWifi(ssid, pass);
ubidots.setCallback(callback);
ubidots.setup();
ubidots.reconnect();

// In loop():
ubidots.add("variable", value);
ubidots.publish("device-label");
ubidots.loop();

Step	Function	Notes
1. Connect	`connectToWifi()`	Blocks until connected
2. Callback	`setCallback()`	Called when a subscribed variable changes
3. Setup	`setup()`	Connects to Ubidots MQTT broker
4. Reconnect	`reconnect()`	Re-subscribes to variables
5. Publish	`add() + publish()`	Send data in every loop iteration
6. Keep alive	`loop()`	Must be called frequently

10 · ESP32 Core 3.x Migration Notes

PWM API (most common break)

	Core 2.x (old)	Core 3.x (current)
Setup	`ledcSetup(channel, freq, resolution)`	`ledcAttach(pin, freq, resolution)`
Attach	`ledcAttachPin(pin, channel)`	Merged into `ledcAttach()`
Write	`ledcWrite(channel, duty)`	`ledcWrite(pin, duty)`

Migration: Core 3.x assigns channels automatically per pin. The old channel-based approach no longer compiles. Every PWM project in this kit uses the 3.x API.

Other Changes

Area	Change
`analogSetAttenuation()`	Now takes an enum (`ADC_11db`), not an integer. Default is 11 dB (full 0–3.3 V range).
`tone()` / `noTone()`	Built-in. No library needed. Signature: `tone(pin, freq, duration)`.
`ledcSetup()`	Removed entirely. Use `ledcAttach()` instead.
`ledcAttachPin()`	Removed entirely. Use `ledcAttach()` instead.
`analogReadResolution()`	12 bits is the default. No need to call `analogReadResolution(12)`.