The Board
A tour of the Arduino Uno R3 — the blue board with the silver USB port. Everything has a label; here's what the labels mean.
The headline features
- USB port — connects to your computer. Uploads programs and powers the board. One cable does everything while you're learning.
- Reset button — restarts your program from the beginning. Your best friend during debugging.
- Power jack / Vin pin — for running without USB (7–12V recommended into the jack).
- The main chip — the ATmega328P microcontroller, the brains. (On official Unos it's the long black chip you can actually pop out and replace.)
The pins: where wires go
Pins are the numbered holes along the edges. They fall into groups:
| Group | Pins | What they do |
|---|---|---|
| Digital | 0–13 | Read/write on/off signals. Pins 3, 5, 6, 9, 10, 11 (marked ~) also do PWM "analog" output |
| Analog in | A0–A5 | Read varying voltages (sensors, knobs) |
| Power | 5V, 3.3V, GND | Power your circuits. GND (ground) is the reference everything measures against |
| Special | TX/RX (0,1), SDA/SCL, etc. | Serial and sensor buses — later chapters |
The built-in LED is on pin 13 (marked L on the board). Your
first program blinks it — no wiring needed.
Three rules of pins
- Never connect 5V directly to GND. That's a short circuit — the board will get hot and unhappy. (Resistors exist to prevent exactly this.)
- Digital pins handle ~20mA each, 200mA total. An LED is fine. A motor is not — motors get their own power and a transistor or driver board. (More in Power and Wiring.)
- Pins 0 and 1 are shared with USB serial. If uploads fail mysteriously, disconnect whatever's wired to pins 0/1 first.
Clones vs. official boards
Official Unos cost ~$25; clones cost ~$10 and work identically for learning. The one gotcha: some clones use a different USB-serial chip (CH340 instead of the official one) and need a driver on some systems. If your computer doesn't see the board, that's the first thing to check — see Troubleshooting.
Next: Getting Set Up