The Arduino Uno has a row of pins along each side, and knowing what each does saves you from guesswork and fried components. Here’s the plain-English map.
The pin groups
| Pins | Type | Use them for |
|---|---|---|
| 5V, 3.3V, GND, VIN | Power | Powering sensors and modules |
| 0–13 | Digital I/O | On/off signals: LEDs, buttons, relays |
| 3, 5, 6, 9, 10, 11 (~) | PWM | ‘Fake analog’: dimming, servo, tone |
| A0–A5 | Analog input | Reading dials, LDRs, analog sensors |
| A4 / A5 | I2C (SDA/SCL) | LCDs, many sensors |
| 0 / 1 (RX/TX) | Serial | Bluetooth, serial comms |
Power pins
5V and 3.3V supply power to your components; GND is the common ground (there are three — use any). VIN can take an external 7–12V supply. Rule of thumb: most 5V modules connect to 5V and GND.
Digital pins (0–13)
These handle simple on/off — either reading a HIGH/LOW (a button pressed or not) or writing one (an LED on or off). You use digitalRead() and digitalWrite(). Pin 13 has a built-in LED, which is why the Blink example uses it.
PWM pins (the ~ ones)
Six digital pins are marked with a ~ (3, 5, 6, 9, 10, 11). These can fake an analog output with analogWrite() — dimming an LED, setting motor speed or driving a servo. If a tutorial says “use a PWM pin”, it means one of these.
Analog input pins (A0–A5)
These read a varying voltage — a potentiometer, an LDR, a soil sensor — and give a number from 0 to 1023 via analogRead(). Note A4 and A5 double as the I2C pins (SDA/SCL) used by LCDs and many sensor modules.
Special pins
RX (0) and TX (1) are the serial pins — used by Bluetooth modules and for talking to your computer. Avoid using them for other things while uploading code. RESET restarts the sketch.
Frequently asked questions
Which pins can I use for an LED?
Any digital pin (2–13). Use a PWM (~) pin if you want to dim it.
Can I get more than 6 analog inputs?
On the Uno, no — it has A0–A5. The Nano adds A6 and A7.
What are SDA and SCL?
The I2C bus (on A4/A5). I2C LCDs and many sensors use just these two pins to communicate.
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