<a href="https://leebinder.com/learning-arduino-topics/">Arduino</a> Voltages: 3.3V, 5V & 12V — What Goes Where?

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Arduino Voltages: 3.3V, 5V & 12V — What Goes Where?

Power rails and logic levels can be confusing. Here’s a friendly guide to keep things safe and working.

💡 Big idea: Power rails (like 12 V) are for motors/LED strips, while logic levels (3.3 V or 5 V) are for the Arduino’s brain and data pins. Mixing them without care can damage parts.

🧠 Logic levels

5 V logic
Common on Arduino Uno/Nano. Pins expect 0–5 V signals.

3.3 V logic
Common on many modern boards and sensors. Do not feed 5 V signals to 3.3 V-only pins.

🔋 Power rails

5 V rail: Powers 5 V sensors, small servos (watch current), and the board itself via USB.
3.3 V rail: Powers low‑power sensors. Usually limited current — check your board’s regulator spec.
12 V rail: Used for motors, relays, LED strips. Never connect 12 V directly to I/O pins.

🔀 Level shifting & protection

Use a logic‑level shifter or resistor divider when a 5 V pin talks to a 3.3 V device.
For noisy loads (motors, relays) add flyback diodes, separate grounds, and decoupling capacitors.
Prefer VIN (7–12 V) to feed the onboard regulator, or a quality 5 V supply into 5V pin (bypasses regulator).

🧪 Quick compatibility table

✔ 5 V Arduino ↔ 5 V sensor: direct connect.
✔ 5 V Arduino → 3.3 V sensor: use level shifter/resistors.
✔ 3.3 V Arduino ↔ 5 V sensor: use level shifter (both directions for I²C/SPI where required).
❌ 12 V to any logic pin: never.

🛡️ Safety checklist

Share a common GND between power supplies and the Arduino.
Measure rails with a multimeter before powering the whole system.
Check sensor/module voltage ratings twice; many have 3.3 V‑only logic.