Why Does My Arduino Reset When the Motor Turns On?

Short answer: the motor is pulling too much current or injecting noise into the power line. The Arduino sees a voltage drop and resets.

Common causes

Voltage drop: Motor startup current drags the 5V line down.
Noisy power: Motors create electrical noise that can crash the board.
Shared supply: Motor and Arduino run from the same weak source.

Fast fixes that work

Power the motor from a separate supply.
Connect motor ground to Arduino ground.
Use a motor driver or transistor, not a direct Arduino pin.
Add a flyback diode across the motor leads.

Quick check

If your Arduino resets only when the motor starts, it is almost always a power issue.

Why a separate supply helps

The Arduino needs a steady 5V. Motors are spiky loads. A separate supply keeps the motor spikes off the Arduino power rail while a shared ground keeps the signal stable.

What not to do

Do not power a motor from the Arduino 5V pin.
Do not skip the flyback diode on DC motors.
Do not run motors from the same USB port that powers the Arduino.

Bottom line

Resets happen because the motor is stressing the power supply. Use a separate motor supply, share ground, and add a flyback diode.