Make things move. Why a pin can't run a motor, motor drivers and the H-bridge, two supplies with one ground, and choosing a battery, then LiPo safety before any LiPo is used. Years 9–10, after Arduino.
Why a pin can't, and what can: transistors, drivers and a shared ground.
A pin is a signal, not a power supply: it can give about 20 mA, and a motor needs hundreds. The pin switches a transistor, and the transistor switches the motor's own battery. A diode across the motor catches the spike when it switches off.
A motor driver (an H-bridge) lets two pins choose the direction: IN1 HIGH and IN2 LOW is forwards, swapped is backwards, the same is stop. A third, PWM pin on ENA sets the speed with analogWrite. Two jobs, two kinds of signal.
The motor gets its own battery, but the grounds must be joined: a signal is a voltage compared with ground, so the driver can't read the Uno's pins without a shared 0 V. Powering the motor from the Uno's own 5V pulls it down when the motor starts, and the Uno resets.
Voltage, capacity and run time, and LiPo safety.
Voltage (V) is how hard a battery pushes; capacity (mAh) is how much charge it holds. Run time ≈ capacity ÷ current, and plan on using about 80% of it. A battery for a robot has to meet every criterion: voltage in range, enough run time, light enough to carry.
LiPo packs are light and give big currents, but they store a lot of energy in a soft pouch. Damaged, over-charged or run-flat cells can swell and catch fire. The rules: inspect every time, charge on a balance charger in a fire-safe bag while someone watches, never run below the cutoff, store at storage charge, and retire any pack that swells. Required before any LiPo use; your teacher signs it off.
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