# Driving Motors and Relays Safely — Arduino & Embedded C

Source: https://www.geekswithgeeks.com/en/arduino-embedded-c/drive-loads

> Transistors, MOSFETs, flyback diodes and motor drivers.

## GPIO pins signal, they do not power

A GPIO pin can supply only a few tens of milliamps (on the ATmega328P, 40 mA absolute maximum per pin, less in practice). A small DC motor can draw hundreds of milliamps and far more when starting or stalled, and a relay coil often needs more than a pin can safely give. **Never power a motor, relay coil or solenoid directly from a GPIO pin.** Use the pin to switch a **transistor**: an NPN BJT with a base resistor, or better a **logic-level N-channel MOSFET** whose gate fully turns on at your logic voltage (check the datasheet; many MOSFETs need 10 V on the gate). Inductive loads (motors, relays, solenoids) produce a voltage spike when switched off, so put a **flyback diode** across the load, reverse-biased. For direction and speed control, use an **H-bridge motor driver** module rated for your motor's voltage and current.

## Moving things without frying the board

GPIO pins signal; transistors, drivers and a proper supply do the heavy lifting.

![Three ideas: switching loads safely, servos and steppers, power budgets and common ground.](assets/figures/arduino-embedded-c/section-5-map.svg) — Figure 5.1 — Microcontroller signal, driver stage and external power to the load.

## Low-side MOSFET switch for a DC motor

Wiring description; component choices are examples, verify ratings in each datasheet.

```text
Motor supply + (e.g. 6-12 V, separate from the board) -> motor terminal A
Motor terminal B -> MOSFET drain
MOSFET source -> GND (shared with Arduino GND)
Arduino PWM pin -> ~100-220 Ohm resistor -> MOSFET gate
MOSFET gate -> ~10 kOhm resistor -> GND (keeps motor off during reset)
Flyback diode across the motor: cathode to supply +, anode to drain
Decoupling: capacitor near the motor supply input

Never: motor between a GPIO pin and GND
```

## PWM speed control through the MOSFET

Same code style as LED dimming; the MOSFET carries the current. Uno R3 pin 9.

```cpp
const uint8_t MOTOR_PIN = 9;   // PWM pin driving the MOSFET gate

void setup() {
  pinMode(MOTOR_PIN, OUTPUT);
  analogWrite(MOTOR_PIN, 0);    // start stopped
}

void loop() {
  int pot = analogRead(A0);              // 0..1023
  analogWrite(MOTOR_PIN, pot / 4);       // map to 0..255 duty
}
```

## Relay modules and mains

Prefer ready-made relay modules with a driver transistor and diode. Do not work on mains voltage unless you are qualified; use low-voltage loads for learning.

**Quiz:** Why is a flyback diode placed across a DC motor or relay coil?

- [ ] To make the motor spin faster
- [x] To safely absorb the voltage spike produced when the inductive load is switched off
- [ ] To convert PWM into true analog voltage
- [ ] To raise the logic level from 3.3 V to 5 V

*Answer:* To safely absorb the voltage spike produced when the inductive load is switched off. Inductors resist sudden current changes and generate a reverse spike that can destroy the transistor.