Unit C - Actuators & Displays

12. Transistors, MOSFETs & Relays

Switch loads safely without overloading an Arduino pin.

Estimated time 4 hours

Learning outcomes

Explain why loads need drivers
Wire a logic-level N-MOSFET low-side switch
Place a flyback diode correctly
Use a relay module only with low-voltage loads

Parts and preparation

Uno, logic-level N-MOSFET, 220 ohm gate resistor, 10 k ohm gate pull-down, diode, low-voltage DC fan or lamp and external supply.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Transistors, MOSFETs & Relays instructional connection diagram — Open the diagram for a larger view. Trace every connection before building.

Driver purpose

Motors, relays and lamps often require more current or voltage than a pin can supply. A transistor uses a small control signal to switch load current from an external supply.

Logic-level MOSFET

An N-channel enhancement MOSFET can act as a low-side switch. Choose a device specified for low RDS(on) at 4.5 V or lowernot only a threshold voltage.

Flyback diode

An inductive load produces a voltage spike when switched off. Place a diode reverse-biased across the coil: cathode to positive, anode to the switched side.

Relay limits

A relay provides isolation between contacts and coil, but contact ratings still matter. This course uses only extra-low-voltage DC; do not wire mains.

Wiring and safe build sequence

Breadboard wiring for lesson 12: Transistors, MOSFETs & Relays — Breadboard layout for this lesson. Match colours and pins before powering the circuit. Click the image for a larger view.

D9 -> 220 ohm -> MOSFET gate
10 k ohm from gate -> GND
MOSFET source -> common GND
Load between external +V and drain
Diode across inductive load
External supply GND -> Uno GND

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

const byte loadPin = 9;

void setup() {
  pinMode(loadPin, OUTPUT);
}

void loop() {
  for (int power = 0; power <= 255; power++) {
    analogWrite(loadPin, power);
    delay(10);
  }
  analogWrite(loadPin, 0);
  delay(1000);
}

How the code works

The Arduino drives only the gate; the external supply provides load current.
PWM can control a DC fan or lamp when the driver and load support it.

Test and record evidence

Expected result: The low-voltage load increases in power, switches off, then repeats.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Confirm MOSFET pin order from its datasheet.
A non-logic-level MOSFET may overheat at 5 V gate drive.
Common GND is essential.

Extension challenge: Add a thermistor threshold to control the driven fan, with hysteresis to prevent rapid switching.

Check your understanding

Q1. Why not power a fan from an I/O pin?

Show answer

Its current and inductive behavior exceed the pins safe capability.

Q2. How is a flyback diode oriented?

Show answer

Reverse-biased during normal operation: cathode to positive.