1.16 Relay Module Control

Imagine controlling a lamp, fan, or motor with your Pico - that’s exactly what relay modules do! A relay is like a remote-controlled switch that lets low-voltage electronics (like our Pico) safely control high-voltage devices (like household appliances).

Think of it as a “digital hand” that can flip a physical switch. When the Pico sends a signal, the relay clicks and switches the connected device on or off. This is the foundation of home automation - smart lights, automatic fans, timed appliances, and IoT devices!

Safety first: Our relay module includes protection circuits to ensure safe operation.

Warning

Modification of electrical appliances comes with great danger, do not try it lightly, please do it under the guidance of professionals.

Here we only use a simple circuit powered by a breadboard power module as an example to show how to control it using relay.

Component List

Raspberry Pi Pico W x1
MicroUSB cable x1
830 Tie-Points Breadboard x1
Jumper Wire Several
Relay Module x1

Component knowledge

Relay Module 

Connect

Code

Note

Open the 1.16_relay_module_control.ino file under the path of Ultimate-Starter-Kit-for-Pico-W\Arduino\1.Project or copy this code into Thonny, then click “Run Current Script” or simply press F5 to run it.
Or copy this code into Arduino IDE.
Don’t forget to select the board(Raspberry Pi Pico) and the correct port before clicking the Upload button.

After running the code, you’ll hear the satisfying “click-clack” sound of the relay switching every few seconds! The serial monitor shows automatic timing cycles with countdowns - “Relay ON for 2s, turning OFF in 1s”. This demonstrates how you could control any device with precise timing: auto-shutoff lights, timed watering systems, or scheduled appliances.

The following is the program code:

/*
  Automatic Relay Controller

  Controls a relay to automatically switch external circuits
  on and off with timing control and status monitoring.
*/

// Pin and timing constants
const int RELAY_PIN = 15;           // relay control pin
const int ON_TIME = 3000;           // relay on duration (milliseconds)
const int OFF_TIME = 2000;          // relay off duration (milliseconds)

// Variables for timing and status
unsigned long lastSwitchTime = 0;
bool relayState = false;
int cycleCount = 0;

void setup() {
  // Set up relay pin
  pinMode(RELAY_PIN, OUTPUT);

  // Initialize serial communication
  Serial.begin(115200);
  Serial.println("=== Automatic Relay Controller ===");
  Serial.println("Relay switches external circuit ON/OFF automatically");
  Serial.println("Can control lights, motors, or other AC/DC devices");
  Serial.println();

  // Start with relay OFF
  digitalWrite(RELAY_PIN, LOW);
  lastSwitchTime = millis();
  Serial.println("Starting cycle... Relay: OFF");
}

void loop() {
  // Check if it's time to switch relay state
  checkRelayTiming();

  // Show continuous status every 5 seconds
  showPeriodicStatus();
}

// Function to handle relay timing and switching
void checkRelayTiming() {
  unsigned long currentTime = millis();
  unsigned long elapsedTime = currentTime - lastSwitchTime;

  // Check if it's time to switch state
  bool shouldSwitch = false;

  if (relayState && elapsedTime >= ON_TIME) {
    // Relay is ON and ON_TIME has passed - turn OFF
    shouldSwitch = true;
  } else if (!relayState && elapsedTime >= OFF_TIME) {
    // Relay is OFF and OFF_TIME has passed - turn ON
    shouldSwitch = true;
  }

  // Switch relay if needed
  if (shouldSwitch) {
    relayState = !relayState;
    digitalWrite(RELAY_PIN, relayState ? HIGH : LOW);
    lastSwitchTime = currentTime;

    if (relayState) {
      cycleCount++;
    }

    // Display switch event
    Serial.print("Cycle #");
    Serial.print(cycleCount);
    Serial.print(" - Relay switched: ");
    Serial.println(relayState ? "ON" : "OFF");

    if (relayState) {
      Serial.println("External circuit powered - Device running");
    } else {
      Serial.println("External circuit off - Device stopped");
    }
    Serial.println();
  }
}

// Function to show periodic status updates
void showPeriodicStatus() {
  static unsigned long lastStatusTime = 0;
  unsigned long currentTime = millis();

  // Show status every 5 seconds
  if (currentTime - lastStatusTime >= 5000) {
    unsigned long timeInState = currentTime - lastSwitchTime;
    unsigned long remainingTime;

    if (relayState) {
      remainingTime = ON_TIME - timeInState;
      Serial.print("Status: Relay ON for ");
      Serial.print(timeInState / 1000);
      Serial.print("s, turning OFF in ");
      Serial.print(remainingTime / 1000);
      Serial.println("s");
    } else {
      remainingTime = OFF_TIME - timeInState;
      Serial.print("Status: Relay OFF for ");
      Serial.print(timeInState / 1000);
      Serial.print("s, turning ON in ");
      Serial.print(remainingTime / 1000);
      Serial.println("s");
    }

    lastStatusTime = currentTime;
  }
}