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2.16 Control Another Circuit with a Relay

In this lesson, we will learn how to control another circuit using a relay and the Raspberry Pi Pico 2. A relay acts like a switch controlled by a low-voltage circuit (like Pico) to operate a high-voltage circuit. For example, you can use a relay to turn on a lamp or any other device, making it possible to automate electrical appliances.

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	1(220Ω), 1(1KΩ)	BUY
6	Transistor	1(S8050)	BUY
7	Diode	1
8	Relay	1	BUY
9	LED	1	BUY
10	Power Supply Module	1
11	9V Battery	1

Circuit Diagram

sch_relay_1

Relay Activation:
- The relay’s coil is energized by the transistor when the Pico outputs a high signal (3.3V) to GP15.
- The transistor allows current to flow through the relay, activating the switch inside.
- The relay makes a “click” sound when switching, indicating the control of the load circuit.
Flyback Diode:
- The diode is placed across the relay coil to protect the transistor from voltage spikes that occur when the relay is turned off.

Wiring Diagram

wiring_relay_1

Writing the Code

Note

You can open the file 2.16_relay.ino from newton-lab-kit/arduino/2.16_relay.
Or copy this code into Arduino IDE.
Select the Raspberry Pi Pico 2 board and the correct port, then click “Upload”.

const int relayPin = 15;  // GPIO pin connected to the transistor base

void setup() {
  pinMode(relayPin, OUTPUT);
  digitalWrite(relayPin, LOW);  // Ensure the relay is off at startup
}

void loop() {
  // Turn the relay on
  digitalWrite(relayPin, HIGH);
  Serial.println("Relay ON");
  delay(2000);  // Wait for 2 seconds

  // Turn the relay off
  digitalWrite(relayPin, LOW);
  Serial.println("Relay OFF");
  delay(2000);  // Wait for 2 seconds
}

After uploading the code, you should hear a “click” sound from the relay every 2 seconds as it switches on and off.

Understanding the Code

Defining the Relay Pin:

Assigns relayPin to GPIO 15, which controls the transistor and thus the relay.
```
const int relayPin = 15;  // GPIO pin connected to the transistor base
```

Setting Up the Pin Modes:

Sets relayPin as an output. Initializes the relay in the OFF state.

void setup() {
  pinMode(relayPin, OUTPUT);
  digitalWrite(relayPin, LOW);  // Ensure the relay is off at startup
}

Controlling the Relay:

Sets relayPin HIGH to turn on the transistor, energizing the relay coil.
Waits for 2 seconds.
Sets relayPin LOW to turn off the transistor, de-energizing the relay coil.
Waits for another 2 seconds.
Repeats the cycle indefinitely.

// Turn the relay on
digitalWrite(relayPin, HIGH);
Serial.println("Relay ON");
delay(2000);  // Wait for 2 seconds

// Turn the relay off
digitalWrite(relayPin, LOW);
Serial.println("Relay OFF");
delay(2000);  // Wait for 2 seconds

Experimenting Further

Set a Timer: Modify the code to turn the relay on for 10 minutes and then automatically turn it off.
Control Home Appliances: With appropriate guidance, you can connect high-voltage devices to the relay for automation tasks such as turning lights or fans on and off.
- The circuit should look like this: To demonstrate controlling an external circuit safely, we’ll add an external 5V power supply (through a breadboard power module) to power an LED. This simulates how you could control higher voltage devices (like home appliances) using the relay. Here’s how to modify the circuit:
- Code to Control the Relay:
```
const int relayPin = 15;  // GPIO pin connected to the transistor base

void setup() {
  pinMode(relayPin, OUTPUT);
  digitalWrite(relayPin, LOW);  // Ensure the relay is off at startup
}

void loop() {
  // Turn the relay on
  digitalWrite(relayPin, HIGH);
  Serial.println("Relay ON");
  delay(2000);  // Wait for 2 seconds

  // Turn the relay off
  digitalWrite(relayPin, LOW);
  Serial.println("Relay OFF");
  delay(2000);  // Wait for 2 seconds
}
```
  When the relay is activated (GP15 outputs high), the Normally Open (NO) and Common (C) pins of the relay connect, allowing the external 5V power to flow through the LED. The LED will light up, simulating how a relay can control an external appliance.
  
  When the relay is deactivated (GP15 outputs low), the Normally Open (NO) pin disconnects from the Common (C) pin, cutting off the external power, and the LED turns off.

Safety Considerations for Controlling Real Appliances

This example uses an LED and a 5V power source to demonstrate relay control. If you are controlling higher voltage devices (like household appliances), ensure:

Proper Voltage Rating: Use a relay rated for the appropriate voltage and current for your appliance.
Isolation: For safety, ensure proper isolation between the low-voltage control circuit (like the Pico) and the high-voltage appliance circuit.
Fuse Protection: Consider adding fuses or circuit breakers to protect against short circuits or overloads.
Professional Guidance: When working with high-voltage circuits, always seek professional guidance to ensure safe operation.

This project can serve as the basis for home automation, such as controlling lamps, fans, or other devices based on timers or sensors connected to the Raspberry Pi Pico 2.

Using the NC Terminal

If you connect your controlled circuit between COM and NC:
- The circuit will be closed (ON) when the relay is not energized.
- The circuit will be open (OFF) when the relay is energized.
- Example: Controlling an External Device
- Warning: Do not attempt to control high-voltage devices without proper knowledge and safety precautions.
If you want to control a small DC motor or another device:
- Replace the LED with the device you want to control.
- Ensure the device’s voltage and current requirements are compatible.
- Provide an appropriate power supply for the device.
- Connect the device in series with the relay’s COM and NO (or NC) terminals.

Conclusion

In this lesson, you’ve learned how to control another circuit using a relay and the Raspberry Pi Pico. By using a transistor to switch the relay coil, you’ve safely controlled a higher-current circuit without overloading the Pico’s GPIO pins. Understanding how to use relays opens up many possibilities for controlling various devices and appliances in your projects.