Lesson 7 Relay

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As we know relay is a device which is used to provide connection between two or more points or device in response to the input signal applied. In another words relay provide isolation between the controller and the device as we know devices may work on AC as well as on DC. However, they receive signals from microcontroller which works on DC hence we require a relay to bridge the gap. Relay is extremely useful when you need to control a large amount of current or voltage with small electrical signal.


– 1 * Raspberry Pi

– 1 * Breadboard

– 1 * Relay

– 1 * LED

– 1 * Resistor (220Ω)

– 1 * Resistor (1KΩ)

– 1 * NPN Transistor

– 1 * Diode (Rectifier)

– Several jumper wires



There are 5 parts in every relay:

1. Electromagnet – It consists of an iron core wounded by coil of wires. When electricity is passed through, it becomes magnetic. Therefore, it is calLED electromagnet.

2. Armature – The movable magnetic strip is known as armature. When current flows through them, the coil is it energized thus producing a magnetic field which is used to make or break the normally open (N/O) or normally close (N/C) points. And the armature can be moved with direct current (DC) as well as alternating current (AC).

3. Spring – When no currents flow through the coil on the electromagnet, the spring pulls the armature away so the circuit cannot be completed.

4. Set of electrical contacts – There are two contact points:

. Normally open – connected when the relay is activated, and disconnected when it is inactive.

. Normally close – not connected when the relay is activated, and connected when it is inactive.

5. Molded frame – Relays are covered with plastic for protection.

Working of Relay

The working principle of relay is simple. When power is supplied to the relay, currents start flowing through the control coil; as a result, the electromagnet starts energizing. Then the armature is attracted to the coil, pulling down the moving contact together thus connecting with the normally open contacts. So the circuit with the load is energized. Then breaking the circuit would a similar case, as the moving contact will be pulled up to the normally closed contacts under the force of the spring. In this way, the switching on and off of the relay can control the state of a load circuit. 

Schematic diagram: 

Experimental Procedures

Step 1: Build the circuit 

For C language users:

Step 2: Open the code file

    cd /home/pi/SunFounder_Super_Kit_V3.0_for_Raspberry_Pi/C

Step 3: Compile the Code

    make 07_relay

Step 4: Run the executable file above

    sudo ./07_relay

For Python users:

Step 2: Open the code file

    cd /home/pi/SunFounder_Super_Kit_V3.0_for_Raspberry_Pi/Python

Step 3: Run

    sudo python 07_relay.py

Now, connect a device of high voltage, and the relay will close and the LED will light up; connect one of low voltage, and it will open and the LED will go out. In addition, you can hear a ticktock caused by breaking normally close contact and closing normally open contact. 

C Code

* Filename 		: 4N35.c
* Description 	: Make a relay to contral led blinking
* Author 		: Dream
* E-mail 		: support@sunfounder.com
* Website 		: www.sunfounder.com
* Update 		: Dream    <2016-07-26>
* Detail		: <update details>
#include <wiringPi.h>
#include <stdio.h>

#define RelayPin 0

int main(void){
	if(wiringPiSetup() == -1){ //when initialize wiring failed, print messageto screen
		printf("setup wiringPi failed !");
		return 1; 
	pinMode(RelayPin, OUTPUT);   //set GPIO0 output

	printf("|                 Relay                |\n");
	printf("|    ------------------------------    |\n");
	printf("| GPIO0 connect to relay's control pin |\n");
	printf("| led connect to relay's NormalOpen pin|\n");
	printf("|  5v connect to relay's com pin       |\n");
	printf("|                                      |\n");
	printf("|      Make relay to contral a led     |\n");
	printf("|                                      |\n");
	printf("|                            SunFounder|\n");

		// Tick
		printf("......Relay Close\n");
		digitalWrite(RelayPin, LOW);
		// Tock
		printf("Relay Open......\n");
		digitalWrite(RelayPin, HIGH);

	return 0;

Python Code

#!/usr/bin/env python

import RPi.GPIO as GPIO
import time

# GPIO0 connect to relay's control pin
# led connect to relay's NormalOpen pin
# 5v connect to relay's com pin
# Set #17 as contral pin
relayPin = 17

# Define a function to print message at the beginning
def print_message():
	print ("========================================")
	print ("|                 Relay                |")
	print ("|    ------------------------------    |")
	print ("| GPIO0 connect to relay's control pin |")
	print ("| led connect to relay's NormalOpen pin|")
	print ("|  5v connect to relay's com pin       |")
	print ("|                                      |")
	print ("|      Make relay to contral a led     |")
	print ("|                                      |")
	print ("|                            SunFounder|")
	print ("========================================\n")
	print 'Program is running...'
	print 'Please press Ctrl+C to end the program...'
	raw_input ("Press Enter to begin\n")

# Define a setup function for some setup
def setup():
	# Set the GPIO modes to BCM Numbering
	# Set relayPin's mode to output, 
	# and initial level to High(3.3v)
	GPIO.setup(relayPin, GPIO.OUT, initial=GPIO.HIGH)

# Define a main function for main process
def main():
	# Print messages
	while True:
		print '...Relay close'
		# Tick
		GPIO.output(relayPin, GPIO.LOW)
		print 'Relay open...'
		# Tock
		GPIO.output(relayPin, GPIO.HIGH) 

# Define a destroy function for clean up everything after
# the script finished 
def destroy():
	# Turn off LED
	GPIO.output(relayPin, GPIO.HIGH)
	# Release resource

# If run this script directly, do:
if __name__ == '__main__':
	# When 'Ctrl+C' is pressed, the child program 
	# destroy() will be  executed.
	except KeyboardInterrupt: