Relay Control Circuit Using 8051 Microcontroller

Introduction

A relay control circuit is essential for controlling high-power devices using a low-power microcontroller. This project demonstrates how to use the 8051 microcontroller to control a relay, which can then switch on or off an external device such as a light bulb, motor, or fan. Relays act as electrically operated switches that isolate the control circuit from the high-power load, providing safety and ease of control.

Components Required

• 8051 Microcontroller (e.g., AT89S52)

• Relay Module (5V)

• Transistor (e.g., BC547)

• Diode (e.g., 1N4007)

• Resistor (1kΩ)

• LED (optional for indication)

• 16x2 LCD Display (optional for displaying status)

• Push Button

• Capacitors (33pF, 100μF)

• Crystal Oscillator (11.0592 MHz)

• Breadboard and Connecting Wires

• Power Supply (5V)

Circuit Diagram

The relay is controlled by a transistor, which is driven by the 8051 microcontroller. The diode is used to protect the transistor from back EMF generated by the relay coil. A push button is used to toggle the relay state.

+5V ----- +5V

          |

          |

         RELAY MODULE

         +---+

   +5V --|VCC| 

         |    |

   GND --|GND | 

         | IN |------- Collector (BC547)

         +---+

                |

                |

               Emitter (BC547) ----- GND

               Base (BC547) -------- R1 (1kΩ) -------- P1.0 (8051)

8051

+---+

|   |

| P1.0 (Relay Control)

| P3.2 (Push Button)

|   |

+---+

DIODE

Cathode to Collector (BC547)

Anode to Relay IN

PUSH BUTTON

One end to P3.2

Other end to GND

Pin Connections

• Relay Module:

• VCC to +5V

• GND to Ground

• IN to Collector of BC547

• Transistor (BC547):

• Collector to Relay IN

• Emitter to Ground

• Base to P1.0 of 8051 through 1kΩ resistor

• Diode (1N4007):

• Anode to Collector of BC547

• Cathode to VCC of Relay

• Push Button:

• One end to P3.2 of 8051

• Other end to Ground

• LED (Optional):

• Anode to P1.1 of 8051 through 1kΩ resistor

• Cathode to Ground

• 8051 Microcontroller:

• Connect crystal oscillator and capacitors for clock generation

Software Implementation

The code is written in C using Keil uVision IDE. It involves initializing the ports, reading the push button state, and toggling the relay state accordingly.

#include <reg51.h>

sbit RELAY = P1^0; // Relay control pin

sbit BUTTON = P3^2; // Push button pin

sbit LED = P1^1; // LED pin (optional for indication)

void delay(unsigned int count) {

    int i, j;

    for(i=0; i<count; i++)

        for(j=0; j<1275; j++);

}

void main() {

    unsigned char button_state = 0; // Variable to store button state

    unsigned char relay_state = 0; // Variable to store relay state

    while(1) {

        if(BUTTON == 0) { // If button is pressed

            delay(20); // Debounce delay

            if(BUTTON == 0) { // Confirm button press

                button_state = 1;

            }

        }

        else if(button_state == 1) { // If button was pressed and now released

            relay_state = !relay_state; // Toggle relay state

            RELAY = relay_state; // Set relay output

            LED = relay_state; // Set LED output (optional)

            button_state = 0; // Reset button state

        }

        delay(100); // Small delay for debouncing

    }

}

Explanation

1. Initialization:

2. Ports Configuration: RELAY is configured as an output to control the relay. BUTTON is configured as an input to read the push button state. LED is optional for visual indication of the relay state.

3. Button Debouncing:

4. A delay is used to debounce the button press to avoid multiple toggles due to mechanical bounce.

5. Relay Control:

6. The relay state is toggled each time the button is pressed and released. The new state is set to the RELAY pin, and the LED is updated accordingly.

Conclusion

This project demonstrates the use of the 8051 microcontroller to control a relay, enabling the control of high-power devices through a low-power microcontroller. The system uses a transistor to drive the relay, providing isolation between the microcontroller and the high-power load. This project is a great way to learn about interfacing relays with microcontrollers and controlling external devices safely and efficiently.

Want us to guide you through your project or make the project for you ?

Order Projects

Create Various Projects

Check out our Free Arduino Projects Playlist - Arduino Projects 

Check out our Free Raspberry Pi Projects Playlist - Raspberry Pi Projects 

Check out our Free TinkerCAD Projects Playlist - TinkerCAD Projects 

Check out our Free IoT Projects Playlist - IoT Projects 

Check out our Free Home Automation Projects Playlist - Home Automation Projects 

Check out our Free NodeMCu Projects Playlist - NodeMCu Projects