8051 – Relay Interfacing by using ULN2803

In our previous delay, we have seen DC motor interfacing with 8051. In this tutorial, we will learn Relay interfacing with 8051. Relay is a very important component to interface the heavy appliances with the help of a microcontroller.


Relays are devices that allow low power circuits to switch a relatively high Current/Voltage ON/OFF. A relay circuit is typically a smaller switch or device which drives (opens/closes) an electric switch that is capable of carrying much larger current amounts.


Current flowing through the coil of the relay creates a magnetic field that attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts.

Construction and working

Relays are made up of an electromagnet and a set of contacts generally based on Single Pole Double Throw (SPDT) or Double Pole Double Throw (DPDT) switching method. It has 3 pins to perform a function –

  • COM = Common, always connect to NC; it is the moving part of the switch.
  • NC = Normally Closed, COM is connected to this when the relay coil is off.
  • NO = Normally Open, COM is connected to this when the relay coil is on.

You can easily understand the concept of Relay by looking at the below image.

Suggestion to read

Before getting into Interfacing, You should know about the Relay Driver (ULN2803) and its need and work.

Relay Interfacing with 8051

Circuit Diagram

In this tutorial, I’m connecting One Relay in Port 0.


If you want to make Relay ON, Just give the High Signal (1) to that pin that is connected to ULN2803. The concept of this code is just to turn On the Relay for Some time and Turn Off the relay for some time.


sbit relay=P0^0;

void main()
    unsigned int i;
    while(1) {
        relay=1;                //Realy ON
        relay=0;                //Relay OFF


[ Please find the output image ]

That’s all guys… So, Programming wise it’s very simple. It is like a LED program. But Hardware wise we should be careful. Because we might connect AC power supply devices here. So Be careful guys. Here I have given some Tasks. Please try that. and you may connect Fan, AC Motor, Lamp, etc to relay.


  • Program to display the status of the relay in LCD display
  • Program to control the relay through serial communication

In our next tutorial, we will see how to interface the keypad to 8051.

You can also read the below tutorials.

Linux Device Driver TutorialsC Programming Tutorials
FreeRTOS TutorialsNuttX RTOS Tutorials
RTX RTOS TutorialsInterrupts Basics
I2C Protocol – Part 1 (Basics)I2C Protocol – Part 2 (Advanced Topics)
STM32 TutorialsLPC2148 (ARM7) Tutorials
PIC16F877A Tutorials8051 Tutorials
Unit Testing in C TutorialsESP32-IDF Tutorials
Raspberry Pi TutorialsEmbedded Interview Topics
Reset Sequence in ARM Cortex-M4BLE Basics
VIC and NVIC in ARMSPI – Serial Peripheral Interface Protocol
STM32F7 Bootloader TutorialsRaspberry PI Pico Tutorials
STM32F103 Bootloader TutorialsRT-Thread RTOS Tutorials
Zephyr RTOS Tutorials - STM32Zephyr RTOS Tutorials - ESP32
AUTOSAR TutorialsUDS Protocol Tutorials
Product ReviewsSTM32 MikroC Bootloader Tutorial
VHDL Tutorials
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
Table of Contents