I recently got a relay module from Ebay for a good price (I think) of $14.5. I

decided to use my laptop to send command over the uart cable to control a simple

motor. To do that, I used VS C# app since they support serialPort. Once the MCU

decodes the uart command, it will pull the IO pin low or high depending on the

user command.

Here is the code for the MCU:

#define F_CPU 14745600

#include <stdio.h>
#include <math.h>

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <inttypes.h>

#include "../libnerdkits/delay.h"
#include "../libnerdkits/lcd.h"
#include "../libnerdkits/uart.h"
//#define MAX_STRING_LEN 10
//
uint8_t baudrateH, baudrateL;

int8_t SetBaudRate(uint16_t baud)
{   uint16_t baudrate;

    baudrate = (float)F_CPU/16/baud-1;
    baudrateH = baudrate >> 8;
    baudrateL = baudrate & 0xff;
    UBRR0H = baudrateH; //(F_CPU/16/BAUD-1)>>8;
    UBRR0L = baudrateL; //(F_CPU/16/BAUD-1)&&0xff;
    UCSR0B = (1<<RXEN0)|(1<<TXEN0); // enable uart
    UCSR0C = (1<<UCSZ00) | (1<<UCSZ01); //8 bits
    return(1);
}
int main() {
  // start up the LCD
  lcd_init();
  FILE lcd_stream = FDEV_SETUP_STREAM(lcd_putchar, 0, _FDEV_SETUP_WRITE);
  lcd_home();

  // start up the serial port
  uart_init();
  FILE uart_stream = FDEV_SETUP_STREAM(uart_putchar, uart_getchar, _FDEV_SETUP_RW);
  stdin = stdout = &uart_stream;

  //int i=0;
  //uint16_t baud = 9600;
  char x =' ';
  //char msg[MAX_STRING_LEN];
  SetBaudRate(9600);
  // set output
  DDRC|=0xFF;
  PORTC|=0xFF;
  DDRB|=0xFF;
  PORTB|=0xFF;
  while(1) {

      if (uart_char_is_waiting()){
            x=  getchar();
      }// end if
            lcd_line_one();
            fprintf_P(&lcd_stream, PSTR("You Entered: %c       "),x);

      if(x=='1'){
        PORTC&=~(1<<PC5);
      }
          else if(x=='2'){
        PORTC&=~(1<<PC4);
      }
          else if(x=='3'){
        PORTC&=~(1<<PC3);
      }
          else if(x=='4'){
        PORTC&=~(1<<PC2);
      }
          else if(x=='5'){
        PORTC&=~(1<<PC1);
      }
          else if(x=='6'){
        PORTC&=~(1<<PC0);
      }
          else if(x=='7'){
        PORTB&=~(1<<PB5);
      }
          else if(x=='8'){
        PORTB&=~(1<<PB4);
      }
      else{
            lcd_line_two();
            fprintf_P(&lcd_stream, PSTR("Reset All Relays "));
              PORTC|=0xFF;
              PORTB|=0xFF;
      }

  }// end while

  return 0;
}

Here is what it look like in action

relays in action