Temperature & Humidity Sensor

Timer/Counter / By /

DHT11 digital temperature and humidity sensor is a composite Sensor contains a calibrated digital signal output of the temperature and humidity. The sensor includes a resistive sense of wet components and an NTC temperature measurement device and connected with a high-performance 8-bit microcontroller.

RT: Typical application circuit recommended in the short cable length of 20m on the 5.1kΩ pull-up register, the resistance of greater than 20m under the pull-up resistor on the lower actual situation. To get the temperature and humidity data we need 3 steps. Let’s look at all steps one by one

Here the pulse width of 26µs ~ 28µs indicates a bit 0 and 70µs represent bit 1. We measure the pulse using External Interrupt and timer. We enable external interrupt at any edge. Let’s look at the program below for simplicity-

 void init_dht11(void)
  {
    DDRD|=(1<<DDD2);       //DD2 for read data
    PORTD|=(1<<PORTD2);    //Output high
    PORTD&=~(1<<PORTD2);   //Ouput Low for 18ms
    _delay_ms(18);
    PORTD|=(1<<PORTD2);    //Output high for relise bus
    DDRD&=~(1<<DDD2);      //Input sate declear for interrupt triger
    while(PIND&(1<<PIND2));//Wait for pull the data line low
    while(!(PIND&(1<<PIND2)));//wait 80us low
    while(PIND&(1<<PIND2)); //wait 80us high
    while(!(PIND&(1<<PIND2)));//wait 50us low
    sei();
    //SREG|=(1<<I);	       //Global interrupt enable
    GICR |=(1<<INT0);	   //External Interrupt Request 0 Enable
    MCUCR|=(1<<ISC00);	   //any edge of INT0 generates an interrupt request
 }

Let’s use timer0 with prescaler 8. So the new frequency & time of timer0 is

             ƒtimer0 = 8MHz/8 = 1MHz  & Ttimer0 = 1µs

TCNT0 value will determine the pulse width value. 

Look at the interrupt service routine function for more details –

 uint8_t start_flag=1,millisecond;
/**** Interrupt service routine ********************/
ISR(INT0_vect)
{ 
 if(start_flag){
		pulse=TCNT0;
		TCCR0=0; // Timer stop
		if(pulse<30) sample[count]=0;
        else if(pulse>50 && pulse<75) sample[count]=1;
        count++;
        start_flag=0;
    }else{ // start timer with flag clear
    TCCR0|=(1<<CS01);                       //prescaler 8
    TCNT0=0;                                //intialize timer value
   start_flag=1;
  }
if(count>40) 
	{
		count=0;
		start_flag=1;
	}
}

We have found the bit, now it is time for calculation-

Constant variable: const is used with a data type declaration or definition to specify an unchanging value.

  • Example-  const int five=5;

Const objects may not be changed.

Volatile variable: Volatile specifies a variable whose value may be changed by process outside the current program. One example of a volatile object might be a buffer used to exchange data with an eternal device.

In our program we declare volatile uint16_t data_packet[40]; and volatile float humidity,temperature; global variable which can be used in any .c file.

Let’s back to our main program. Sensor transmit a total of 40bits containing the temperature and humidity.

So that the humidity is byte1+byte2*0.1 and in temperature calculation the 7th bit of fractional data byte represent the positive and negative sign. If 7th bit is 1 temperature is positive and if it is 0 then temperature is positive.

/************************************************************************************************
*************************************************************************************************
This program will read the temperature and humidity data and display in LCD
             The value sould be update in every 5s
 
*************************************************************************************************
		Subeer Kumar Sarkar
		Electrical & Electronic Engineer
************************************************************************************************
************************************************************************************************
***********************************************************************************************/
#include<avr/io.h>
#include<util/delay.h>
#include<avr/interrupt.h>
#include"dht11.h"
#include"lcd.h"
char str[40],deg=223,hum=37;
int main(void)
{
LCD_INIT();         //LCD intilize
LCD_write_string(1,1,"Temp and Humidity");
timer2_250ms();
init_dht11();
_delay_ms(1000);
while(1)
{
 for(uint8_t i=0;i<8;i++)
 {
   humidity|=(sample[i]<<(7-i));
   temperature|=(sample[16+i]<<(7-i));
 }
 sprintf(str,"HR:%02d%c T:%02d%cC  ",humidity,hum,temperature,deg);
 LCD_write_string(1,2,str);
 temperature=0;humidity=0;
 _delay_ms(1000); //delay for LCD update
}
return 0;
}

Here timer2 is initialized at 250ms to call sensor at every 5s. So the data will be update in every 5s. One interesting thing that we use %02d to display only 2digit of temperature and humidity data. 

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Interfacing Temperature and Humidity sensor.rar