Interfacing Temperature Sensor with Microchip PIC16F876A

The tutorial aims at providing the necessary information for interfacing an analog type temperature sensor with a Microchip PIC Microcontroller. PIC (Peripheral Interface Controllers) was introduced in 1985. The PIC16F876A has 8K of Flash Program Memory, 368 bytes of Data Memory (RAM) and many other attractive features. Some features are ADC, USART, and 14 Interrupts all in 28 PDIP Package.

The Analog temperature sensor used is LM35. It has a transfer function of 10mv/’c.  The output of LM35 is analog voltage which varies with changes in temperature. This analog voltage is digitized using the On-Chip 10bit A/D Converter and the value is displayed on a 2x16 LCD.

It is possible to switch On/Off an external application based on temperature value.

The LCD is based on HD44780 controller. The programming has been done using the MikroC compiler from Mikroelektronika (www.mikroe.com). The demo version has a 2KB Hex Output limit, fortunately it is more than enough for our requirement.

 

 LM35

 

 

Circuit Diagram

 

 

 

 

Program

 

int t1,temp;

char *text[6];

void main()

{

adcon1=14;

lcd_init(&portb);

lcd_out(1,1,"Temperature");

lcd_out(2,8,"'C");

while(1)

{

t1=adc_read(0);

//temp=0.245*t1;          // For TMP37 Sensor 20mv/’c

temp=0.245*t1*2;        // For Lm35 Sensor 10mv/’c

inttostr(temp,text);

lcd_cmd(lcd_cursor_off);

lcd_out(2,1,text);

delay_ms(100);

}

}

 

The program is self explanatory, however let me explain you the calculation done. In a while loop. The Input channel 0 is read and the digitized value is obtained. Now the smallest digitized value is equal to Vref/((2^10)-1). Internal Vref is 4096mV but we will consider 5000mV for the ease of calculation.  Multiplying the value obtained above with the digitized value will give us the analog voltage. Since the transfer function of Lm35 is 10mV/’c, we can obtain the temperature.