Hi,

I am doing my first avr exp using a temp sensor LM35 to blink blue green or red led according to the temp of the room.

• Connected the leds to PORT B of atmega8L uC and temp sensor to ADC 3 input.
• Using internal oscillator 1MHz as clk, internal reference voltage of 2.56 as reference voltage in FREE RUNING MODE.
• ADLAR = 0 i.e. right adjusted. 
• Connected a 0.1uF decoupling capacitor at VREF pin as per datasheet, ¡°Internal 2.56V Voltage Reference with external capacitor at AREF pin¡±.
• Using prolight leds 1W (for good intensity), transistor in saturation with Ic around 10mA.

According to what I hv understood with temp sensor 10.0 mV/¢ªC scale factor, output voltage of sensor is input voltage of ADC ¡¦...

                                                ADC = ( Vin * 1024 / Vref )

If room temp= 30¢ªC, Vin = 300mV = 0.3V, So ADC = (.3*1024/2.56)=120. So I hv divided the result by 4.

Fig b.png

Here is my code that I hv written in AVR Studio4

#include<avr\io.h>

#define F_CPU 1000000UL

#include <util/delay.h>

int main (void)

{

  unsigned int TEMP;

  ADMUX= (1<<REFS1)| (1<<REFS0)| (1<<MUX1)| (1<<MUX0);

  ADCSRA= (1<<ADEN)| (1<<ADFR);

  DDRB|=0xFF;

  while(1)

  {

    TEMP=ADCL+ADCH*256;           // ADCH is read only to update ADC data register, otherwise no need

     TEMP=TEMP/4;

     if(TEMP<=20) 

     PORTB=1<<PB1;                          //BLUE

    else if(TEMP>20&&TEMP<=28)

    PORTB=1<<PB0;                           //GREEN

    else if(TEMP>28&&TEMP<=35)

    PORTB=1<<PB3;                           //RED

   else

    PORTB= ((1<<PB1)| (1<<PB0)| (1<<PB3));

  }

}

But I don¡¯t know what the problem is; uC only blinks the led which satisfies the minimum criteria for temp i.e. blue. Even if I multiply the final TEMP variable by 10000 and keep min temp criteria for blue led to blink as less than 2¢ªC, then also only blue led blinks. It seems the uC is taking no voltage as input.

I hv checked that if I write (PORTB=1<<PB0), then green led blinks or (PORTB=1<<PB3), then red led blinks instead of (PORTB=1<<PB1)

I have connected output pin of sensor directly to uC, is there a problem in it or do I need to use more decoupling capacitors somewhere in the circuit.