Thermometer using Microcontroller

Introduction

• This article is about a simple 0-100°C digital thermometer using Microcontroller.
• The circuit is based on LM35 analog temperature sensor, ADC 0804 and AT89S51 microcontroller.
• LM35 is an analog temperature sensor IC which can measure a temperature range of -55 to 150°C. Its output voltage varies 10mV per °C change in temperature. ADC 0804 is used to convert the analogue output voltage of the LM35 to a proportional 8 bit digital value suitable for the microcontroller.
• The microcontroller accepts the output of ADC , performs necessary manipulations on it and displays it numerically on a 2 digit seven segment LED display.

Circuit Diagram

Circuit Diagram Description

Output of the LM35 is connected to the +Vin (pin 6) of the ADC 0804. Resistor R13 and preset R14 is used to provide an external reference voltage of 1.28V to the Vref/2 pin  ( pin 9) of the ADC 0804 and with this reference voltage, the step size of the ADC will be 10mV and span will be 0-1 V. This means that for a 10mV input the digital out of ADC will be 1 (1 in decimal also), for 20mV it will be 10 (2 in decimal), for 30mV it will be 11 (3 in decimal) and so on. The microcontroller accepts this data and puts it on the seven segment display. Digital out of the ADC (D0 to D7) are connected to P1 (P1.0 to P1.7) of the microcontroller. This is the line through which the microcontroller accepts data from the ADC. The control pins CS, RD, WR and  INTR are connected to P3.7, P3.6, P3.5 and P3.4 of the microcontroller. This is the data path through which the microcontroller sends chip select (CS), read (RD) write (WR) signals to the ADC and receives INTR signal from the ADC. Data lines (a to h) of the multiplexed seven segment display are interfaced to P0 (P0.0 to P0.7) of the microcontroller. Activation signals for the segment driver transistors Q1 and Q2 are available from P3.2 and P3.1 pins of the microcontroller. 

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