Tag Archives: Thermometer


A Digital temperature meter using an LM35 temperature sensor

Introduction

A digital thermometer is a good choice of project for beginners who just stepped in to the world of microcontrollers because it provides an opportunity to learn using sensors to measure the real world signals that are analog in nature. This article describes a similar project based on a PIC16F688 microcontroller and an LM35 temperature sensor. LM35 is an analog sensor that converts the surrounding temperature to a proportional analog voltage. The output from the sensor is connected to one of the ADC channel inputs of the PIC16F688 microcontroller to derive the equivalent temperature value in digital format. The computed temperature is displayed in a 16×2 character LCD, in both °C and °F scales.

Theory

The LM35 series of temperature sensors are produced by National Semiconductor Corporation and are rated to operate over a -55 °C to 150°C temperature range. These sensors do not require any external calibration and the  output voltage is proportional to the temperature. The scale factor for temperature to voltage conversion is 10 mV per °C. The LM35 series sensors come in different packages. The one I used is in a hermatic TO-46 transistor package where the metal case is connected to the negative pin (Gnd).

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AVR displays body temperature on a Nokia 3310 LCD

This project describes how to measure temperature with Atmega8 and a thermistor and display it on a Nokia 3310 LCD. A thermistor is a device that changes its resistance with temperature. With a proper resistor divider network, the temperature can be measured by measuring the voltage across the thermistor. The voltage across the thermistor is measured by the on-chip ADC of Atmega8. The necessary coefficients to convert the resistance to temperature are given in the manufacturer’s datasheet of the thermistor. The thermistor used in this project is Vishay NTCLE100E3103JB0 (found at Sparkfun.com).

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