LCD Thermometer Circuit with LM35


The LM35 of National Semiconductors that is used in this project is a precision centigrade temperature sensor, which has an analog output voltage. It has a range of -55ºC to +150ºC and an accuracy of ±0.5º C. The output voltage is 10mV/ºC. The output voltage is converted by the AD convertor of the AT Mega8. The temperature is displayed on an LCD module. This is the figure of the circuit.

In this example the thermometer has a range of 0ºC to 40ºC and a resolution of 0.5ºC. If you want to have a read out in Fahrenheit you can use the LM34. The software for this project is written in BASCOM AVR. The BASCOM AVR compiler has build in commands for reading out the ADC port of a AVR microcontroller. The result is displayed on a LCD module in a discrete value of the temperature and in a bar-graph. The AT Mega8 has a A/D converter which can give an output of 210 = 1024 discrete values. When a 5V supply is used you have a resolution of 5000mV/1024 = 4.8mV. Because the LM35 has a output of 10mV/C the resolution of the thermometer is 10mV/4.8mV ~ 0.5ºC. The LCD module has 20 columns. In the scale of 0ºC to 40ºC every column represents 2ºC. [Circuit’s Source: National Semiconductor, Inc].


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