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Project Help and Ideas » Temperature sensor project

April 25, 2010
by akeller74
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I am having difficulty getting the temperature sensor project to read negative values on the LCD. Any suggestions would be very helpful.

April 25, 2010
by mrobbins
(NerdKits Staff)

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Hi akeller74,

Welcome to the forums!

The microcontroller's analog-to-digital converter (ADC) is only capable of measuring voltages between AGND and AREF, or in this case, 0.0 and 5.0 volts. It assigns these voltages to the ADC readings 0 through 1023. No negative readings are possible in this configuration. The ADC results are an "unsigned" 16-bit integer, where "unsigned" means they do not even contain information about plus/minus. So, if you're trying to measure <0 degrees Fahrenheit, you're out of luck with this setup.

(There do exist "signed" ADCs which can provide you a conversion through 0. If you bought one of those chips, plus modified the LM34 configuration a bit to be able to go negative, you could make it work.)

If instead you're trying to do something like convert degrees Fahrenheit to Celcius and then display values 0 to 32 deg F as their negative Celcius quantities, that should be doable with just some code changes.

Hope that helps -- let us know!


April 25, 2010
by akeller74
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Mike, Thank you for the quick response. Your response makes sense, the MCU I used last semester in class was able to do both signed and unsigned. I guess I didn't realize I needed to look for that when I purchased this setup. Thank You again. I will make do with what I have. I may come back for help with modifying the code to go to Celsius.

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