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LMT84: I Simply Do Not Understand How to Read from Arduino UNO.

John Weed33
Prodigy 70 points
Part Number: LMT84

Anyone,

I recently needed to get the temperature from an analog temperature device; my first attempt was the LM35C which worked great except that it didn't go into the negative temperature ranges. So, I threw some greenbacks at the LMT84LPM with the expectation that I could pretty much just swap out the two devices and change the math to come up with the temp. Well, that was a no go. Did a good bit of research on-line and read the data sheet, all of which lead me nowhere. In fact, no matter what I do, the value returned from the Arduino's Analog pin is 186. If I pinch the LMT84 between my thumb and forefinger I can get it to change, downwards, which is typical of the response to an increase in heat. However, I am never able to get results that are in the range of the data sheet. 

A side note: The data sheet indicates the use of  capacitors and resistors on the output pin. Whether I use the components or not, I still get the 186 value. 

VR/JW

  • 0
    TI__Mastermind 41155 points
    Hi John,

    Could you check the voltage at the VOUT pin using a voltmeter?

    Thanks,
    Ren
  • 0 in reply to Ren Schackmann
    Prodigy 70 points

    Yes, and thank you again!

    The voltage is 0.918VDC, whether I use the Arduino's 5.0 or 3.3 VDC supplies. If I hold the LMT84 the voltage starts to drop from there.
    I'd say that the value measured by the multimeter, if multiplied by 1000, is almost spot on with the table in the data sheet. Which I guess it should be. At the same time, the Arduino UNO pin is reading 186.

    Update: I changed the Arduino program and added the analogReference(INTERNAL) statement just prior to the loop to begin reading from the pin connected to the LMT84. I now get values in the 856 range, close to the 914-916 range of the multimeter value of the Vout pin (x1000). 



    VR

  • 0 in reply to John Weed33
    TI__Mastermind 41155 points
    Hi John,

    Based on your update, is your issue resolved? Do you have any further questions?

    Thanks,
    Ren
  • 0 in reply to Ren Schackmann
    Prodigy 70 points

    Ren,

    Many thanks. Sorry for the delayed response. I've still not figured out how to read the output of the LMT84LPM into the Arduino Uno. I'm sure it has to do with the reference voltage, which I can set to 1.1VDC. But the values being read are off significantly; I'm just not smart enough in the microprocessor arena to figure it out. I'm getting values of ~860 (resolves to about 31C), where the temperature is actually 21C. I guess I can work it mathematically to make adjustments. I would like to know if there are any mechanical means available to correct the values being read.

    Thanks again!!!

    VR/JW

  • 0 in reply to John Weed33
    TI__Mastermind 41155 points
    Hi John,

    The voltages that you shared previously indicate the LMT84 is working correctly. Unfortunately, I won't be able to help you with your Arduino as it's outside my expertise.

    Thanks,
    Ren
  • 0 in reply to Ren Schackmann
    Prodigy 70 points
    Much appreciated!