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Temperature sensor?

Discussion in 'General Electronics' started by Greg T, Jun 7, 2005.

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  1. Greg T

    Greg T Guest

    Gidday,

    After losing faith in computer motherboard sensors, I'm trying to design
    an accurate (+/- .2 degC) electronic temperature sensor that will plug
    into a computer. I'm mostly interested in measuring the ambient
    temperature in my server room.

    Sensors like the LM75 look ideal but I haven't come across any decent
    schematics that can interface with them. I've yet to decide whether to
    use USB, RS-232 or IEE-1284 as the external interface.

    Can anyone point me in the right direction?

    Thanks,
    Greg
     
  2. The LM75 is only accurate to 2C with a resolution of .5C, so I don't
    think you can achieve your goal with that part. You might want to look
    at the LM35 since it will at least make more resolution available. You
    should be able to easily achieve .1C resolution and excellent accuracy
    with some calibration.

    BTW, Dallas makes some 1-wire devices that have digital readout with up
    to 1/16C resolution and very good accuracy (without calibration).
     
  3. Greg T

    Greg T Guest

    Ah yes, my mistake. It was the LM35 I was thinking of.

    Does anyone know of a site that provides schematics for such projects?

    thanks,
    Greg
     
  4. The datasheet is a good place to start as it contains some reference
    circuits. Since the LM35 outputs a voltage that is proportionate to the
    temperature, you will need an external ADC (12 bits would be a good
    choice) and associated reference voltage (4.096V makes things easy) to
    measure it. You will also need some kind of I2C or SPI capable device
    to talk to the ADC. I would use a microcontroller, but there are other
    ways.

    I have easily achieved .1F resolution using an LM34 and a PIC with only
    a 10 bit ADC. I was able to accomplish that by using a Vref of only
    1.024V. Currently that means that I can only measure temps up to
    102.3F. But since my house should never be any hotter than that, it
    works for me. If I needed a higher max temp capability, I could use an
    op-amp to offset the output of the LM34 and slide the 102.3F range up or
    down as required. For example, by subtracting 500mV from the output of
    the sensor, I can increase the max temp limit to 152.3F but still
    maintain .1F resolution. Of course the lower limit would now be 50F
    instead of 0F. A 12 bit ADC would be a better choice since I could
    maintain .1F resolution AND utilize the full temperature range of the
    sensor.
     
  5. Robert Baer

    Robert Baer Guest

    However, Dallas is another name for Maxim.
    And Maxim has a bad habit of advertising vaporware.
    Many distributors and companies have blacklisted them as a source due
    to the problem of unavailable parts.
     
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