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Thread: 2 wire sensors

  1. #1
    Constant Bitrate
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    2 wire sensors

    This is a stupid question that I should know the answer to but bear with me as I have spent as many years trying to forget this stuff since retiring than I spent learning it. In an automotive application you typically have 2 wire and 3 wire sensors, 2 wire sensors like thermistors for measuring temperatures are common. your temp sensor is a 3 wire sensor which makes it easy to hook up to a board that is set up for 3 wire analog inputs, but just the same, a 2 wire temp sensor is an analog input also.
    Now if I wanted to utilize my already present ECT sensor can I just use the 5vdc and sensor return, disregarding the GND, or do I need to build some other circuit to interface it?

  2. #2
    Fusion Brain Creator
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    http://www.mp3car.com/vbulletin/docu...uspension.html

    the fusion brain senses voltage, 2 wire sensors are variable resistance so they must be wired as part of a voltage divider

  3. #3
    Constant Bitrate
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    So looking at the schematic you provided in the link then my 2 wire resistive sensor would go between the GND and the Input terminals on the brain and the 5vdc terminal would connect to a resistor of which the other end would tie in with the input terminal right?
    also is the value of R1 a constant or is this going to be something that depends on the resistive range of the thermistor and if so is there a formula to determine the proper value?

  4. #4
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    "also is the value of R1 a constant or is this going to be something that depends on the resistive range of the thermistor and if so is there a formula to determine the proper value?"

    the latter, and you can certainly optimize, but a nice rule of thumb is R1 = highest resistance the sensor can be

  5. #5
    Constant Bitrate
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    So if I have a thermistor that reads roughly 13k at ambient temp and 60k at freezing, I should be able to replace R1 with a 100k pot and adjust that to get the most accurate temp readings across the range I will be using and then replace it with a resistor valued approximately at the resistance I find with the pot?

  6. #6
    Fusion Brain Creator
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    yup, that'd work

  7. #7
    Constant Bitrate
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    just one other problem, what if my thermistor is a negative temperature coefficient device?

  8. #8
    Fusion Brain Creator
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    why is that a problem?

  9. #9
    Constant Bitrate
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    because as the temperature rises the voltage goes down. with room temp I show about 60 degree F, if I stick the thermistor in ice water It jumps to 125!

  10. #10
    Fusion Brain Creator
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    Quote Originally Posted by jimp View Post
    I show about 60 degree F, if I stick the thermistor in ice water It jumps to 125!
    how are you converting to temperature?

    it's not a problem, you just need a new formula to convert to temperature, utilizing the negative coefficient

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