Need help converting temp to frequency

Discussion in 'The Projects Forum' started by 123troy, Jun 25, 2013.

  1. 123troy

    Thread Starter New Member

    Jun 25, 2013
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    Hello,

    I am working on project that takes the body's temperature, but the output needs to be in the form of frequency.

    I am looking at using a thermopile, LM358 op amp and a AD7740 voltage to frequency converter. Using the datasheet for the AD7740 and this link for the thermopile to LM358, http://www.instructables.com/id/Thermopile-Sensor/step3/Op-Amp/

    I have created the schematic below, but I have some doubts if this will work. I have concerns in that the AD7740 needs a power in, so where does the battery get grounded? Also, Is there another temperature sensor that can be used to simplify this circuit?

    [​IMG]

    I do apologize if I am asking redundant questions or if I am way off. I have been removed from electronics for the past 15 yrs or so and I am trying to get back in and reading everythnig that I can.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,142
    1,790
    Where are the power and GND connections for the LM358? That would be a good place to connect the battery ground. Are you sure the LM358 will operate from a single supply at such a low voltage?

    Oh..., and the LM358 and the voltage to frequency converter should also probably have the same ground.
     
    Last edited: Jun 25, 2013
  3. 123troy

    Thread Starter New Member

    Jun 25, 2013
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    0
    Sorry about that PH1, is actually the thermopile (I was not able to find a thermopile in Eagle). The thermopile produces millivolts. Is that enough to power the LM?
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I would look into using an LM34 and an LM131 as shown in the attachment taken from page 8 of the TI LM34 datasheet.
     
  5. 123troy

    Thread Starter New Member

    Jun 25, 2013
    3
    0
    Thanks for the help.

    Trace, I looked at that but I am looking to design something using a 3 volt lithium button battery.

    Taking Papa's help I tweaked my schematic to the following:

    [​IMG]

    Thanks
     
    Last edited: Jun 25, 2013
  6. MrChips

    Moderator

    Oct 2, 2009
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    Does the sensor have to be a thermopile?
    How about a thermistor in a 555 timer circuit?
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    With a 2032 battery, a 555 would have to be CMOS.
     
  8. LDC3

    Active Member

    Apr 27, 2013
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    You could try a MAX6577. It provides a frequency output based on temperature. Oh, wait a minute, it's not very accurate so it may not suit your purpose.
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,142
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    No. The LM358 needs at least a positive supply of at leas several volts. This is not the same thing as the input signal which is measured in milivolts. RTFDS! That means Read the ****ing Data Sheet.

    OOOPS...I see you found the datasheet. OK then.
     
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