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first of all I have to say that this is my first Arduino thing. I'm trying to connect a k-type termocouple to my Arduino using an LM2904 Oamp, Yes I know that this is not the best solution but is the only that I have right now due to the pandemic situation, that I can't get a dedicated module as an MAX6675

This is what I have done:

enter image description here note: The Grnd symbol next to 2.6k resistor is a handwritten error, its not connected in the circuit

my problem is that with this circuit I'm having 3.7V on the arduino analog input even before connect the termocouple. After connect it, the read is about 2.6V and it starts to increase as expected when heat is applied.

My problem is if I have 2.6V for 32°C for temperatures over 200°C I will have more than 5V. Any help with this?

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  • Do you need to measure temperatures over 200°C? If not, thats ok. The opamp cannot output a higher voltage than its supply voltage, thus it cannot exceed 5V in your circuit. If you need to measure also that range, you need to change the amplification of your circuit by changing the resistor values
    – chrisl
    Sep 3, 2020 at 6:53
  • The MAX6675 is obsolete. You should be considering the MAX31855 or MAX31856 instead.
    – Majenko
    Sep 3, 2020 at 9:21
  • @chrisl, thanks for yours suggestions, I know that I wold have to change the values of my resistors, but, its normal to have 3.7V in the output even with no inputs? with my current setup I have expect to have near .5V not 2.6V I dont know what is wrong here
    – efirvida
    Sep 3, 2020 at 12:10
  • Why do you think, that you get 0.5V without anything connected? The pin would be floating then, so the opamp will be amplifying the noise, thats flying around.
    – chrisl
    Sep 3, 2020 at 14:18
  • @chrisl no I spect to have near 0.5V for on 32°C which is 1.2mV on the K thermocouple table
    – efirvida
    Sep 3, 2020 at 17:44

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