The circuit above lets the Raspberry Pi read a type K thermocouple via an AD7715 16 bit ADC. The raspberry Pi communicates to the ADC via the GPIO Pins 18,23,24,25,8 & 7 by bit banging the port from python.

The Thermocouple is connected via J1 and filtered by the low pass filters formed by R1 & R2 and C5 and C9. It is then level shifted to within the common mode of the ADC by connecting via J3 to the reference

Cold junction compensation is made by reading U3 an LM75 via the I2C port on the raspberry pi.

When the ADC is read the resulting  reading in mV is converted to Celsius via a look up table, to which the Cold junction temperature is added.

The thermocouple monitor is fully supported in Pilogger allowing a graphical display of the temperature over time.

QTY  PART-REFS           VALUE              

---  ---------           -----              

Resistors

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3    R1,R2,R5            100                

2    R3,R4               10k                

 

Capacitors

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4    C1,C2,C7,C8         100p               

3    C3,C5,C9            100n               

1    C4                  10uF               

1    C6                  1uF                

 

Integrated Circuits

-------------------

1    U1                  AD7715             

1    U2                  REF192             

1    U3                  ADT75               

 

Miscellaneous

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1    CN1                 CONN-DIL26         

1    J1                  TBLOCK-I2          

1    J3                  CONN-SIL2          

1    X1                  CRYSTAL