Sunday, December 12, 2010

K-Type thermocouple sensor I2C interface board

Today I finished with board preparation and soldering:
K-Type thermocouple sensor I2C interface board
Most important part is firmware to turn ATTiny13 into I2C ADC which is not ready yet. Will continue with this task on next week.

Friday, December 10, 2010

K-Type thermocouple measuring

I'm designing the circuit to measure high temperatures (up to 600 °C) with at least 1 degree resolution with TWI (I2C) interface.

Literature on thermocouples:

Circuit design will include LM75 TWI temperature sensor for cold junction compensation, LM358 dual operational amplifier for two-stage amplification and ATTiny13 microcontroller for ADC and TWI interfacing (software slave).

Board layout supposed to be as narrower as possible to be insulated with heatshrink tube. Thermocouple will be connected using 3.5 mm screw terminal. For MCU communication and power supply 4-pin JST connector will be used (2.5 mm step). TWI address of LM75 will be configured using solder jumpers, ATTiny address will be set using TWI protocol and default address and then stored to EEPROM.

Device will support 3.3V and 5V supply.

Dual stage Non-inverting amplifier calculation is the following:

As I need 600 degrees upper level, voltage could be aplified up to 125 times to be below 3.3V for normal operation with this supply voltage as about 25mV corresponds to 602° on k-type probe.

Vo=Vi*(1+R2/R1)

1st stage: R2=10K, R1=1K, Gain=11

2nd stage: R2=10K, R1=1K, Gain=11

Total gain = 11*11 = 121

Total gain of 121 is good for us as it gives 25mV*121=3.025V at 602 °C (25 mV reading).

Measurement resolution considering 10-bit ADC will be:

At 3.3V 3.025/(3.3/1024)=938 steps, about 0.64 °C per step.

At 5V 3.025/(5/1024)=619 steps, about 0.97 °C per step.

I will work on proof of concept today; let's see the results.