Well, I know this solution but it is rather expensive. Minimum set (without power supply) costs 100 EUR. There are ready solutions that are technically better but they are also expnesive - like thse 2 for example (there are other as well): https://ldperformance.co.uk/product/wideband-controller-for-bosch-lsu49/
My aim is to find chep solution for not more than 30-40 EUR overall , including arduino uno. Control of lambda in pellet boiler does not need such high acuracy like for the car engine. The burnig process in the pellet boiler is naturally varying because the fuel is beeing supplied in portions and burns during the pause. Therefore, the lambda changes in very wide range. That will need integration for 1-2 minutes for measuring the average lambda.
I intend to use old and cheap LSM11 lambda sonde (14 EUR) and 16 bit ADC INA226 (2 EUR) for the purpose , or ADC ADS115. LSM11 does not need so strict control of temperature like LSU 4.9 or latest LSU ADV. It needs just 12 V supply to the heatimh element.
I will probably include also channel for measuring temperature of exhaust gases by MAX6675 and K type sensor (thermocouple). The exhaust gases temperature should not fall below 120 deg.C to avoid condensation in chimney. So the regulation of burning process of pellets is compromise between lambda and temperature of exaust gases. PT500 sendsor will be probably also suitable for this purpose.
The ordered from Aliexpress INA226 and LSM11 sensor came yesterday. I will try them soon.
I wrote the code and connected LSM11 O2 sensor to INA226. All works well and display shows the O2 in %. But I found a core problem. The INA226 input impedance is 13 kohm measured both directions by multimeter on in+ and in- . That is not a problem if shunt of 0.1 ohm is there. But if I remove the shunt to measure input voltage of the O2 sensor the 13 kohm input impedance affects significantly the operation of O2 sensor and generated by it output millivolts. So INA226 is not suitable in this case. I will try with ADS1115.
Hi Tom George,
The sensor voltage is in range from -10 mV to +40 mV. The Vbs measure voltage against ground. If I am not wrong, it cannot measure negative voltage, is it? Therefore I remover shunt to measure differential voltage as RIN67630 suggested. In+ in- measure voltage from -80 mV till +80 mV.
I was wrong. I measured the output voltage of the sensor while connected and while is disconnected to the in+/in- . It is almost same. It is even lower when is disconnected. So the 13 kohm input impedance of INA226 does not affect the sensor voltage.
However I expected about -10 mV in open air (20.7% oxygen) as per the sensor diagram, but I measure about -6 mV which slightly vary up/down after the decimal point. It looks that sensor needs some kind of calibration. I will write additional code for sensor calibrate on start up. Calibration activation will be by button when it is exposed to free air. It should wait sensor to worm up and then will measure and store the error.
Then all measurements will be corrected by this error value.
Voltage measurement range as per INA226 manual on in+/in- is -81 till +81 mV. It is differential input.
Measurement range between Vbs and ground is from 0 to 36 v.
