Happy to help in any way I can. What is the version number printed on the AQEv2 Interface Shield? From what I can tell, A6 is the LCD backlight control signal. Depending on the shield version, it’s not unreasonable to believe A7 is not connected. In the most current version of the shield, A7 is used as the SoftwareSerial RX pin for our particulate sensors.
The four pads you are referring to (below the LCD contrast potentiometer and above the LCD connector) is labelled G, 5V, ck, dt. That’s a (5V) I2C header, and the they are, left-to-right, GND, 5V, SCK, and SDA.
The voltage values you are getting are in the ballpark of what I expect from those sensor types. For the conversion of the voltage to a concentration, it is pretty important to include the characterized response to temperature to get an accurate reading (ref1, ref2). So it’s not just as simple as concentration = voltage * slope + offset (I wish!). We’ve come a long way in sophistication since that Egg was built.
The code on github has temperature baseline compensation, but unfortunately it’s not going to do you much good unless you have the means put the unit through a temperature profile and then program in the results. Failing that, yes, you can adjust the CAL_OFFSET values to bring the readings into the positive. Just use your favorite calculator / excel to determine what the offset needs to be to get you a positive result. You can basically ignore the sensitivity as it’s just used to determine the slope.
