Member #1513133

Member Since: March 27, 2019

Country: United States

  • OK, so here is the problem with this design: the output voltage changes as a function of the parameter hFE of the on board transistor given a fixed impedance at the probe (hFE spec 70-300 but I've measured as high as 470) AND hFE is temperature dependent. What does this mean: The probes are not interchangeable given fixed software defined "moisture" thresholds; If you were to calibrate each probe at room temperature and you take a measurement at 100F it will read much "moister" than one taken at 72F. So if you squint your eyes and wave your hands you will see that the output voltage is proportional to hFE and inversely proportional to the probe impedance - simple fix: add a P-FET to short the probes in order calibrate before each measurement (this takes the temperature out of the equation given the next part), move the probe resistor to the base to give operating headroom for control of the P-FET and increase the probe resistor to a value of 10s to 100s of Kohms (small changes in the probe resistance + probe resistor result in a relatively constant linear change in output voltage over a large variation in hFE).

No public wish lists :(