WTH questions in electronics - some with answers

Now and then I ran into schematics that just plainy look wrong, so the WTH questions pop.
This page hosts them, hopefully many would get answers (if not already had).

  1. The capacitive soil moisture sensor from DFRobotics. - July 2016
    WTH - a weird frequency to voltage converter using something that's resembles a LP-filter but has an additional diode and resistor in the circuit
    Verdict: "Not going to work with the values in schematics. Also, non-linear complicated response, know well what you are doing if you go this way"