Measurement of capacitance of soil probe to determine moisture content of soil Uses an LC oscillator at f from 50-100MHz Frequency of oscillation is proportional to the inverse root of capacitance
At these frequencies and temperatures 0 < T < 30 degC the dielectric function of water is ~80; small changes in soil moisture content create a large change in capacitance
There are two physical prototypes, mark I & II:
Mark I Uses a Clapp oscillator and emitter follower with BC547B transistors. In the tank circuit is a toroidal inductor of 13-16 turns and 3x 22pF capacitors. Base input and stray capacitance estimated as 7 and 3 respectively.
Mark II Uses a Motorola VCO MC1648 a toroidal inductor of 9 turns and 10pF cpacitor in the tank circuit (adds to input capacitance of 6pF)
Marks I & II use the same prescaler and counter circuits to convert the high frequency oscillation to a low frequency square wave, reducing the frequency by 1/2^17
The low frequency is measured using a 16MHz 5V Promini and the result sent to a gateway using a cheap 433MHz RF transmitter
Repository contains:
Clapp circuit 181226.log : LTSpice simulation of Clapp oscillator and transmission line
ReadNodeDataNew.R : R analysis of data from soil probe
Schematic_Soil-Sense-Mark-1_buffered-Clapp-circuit_20181225231505.pdf : Schematic of circuit for SoilSenseMark1 from EasyEDA
SketchVxx.cpp : Old - hidden
SoilSenseMark1 : C++ code for ProMini measurement and reporting sequence
SoilSenseMark2 : C++ code for ProMini measurement and reporting sequence
SoilSenseMark2_board.pdf : Vero board design for SoilSenseMark2 from Eagle
SoilSenseMark2_schematic.pdf : Schematic of circuit for SoilSenseMark2 from Eagle
freqCounterV4.brd : Vero board design for SoilSenseMark2 from Eagle
freqCounterV4.sch : Schematic of circuit for SoilSenseMark2 from Eagle