chopnhack
New Member
Hello all, first time on this forum. I have some basic understanding of electronics, a few small projects under the belt and I am looking to undertake the design and construction of a moisture probe for gardening. Before anyone starts typing that there a dime a dozen moisture probes commercially available, this is a hobby pursuit for me and I have some specifications that I am trying to achieve that are not commercially available, so please humor me 
I have researched some journals on moisture measurement, FDR (field domain reflectometry), etc. and it seems that an ideal solution is to use a coated pcb stake with two trace lines acting as a capacitor such that a frequency passing through the traces will be altered by the soil permittivity (The soil is isolated from the traces). Dry ground has a low permittivity value of say 2-6 whereas wet, sandy soil can be anywhere between 20-30. This change affects the capacitance and thus the frequency.
I tried breadboarding a few RC time delay and oscillator circuits to create a frequency, but both my DMM and digital o-scope failed to give reliable readings of the frequency. The breadboard maybe suspect because I also tested a PIC12F675 blink circuit that only worked when the breadboard was held at an angle other than flat
Can anyone make recommendations for low powered, low item count methods of generating a stable frequency? 50-150MHz would be ideal, but I am willing to work with much less or within the confines of the aforementioned PIC IC that I have on hand. Should I be looking to use a crystal oscillator? I saw this schematic, but it was unclear to me why both TRISA and TRISC in his code had RA4, RA5 set as inputs. Shouldn't RA4 be set to output to drive the crystal? I know very little about RF. Thanks in advance for any assistance and direction!
I have researched some journals on moisture measurement, FDR (field domain reflectometry), etc. and it seems that an ideal solution is to use a coated pcb stake with two trace lines acting as a capacitor such that a frequency passing through the traces will be altered by the soil permittivity (The soil is isolated from the traces). Dry ground has a low permittivity value of say 2-6 whereas wet, sandy soil can be anywhere between 20-30. This change affects the capacitance and thus the frequency.
I tried breadboarding a few RC time delay and oscillator circuits to create a frequency, but both my DMM and digital o-scope failed to give reliable readings of the frequency. The breadboard maybe suspect because I also tested a PIC12F675 blink circuit that only worked when the breadboard was held at an angle other than flat
Can anyone make recommendations for low powered, low item count methods of generating a stable frequency? 50-150MHz would be ideal, but I am willing to work with much less or within the confines of the aforementioned PIC IC that I have on hand. Should I be looking to use a crystal oscillator? I saw this schematic, but it was unclear to me why both TRISA and TRISC in his code had RA4, RA5 set as inputs. Shouldn't RA4 be set to output to drive the crystal? I know very little about RF. Thanks in advance for any assistance and direction!
