The 555 timer specs will tell you how it works. THe Cap is charged to a threshold in an oscillator condition with a current source regulated by a reference voltage, internal current pump, external resistance and capacitance. Cm is the external cap in this case connected to ground. You need stainless steel or nickel plated probes.
First make a list of specs of what you want this circuit to do and not to do, be specific in numbers and think details
Consider;
Where is this intended to be used?
How long must it work for?
What is your budget for cost? and time?
What is the purpose of the sensor? details? cheap self powered soil sensor? portable measurement tool?
input possibilities, resistance range vs conditions, like depth of soil to be measured. range of conditions
Outputs? Human interface? Autonomous? Servo control system? hydroponic sensor?
Processing required of sensor. tools avail. power avail?
Static and high voltage protection circuit needed !!!
There are infinite number of solutions
But all depends on what you want, need and are capable of doing.
Fastest way to learn is reverse engineer;
the state of the art commercially,
or patents,
or consumer products,
Then make it work and try to understand it and possibly improve it for your needs.
To make an AC sensor, here using capacitance of the dielectric, which is water embedded in soil, you begin realize it is not ideal and has lossy and galvanic characteristics.
It also changes with depth. So the probe should be insulated and only the end exposed for a certain length so you can measure at desired depths independent of surface watering. Some products use chalk or gypson coated probes to absorb moisture, which works but adds delay time, which is why RF Cap method is better.
If you decide to measure C@f, determine why higher f is better for some conditions, but if you only want cheap and dirty then use DC resistance and expect unreliable results.
If you choose %moistiure to C to F conversion then convert F to % moisture using an F to V converter (eg. tach circuit with pulse charge open switch to diode cap)
I suggest you make a list of the specs for yourself before you start to design ANYTHING. Understand what you need , nice to have and do not want and specify it. Otherwise, you will spin your wheels. like this thread.
Once you go thru this process in detail, the solution looks simple and elegant, but the process to get there is not. That is what design is all about.
That concludes my sermon :lol: