Fun story: we're doing a project at work to do with agriculture.
We bought some pot plants to keep alive and some cheap soil sensors; they're just two electrodes and you pass a current through the soil, measure the resistance and get a proxy for soil moisture. We left the sensor continuously logging on Friday, came back on Monday to find the plant very unhappy and the probe contacts almost completely corroded. Within a day or two it was a stump, never recovered.
Turns out you're not supposed to leave these things powered on 24/7 and capacitative sensors are a much better solution. TIL you can electrocute plants.
This is a really cool idea actually. However you're likely talking sub nanosecond rise times which makes things a little difficult in the signal processing space. For example my scope (knackered old 1971 Tek 475) can barely manage 1.8ns on a properly terminated transmission line which this will likely not be.
I don't remember the name of the peripheral off the top of my head, but there are some PICs that have a very fast time measurement unit in them. I don't know if they're fast enough for soil measurement, but there's app notes around for doing TDR-based liquid level sensing with them.
3$ for the microcontroller, 3$ for the 4 ns comparator, and a few cents for the resistors, capacitors, and 7 GHz transistors (I couldn't find the exact model), and probably some dollars for the rest of the materials such as steel rods and tubes. All in all it looks doable for <50$ easily. That's pretty amazing!
Provided you can get the rise time fast enough, you can use a TDC7200 http://www.ti.com/product/TDC7200 which will give you picosecond timing on the signal.
On the PIC solution you posted, how do you get around the fact that the comparator is only good to 4ns (regardless of whether you can time down to 4ps)? Lots of measurements and look at the stdev?
We bought some pot plants to keep alive and some cheap soil sensors; they're just two electrodes and you pass a current through the soil, measure the resistance and get a proxy for soil moisture. We left the sensor continuously logging on Friday, came back on Monday to find the plant very unhappy and the probe contacts almost completely corroded. Within a day or two it was a stump, never recovered.
Turns out you're not supposed to leave these things powered on 24/7 and capacitative sensors are a much better solution. TIL you can electrocute plants.