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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.

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http://www.edn.com/Pdf/ViewPdf?contentItemId=4433411

Details above. Looks like you can get 3.5ps resolution with an off the shelf PIC after calibration which is pretty impressive!

Wow!

So the price is..

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!

That's the one! I was blown away when I first saw it!

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