Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot

Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by kaybe on Tuesday June 27, 2017 @04:05PM (#14648886)

Put a metal stick in the soil and check the travel time of the reflected signal:

https://en.wikipedia.org/wiki/Time-domain_reflectometer#TDR_...

https://en.wikipedia.org/wiki/Measuring_moisture_content_usi...

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Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by setq on Tuesday June 27, 2017 @06:50PM (#14650269)

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.

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Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by tonyarkles on Tuesday June 27, 2017 @11:43PM (#14651551)

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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Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by setq on Wednesday June 28, 2017 @02:46AM (#14652149)

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!

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Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by kaybe on Thursday June 29, 2017 @08:35AM (#14662711)

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!

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Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by tonyarkles on Wednesday June 28, 2017 @02:26PM (#14656872)

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

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Re:Show HN: GreenPiThumb – A Raspberry Pi Gardening Bot by joshvm on Wednesday June 28, 2017 @09:21AM (#14653806)

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?

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