[chbot] Just when I thought I had a good soil moisture solution...
erringtona at gmail.com
Sat Nov 21 00:52:41 GMT 2015
Is this any use?
On 21 Nov 2015 07:09, "Robin Gilks" <robin at gilks.org> wrote:
> > ... I discovered that rather than resistive I should be looking at high
> > frequency capacitive sensors.
> Looking into this some more and getting my calculator out, I have come up
> with the following figures (perhaps someone has a feel for how close these
> are likely to be!!) based on some of the ideas in the 'tindie' sensor I
> A pair of probes, 100mm long, 20mm apart will exhibit a capacitance of
> about 1pF in air (using 2 methods, I get .4pF and 2pF so in the same
> The dielectric constant of water is 80 (compared to just over 1 for air)
> so saturated soil (50% water) would look like 40pF and dry soil about
> The STM32F103 has a MCU clock output that can run at half the core clock
> speed, i.e. 36MHz so I'll be looking to measure a change of impedance from
> 110 to 440ohms
> Just playing with the idea of having the CPU driving one end of a length
> of CAT5 twisted pair with a differential signal and putting a bridge of
> low capacitance schotttky diodes between the probes and feeding the output
> back on another of the twisted pairs. That keeps all the underground parts
> passive and puts the CPU next to where I want the 2.4GHz module to return
> the results!!
> If I can cut/etch some PCB material then I'll knock something up for the
> next meeting. This is all becoming quite interesting!
> Robin Gilks
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