Re: Soil moisture sensors

[MFHult@xxxxxxxxxxxxxxxxxxxxx]

On Tue, 14 Jun 2005 14:59:12 +0100, "Keith Rathband" <notreal@xxxxxxxxxxx>
wrote in message  <d8mnrl$n4d$1@xxxxxxxxxxxxxxxxxx>:

>I have some more questions about the interface circuit given,
>
>The circuit shows an LT1761 IES-33 3.3V regulator - this seems to obsolete
>with my suppliers, can anyone suggest an alternative.

The LT1761 is/was a low noise, low power device with good transient
performance. emesystems docs suggest that this device was chosen for
known-good performance under "pulse" conditions. If you have a scope, you
could look at the power rail (output) of a substitute device (most any 3.3
regulator -- or higher with commensurate signal output voltage).

But if you are running wires to the watermark, rather than using RF to send
the data, and can provide good DC source, power consumption should not be
an issue. For belt and suspenders engineering , simply add more capacitance
to the input and output terminals of the regulator with another several
ufds of electrolytic and a small cap with good high frequency
characteristics (eg ceramic 0.1 ) Depends in part on connection to
(remote?) power source.

>Also - the circuit shows two 4.7uF ceramic caps, to isolate the sensor
>from the circuit. I'm having difficulties finding suitable ceramic cap of
> this value - can anyone one suggest an alternative ?

You could try audio-grade mylar or non-polar electrolytic capacitors.
Depending on how cold it gets in the winter where you are, you might want
to confirm suitable storage temperature range of electrolytics if you are
going to over-winter the probes and signal conditioner  in the ground. I
suspect emssystems chose ceramic over electrolytic owing to long-term
storage issues and high voltage (think lightning) and ceramic over mylar
owing to size.

Another (better?) way is to omit the input caps in favor of switching
between sensors using (eg) small DIP signal relays. Leave all sensors
unconnected (not grounded) while not measuring and so avoiding long-term DC
offsets that could lead to change in sensor characteristics over time.
Again, this presumes that you are running wires from the sensor to _nearby_
logger or signal input multiplexor.Think lightening (worse case) and ground
currents (ubiquitous).

Or (best) build one 555-input circuit with input cap and switches for
different inputs.

Or just buy their module(s) in appreciation of the nice DIY write-up that
they provide ;-)

Where and how you install the sensor is important. Unless you are doing
this for a PhD dissertation, place the probe(s) in the _root zone_ at a
location where  the plants of concern dry out first. If you place the probe
too high in the soil column, it may record precipitation/irrigation that
never makes it to the roots. Too low and it may not reflect near-surface
moisture variations. One sensor is not very many for this sort of work ..

HTH ... Marc
Marc F Hult