Humidity sensor plans made

Quite a bit of reading has suggested that capacitive FDR (Frequency-Domain Reflectometry) and TDR (Time-Domain Reflectometry) approaches are going to be impractical when very low cost is a requirement. Resistive continues to look pretty unattractive, so I think TDT (Time-Domain Transmissive) is going to be the approach I focus on.

All this would be much easier if the abundant prior work on this topic weren't locked away behind expensive journal paywalls or commercial secrecy agreements. The few people who seem to have done it for hobby purposes mostly show up asking questions on forums and mailing lists; they never seem to write their results up. Needless to say, I'm frustrated, but I'm going to try to do something about it.

So: In TDT, capacitance is measured by sending a pulse or wave down an insulated loop or line through the soil. The capacitance in the line is altered by the amount of water surrounding the line. As the capacitance of a conductor alters the time it takes for a pulse to propagate through the wire, the capacitance can be measured by timing how long a high pulse takes to propagate through the line.

SoWACS: The soil water moisture content measurement systems and sensors mailing list

If you've been following the soil moisture stuff I've been playing with you will have seen me referencing many others' work, both hobbyists and pros. This isn't new stuff, though it's poorly documented on the 'net and hard to find.

It turns out there's a mailing list dedicated to the topic - but you'd be really, really lucky to find it. Check the archives out here, they're seriously informative:

http://groups.google.com/group/sowacs?pli=1

The group moved from its old hosting to Google Groups a while ago, but the best stuff is in the old archive on sowacs.com. It's a bit patchy, frustratingly, and there don't seem to be mbox files of the archives to download, but it's still very informative.

Atmel Microcontroller (non-ATmega/ATtiny compatible) with built-in 433MHz (US: 310MHz) transmitter!

While researching parts for my soil moisture sensors I stumbled across these awesome Atmel microcontrollers:

• ATAM862-4 Microcontroller with UHF ASK/FSK Transmitter (433MHz non-US band)
• ATAM862-3 Microcontroller with UHF ASK/FSK Transmitter (310MHz-330MHz US band)

I was so excited I had to share. At about AU$8 each, these little beasties might make building wireless soil moisture sensors so much easier it's just not funny. The main problem is going to be ordering them, since Jaycar and Element14 don't carry them, and DigiKey has them as non-stock components with 4000 unit minimum volumes. They're 4-bit 8051-architecture micros so they're not going to be compatible with the ATmega or ATtiny range, so I lose the advantage of having the same arch on sensor and control system. For something as relatively simple as sampling an analog temperature and humidity sensor that may not be a big problem.

It may still land up being easier to use an ATtiny for the analog sensor controller and digital sensor data transmitter, so I can use (mostly) the same software tools as for the ATmega on the control board. I could then hook the sensor's ATtiny up to either some wiring for wired service, or to an RF transmitter IC for wireless operation without much if any change to the sensor codebase.

Atmel also have a family of RF receiver ICs (with matching tx modules or transceivers available) so I might be able to avoid the need for a breakout board / shield for the RF receiver support and just make it an optional component in the base design. Things like the ATA5723 /ATA5724/ATA5728 and the ATA5745 /ATA5746 RF receiver ICs could be awfully handy at about AU$4 each ... if I can find someone who'll sell them to me in less than 1,500 unit quantities. If not, there are lots of other highly integrated 433MHz RF receivers and transmitter ICs out there.

The ATA8204P3-TKQY looks particularly suitable; it's a slower and cheaper unit without UHF, but that shouldn't be a biggie for my use. It's cheaper than any of the other units except the ATA8202-PXQW 19 on digi-key, and should do the job fine. It's surface mount so it won't be assembly-friendly, though. An alternative might be the ALPHA-RX433S from RF-Solutions as that's packaged as a little module that'd be a bit saner to solder up.

DIY DC soil moisture sensor - early test successful

On the the progressive difficulty scale of home built soil moisture sensors the bottom of the ladder is a DC soil conductivity sensor that uses simple resistivity measurement.

It took a couple of hours build one of those last night, most of which was spent incompetently attempting to produce a decent solder joint on steel wire and on the cleaned heads of galvanized nails. Anyone who can use a soldering iron without being a hazard to themselves and those around them should be able to whip something like this up in a few minutes.

Interested in soil moisture sensors and irrigation control? Start with the UF/IFAS virual extension series

I've been having ... "fun" ... trying to find a way to build an affordable network of soil moisture sensors that don't require too much looking after.

It's harder than you'd think, but this UF/IFAS Virtual Extension series on soil moisture and irrigation has made it a lot easier to understand the different approaches and sensor types. It'll help you understand the differences between resistive and capacitive soil moisture measurement, introduce alternatives like tensiometers, etc. This is important whether you plan to DIY your sensors or buy off the shelf.