Wireless logging system?

I'm after a some kind of wireless logging system for a property where a number of moisture sensors are located around a central PC that logs the readings. The moisture sensors are in clusters of 3 or 4, once a day a micro (PIC) takes a reading off each sensor and transmits the cluster ID and the set of readings. There would be about 6 or so of these clusters all within about a 100m radius of the logging computer.

While I'm guessing I could use a UHF TX module hanging off the micro and bodge up something on the recieving end, I wanted to check whether there was some kind of system already available that would reduce the amount of code and development to get it going.

Any suggestions?

Thanks Lionel - you're right, that's an excellent design to work off.

The only point of difference is the sensing - my application has a cluster of sensors buried a different depths (30cm, 1.5m and 2.5m for example) to monitor the movement of water from the sub-surface, down through the rootzone and below the rootzone. That said, converting it to work with the different sensor would be trivial.

Out of interest what are you monitoring - a vineyard? Do you use it to save on water useage or to optimise soil moisture for growth / fruit or whatever? regards rob

We do something quite similar with our water level and temperature wireless short range telemetry system. Could fairly simply be adapted to your application. How many do you want?

Regards John

In this application it's apple trees. Water wastage is not a great concern in this particular orchard - although that could change in the future. Rather, the idea is to monitor the rate of percolation of water through the ground to below the root zone - there's no point putting a bunch of water in when the majority is ending up below root level, thus instead of one 8-hour cycle it may be better to do 2 x 4 hours over 2 days etc, depending on the soil and water uptake by the tree.

This is also important when fertilising through the irrigation system as you want the nutrient to hang around the root zone and get sucked up. Beyond that, it would obviosly be useful to get an accurate picture of the moisture levels accross the orchard in order to optimise water use.

If it does work out, I might try it out at a friend's vineyard to see if it has a similar application there.

wireless

That water level system with the neat sensor arrangement, wireless link and high measurement resolution is very impressive John.

In this situation I'm proposing to install up to 6 clusters of 3 or 4 sensors each buied at different depths. Each sensor, in the first instance, will be a gypsum block (2 wires stuck in a lump of plaster) measured using a

555 that generates a frequency proportional to the resistance in the block. A PIC will count the frequency of each sensor and then send the results to the logging computer which will generate a web page showing the data both numerically and graphically.

If the system works, the gypsum blocks will be replaced by Watermark sensors that allegedly last up to 10 years, but cost US$30 each

At the moment, having looked at the project that Lionel posted earlier, I'm leaning towards a transceiver based system where the logging computer polls each client to report, thus avoiding collisions. I think I'll have a bit of a play with the little UHF tranceiver module that Oatley flogs and do a bit of testing - I have to use a micro for the moisture sensing, so it'd be good if I can get it to do the comms as well.

That said, I might very well get jack of trying to get the comms bit to work (which is why I was looking about for some kind of module).

the

within

code

When I was an apprentice i remember working in with the CSIRO to do a similar thing. They were using a transmission line with RF to detect moisture and conductivity. From memory, they used two stainless rods seperated by teflon as the sensor. Might be worth looking into.

The other thing i saw was sensors that measured water flow or sap flow by using a heater and 2 temp sensors. Very interesting stuff. I should of stayed in that industry.

If you poke around Google, you'll find there are a whole range of different techniques for measuring soil moisture - the most accurate seems to be a thing called a neutron probe - I hadn't seen the RF idea in my research, but given the earlier-mentioned project's use of audio to determine moisture levels, it seems there are a large number of valid approaches.

As I mentioned in another post, I am starting with simple gypsum blocks containing bare wires and measuring the resistance between them using an AC signal to avoid electrolysis. The advatage is that the sensors are very cheap, although with limited lifetime buried in the soil, and the circuit to measure the resistance is very simple (555 timer). The required accuracy and resolution isn't terribly high, rather it is more an issue of trends in the data

AC won't stop electrolysis

stainless-steel wires will corrode much slower than copper

Bye. Jasen

work

Keep an eye out for an up coming silicon chip article (Dec or Jan?). A little birdy tells me they are exploring the new RF transmitters from Jaycar.

I guess I'll have to try and track down some twin-core insulated SS wire. Does that exist?

Trust me, the conductors will outlast the gypsum.

But will the resistance change as the conductors corrode?

Keep the duty cycle low. Use AC but only energise the stainless steel wires when you are taking a reading.

Should last ages.

AFAIK unfortunately no, you'd have to make your own...

but you only really need the SS at the ends of the cable, joining stainless to copper in a way that'll withstand the elements can be tricky, one way woulld be to spot weld it and then seal it with "liquid electrical tape" or similar but some of those grease-filled "squeeze" connectors that the telecom guys use should be almost as good if slightly more bulky.

--

Bye.
   Jasen

If you cannot find stainless steel (it is available for use in MIG [metal inert gas] welding machines) then use the next best thing, nichrome as used in heating elements, it is very corrosion resistant. Bye Roy