Month: April 2013

Fix for ratty behaviour of Wiznet 5200 with the OpenKontrol Gateway

I’m a fan of Ciseco’s OpenKontrol Gateway. It doesn’t get anywhere near enough love on their site compared to the flighty Eve. Where it scores is that it’s a cheap way of making stuff happen and I like that.

Ciseco OKG

It makes a dandy data logger, basically get the RTC kit and the SD card socket, slam one of their XRF radios in the left hand socket plus a LLAP thermistor device within 50 yards and you’re all set 🙂 You can add more thermistor devices while the datalogger is running and they get logged too without bumping it. And all for less than about £80 total system cost, what’s not to like? Okay in an ideal world the power consumption could come down a little bit as the OKG is in receive only mode, but it’s livable with – using 8 NiMH AAs gave me a couple of days runtime.

thermistor board with XRF

thermistor board in box

 

This saved our tail with finding out what the soil temperature was like in and outside a polytunnel earlier this year

reading of soil and air temp inside and outside polytunnel

What you can also see in the OKG picture is a Wiznet 5200 Ethernet card. What I didn’t realise is that this damn thing has a microcontroller inside. And it’s reset at the same time as the Arduino, so you get some sort of evil who-dares-wins race to get out of the starting blocks when that reset line goes High. The horse you want to win is the Arduino, so as the Wiznet’s ready to get initialised by the Arduino, but it doesn’t seem to happen that way.

Looks like with some configs the wiznet wins the race, and presumably gets confused when it sees data and stuff. I never got it to work reliably with the SD card datalogging. I the got another OKG, this time without the SD card and RTC option, just for posting LLAP devices from the XRF to Cosm. And this damn thing was about 50% reliable, it  gave me a world of hurt until I discovered this natty fix for this sucker over at Open EnergyMonitor. I am so grateful to them. Basically you dis the reset on the Wiznet and ground the sonofabitch once the Arduino powers up, so you know where it starts from rather than whatever it got to see on the data lines as everything Arduino and SD card and XRF and what have you fired up. Sounds good to me, and it works. I had considered chucking a monostable at this to ground the line for a sec so the Arduino can sort itself out, but caught the Wiznet getting its knickers in a twist after a while having started okay. So the option to deck it if it returns more than five -ve values for etherclient post is necessary. I went off pin 3 of the RH socket which is arduino pin 6 ISTR, but I need to see if there is a genuinely spare pin because one day I might want to use the OKG as an RF to RF gateway which would mean getting both slots into service. I’m open to pinching the red status LED if necessary 😉

The Wiznet seems actually reliable now, and if it does get itself stuffed in an IoT application returning a negative value for a post five times in a row I simply reset it and reinit the device. It’s not the end of the world if five temperature readings get lost, as long as recovery is possible 😉

You still have to patch the Arduino Ethernet library as per this wiznet blog to make it go.

Logging soil and air temperature in a polytunnel

I have used two Dallas DS18B20 sensors to log the soil temperature inside and outside a polytunnel, as well as a free-standing RF thermistor probe for the air temperature.

reading of soil and air temp inside and outside polytunnel
reading of soil and air temp inside and outside polytunnel

The air temperature reading clearly has issues in the day, but the soil temperature readings show the value of a polytunnel. The blue trace, inside, shows the soil temperature to always remain above 5C, which is apparently important for seed germination. The temperature excursions to be smaller, ranging over about 8C rather than 12 for the outside soil temperature, and the outdie soil temperature falls to 0 at one point.

The air temperature suffers in the day from the problem of the greenhouse effect, which is of course the whole point of a polytunnel, but the sensor additionally suffers from not being in thermal balance in the day.

Measuring temperature in a room is easy, and the one-box LLAP sensors do that fine. However, setting them up where the sun shines on them means the sensor gets heated by the sun and no longer accurately reflects the ambient air temperature, because the black case is heated up faster than it radiates / convects heat away, so it’s no longer in thermal equilibrium.

The way to address this is to use an aspirated thermomenter, which has a thermal screen from above to stop solar radiation heating the sensor and a fan to waft air across the sensor – it doesn’t need a high fan speed but it needs some.

However, at night solar radiation isn’t a problem, so the air temperature reading is useful in the night, where it shows the air temperature falling below zero inside the polytunnel from about 9pm to 6am.

The soil sensors are about 5cm below the surface, which is typically where seeds would be planted.