Under Construction

Greenhouse monitoring and control using a Raspberry Pi and an Adafruit Feather

The picture shows a Raspberry Pi connected to an Adafruit Feather (on a breadboard) powered by a battery pack. (It all looks so simple at this stage!)

I wanted to use an Adafruit Feather (an Arduino Uno on steroids) to control various things in my new greenhouse. I could have used many other controllers including a Raspberry Pi. In spite of what I said in my blog about Arduinos and RPIs, I have chosen the Feather because of its ability to run for a very long time (years, perhaps) without crashing (although I have had some problems provoked by a WiFi router crash - more about this later). If the power is cut, it automatically resets when the power is restored (also the Feather incorporates a battery backup facility).

Up to now, I have programmed my Arduinos etc using the IDE running on a Windows PC but recently I tried loading the IDE on a Pi3 and using that. (I tried a Pi Zero first, but it was slow.) It worked very well and didn’t have the annoying habit of losing track of the port being used to program that PCs seem to show (at least on my Windows 7 PC).

I took the Pi and the Feather down to the Greenhouse as an experiment and programmed the Feather from the comfort of the house using VNC (I still can’t get over how wonderfully VNC works!) It the occurred to me that I could put a Pi and A Feather in the same box and get the best of both worlds. I could use the Feathers ports the GPIOs on the Pi to connect stuff and everything could talk to everything else either by serial or WiFi.

Background

The next step was to write out a rough specification for the system. It would need to monitor temperature and soil moisture content and turn on a heater and a watering system. I might also add some weather station type facilities such as rain and sunshine gauges.

The “outputs” will be handled by mains switching relays rated at 16 amps (although the total load will be restricted to 13 amps). This requires some reasonably heavy duty wiring, plugs and sockets etc which inevitably takes up quite a bit of space.

The “inputs” will be I2C temperature sensors, some soil moisture sensors (of my own design - yet to be properly tested!) And a few other types as noted above. These need plugs and sockets which again take up space. A took a guess, on past experience, that I could fit the Feather, relays, drivers etc onto a 160 x 100 mm pcb board and get this, a 5 volt power supply, a Raspberry Pi and the various plugs and sockets  into a box 220 x 190 x 100 mm.

With regard to I2C, I know this is intended for short connections within a piece of equipment but I have used it perfectly well over distances of 10 m or more which should be adequate for what I have in mind.

One other thing I need to be aware of is the possibility of moisture getting into the equipment or the connections. The greenhouse is nominally watertight but slight leaks can occur, there is condensation and, of course, stuff gets watered in a greenhouse!

Design

Block diagram of circuit arrangement

The Rpi and the Feather are connected by a serial/usb programming lead and a direct connection (via a buffering transistor) between GPIO 27 and the reset pin on the Feather. Thus the Rpi can reset the Feather if it crashes. The Feather also receives its power via the usb programming lead.

The Feather can restart the Pi if it has been shutdown (or crashed) as the power supply to the Pi can be interrupted by a relay operated by the Feather. If the Pi is shutdown, power to the usb ports is turned off which would shutdown the Feather were it not for its battery which automatically kicks in when power is lost.

The relays controlling mains powered systems are switched by a MCP23017 port expander which talks to the Feather by I2C.

Temperature sensors comprising TMP102 I2C sensors are also linked to the Feather’s I2C bus.

My dampness sensors connect to individual digital inputs on the Feather via opto-isolators. The sensors produce a train of pulses the repetition frequency of which relates to the dampness in the soil being measured. The Feather measures these rates. The sensors involve probes in the soil and it is probably not a good idea to randomly earth microprocessors where all sorts of unwanted current loops could affect matters. Hence the opto-isolators. An isolated power supply to these sensors is provided by the Lipoly battery which is switched out of the Feather supply, when required, by a two pole relay. I still need to determine whether I can get away with energising all the sensors simultaneously for reasons I will go into later.

The Feather is supplemented by a “Adalogger” “Feather Wing” which adds SD card data storage and a real time clock.

General description

Circuit diagrams and construction details are next:

As an aside, here is how to load the Arduino IDE onto the Rpi. Don’t use apt-get as you get a very old version of the IDE missing all sorts of facilities. This is because of the IDE license documentation not meeting the package manager standards, apparently. Go to the arduino.cc site and download the latest linux arm version. At the time of writing this was 1.8.7-r1 but this will obviously change over time. Then following an update, upgrade and reset, in a terminal enter:

cd Downloads/
tar -xf arduino-1.8.7-r1-linuxarm.tar.xz
sudo mv arduino-1.8.7 /opt
sudo /opt/arduino-1.8.7/install.sh

(Thanks to The Magpie)