Computer Controlled Greenhouse
Submitted by Matt J, Cumbria, UK
This project
actually started because I had just bought a new PC leaving my old one
doing nothing but sitting in a corner. It was still a perfectly good PC
running windows XP at a reasonable speed for all but the most demanding
of applications and I was determined to use it for something. The
something was based on an idea I had some time ago to automate my
greenhouse. I got this idea after fitting electric windows which opened
and closed at the flick of a switch rather than boring old window stays.
It created an opportunity for automating the ventilation function.
As I saw it there were four areas I had to work on to make this project
happen.
1. Decide exactly what I meant by automating
the greenhouse. i.e. the specifications
2. Choose / design the sensors and actuators
3. Decide how to connect all these to the PC
4. Write the software
Seems simple when you say it quickly ! Anyway, setting about this
in pseudo professional style here were my thoughts on the four areas
starting with the specifications I decided on.
What it should
do.
Temperature:
The greenhouse works best when the temperature is not too hot and not
too cold. Sounds simple but in the spring and autumn you can easily have
a wide range of temperatures from the cold in the middle of the night
(especially in March) to the excessive heat of the day when the sun is
shining (greenhouse effect and all that !). Currently, when the sun’s
out and its starting to cook the tomato plants I usually open all of the
windows which has the desired effect. Problem is being there when they
need to be opened and not being at the shops or down the pub. Also at
night remembering to close them again is, shall we just say,
challenging. (Probably amnesia from all that beer down the pub). So, the
windows should open automatically whenever the temperature gets to a
certain level and close again when it falls to a lower level.
Watering:
Surprisingly plants need watering ! Not when you want to water them but
actually when the soil they are in starts to dry out. Too many plants
are ruined or their growth reduced due to under or over watering.. I’ve
tried over the years with various drip delivery systems but they
inevitably deliver either too much or too little depending on the
ambient conditions and I can never adjust them to get it just right.
They also suffer from the hard water in our area causing blockages due
to scale build-up. The only solution is to monitor the “dryness” of the
soil and automatically deliver water when it gets to an appropriate
dryness. (this sounded like it was going to be a tough part of the spec
to meet).
Humidity:
As any greenhouse gardener will tell you the humidity of the air plays
an important role in many aspects of plant welfare. Some plants are very
tolerant of an arid dry air while others suffer in this environment.
Whilst not as important as the soil dryness I felt that most of my past
success with some plants was down to my attention paid to humidity and
it should be included in any automated system. Therefore I needed to
spray a mist of water into the air whenever the humidity falls below a
certain measured level. (again sounded tough).
Connecting to the
PC
I decided that the method
of connecting my (as yet unspecified) sensors and actuators to the PC
should be done first in case I had to choose specific devices to match
the interface unit. I opted for a USB board product called a
DigiBee which provided a convenient
way of connecting 16 inputs and 16 outputs to the PC. I also purchased a
switching adaptor board to go with it called a
BeeDriver. Although I could have
probably made the switching adaptor myself, it offered a convenient way
of directly connecting higher current devices via screw terminals. I had
considered finding an analogue input device for measuring temperature
etc. but decide this was a bit of an overkill since all I wanted to know
was when a pre-determined temperature threshold had been reached. Before
I firmly decided on the digi-bee I had to be sure I could use this
approach for the other inputs. What follows are my designs for all of
the required sensors, confirming that all I needed was digital type
inputs..
Input Sensors
Temperature Measurement.
The temperature sensing
device used was a simple 5k thermistor purchased from an online store
(25p). The circuit I used is shown below.
The principle of the circuit is very simple. The thermistor forms one
half of a voltage divider which is connected to one of the inputs to a
voltage comparator (1/4 LM339 also from online store(30p) ). The other
input to the comparator is a fixed voltage set by the potentiometer.
Whenever the thermistor decreases in resistance due to a rise in
temperature by enough to make it’s voltage divider produces a voltage
greater than the reference one, the comparator output flips from 0 to
5v. This output is connected to one of the digi-bee inputs to be read by
the computer. By setting the fixed voltage reference potentiometer I
could set the desired switching temperature. I considered using just one
threshold (i.e. comparator) but eventually decided on two to allow me to
have two levels. One for detecting too hot and one for too cold, with an
assumed okay region in between. This avoids the constant on-off
switching around the desired temperature. One LM339 was all that was
needed since it has 4 comparators in the same DIL package.
Humidity
Measurement
Getting a measure of humidity was not as difficult as I had anticipated.
I used an SRHR233 sensor (from RS Components (£3.40)) which is a
resistive humidity sensor which operates very like a thermistor. i.e.
the more humid the air, the lower the resistance. This allowed me to use
the same circuit I had used for the thermistor with just a change in the
resistance values. Again I decided to use two comparators, one for too
humid and one for too dry. This used up the two remaining comparators on
the LM339 device. (not bad value for 30p). circuit shown below..
Soil Dryness Measurement
Again the object here is to determine when the
soil has dried out enough to warrant watering rather than actually
measuring it’s “dryness”. The principle used was based on the fact that
dry soil has a much higher electrical resistance than moist soil. To
evaluate this I inserted two lengths of copper wire about 10cm long and
1cm apart, into a plant pot and connected my meter to them to measure
resistance. The copper wire was simply stripped out of some heavy gauge
twin and earth cable I had lying around my garage.
I measured the resistance at about 80K with the soil fairly
moist and , when left to dry out, the resistance increased quite sharply
to several MegaOhms. Virtually an open circuit. This made the detection
circuit fairly easy to design and in fact, was yet again based on the
good old LM339 comparator. Essentially it is the same as the circuit
used for humidity measurement except the humidity detector is replaced
by two copper wires in the plant pot.
Once I had the basic moisture detection sensor working I realised that I
should really use a number of them since, like most greenhouses, I have
a large number of separate pots, troughs and tubs. It would however be
impractical to give every pot it’s own sensor so I decided that the best
approach was to put one sensor into each type and size of pot. My
assumption was that pots of similar size with similar plants would dry
out at similar rates. I also had to try and arrange these pots together
in these similarity groups to make it easier to apply watering when that
particular group needed it. In the end I managed to “make do” with just
four groups (matching the number of comparators on the second LM339).
Control Outputs
Water Valves
The controlled outputs in
my system were the Motorised windows (3 off) and Solenoid operated water
valves (5 off). As far as driving these is concerned it was simply a
case of connecting them to some small pcb relays which were then
connected directly to the BeeDriver terminals of the digi-bee, taking
care to get the polarity correct as shown in the diagram below.
The solenoid operated water valves served two purposes. One was to turn
on the plant watering system. The 4 valves corresponded to my 4 groups
of “dryness monitored” plants. This allowed me to water only the group
that needed it, when it needed it. The fifth water valve was to provide
the humidity control. When on , this delivered water to an atomising
spray mounted centrally in the apex of the greenhouse. It provided a
very fine mist of water vapour, which actually proved quite effective in
maintaining a good level of humidity.
Motorised Windows
When I originally installed
the motorised window openers some years ago, I arranged one switch to
operate all three windows together. Although I could have left it like
this and just used the one control I decided to go one step further by
having independent control (partly influenced by the fact that I still
had 9 unused outputs on the Bee Driver unit). This would allow me to
vary the amount of ventilation/cooling in three stages.
The connection of the motors was identical to the water valve solenoids
except that each motor required two relays; one for forward and one for
reverse operation. The motorised openers had their own built in over run
limit so they automatically disconnected the appropriate input when the
window was fully open or fully closed. This meant that all I had to do
was ensure they were turned on for “long enough” to reach that limit.
This was about 15 seconds (a job for the software).
Connecting the DigiBee to the computer was (in keeping with
USB devices) very simple. There wasn’t even a driver to install since
Windows XP already has the necessary HID driver installed as standard. I
installed the software which was supplied with the digi-bee (LogicLab)
which offered a quick way to get up and running with basic combinational
logic applied to inputs and outputs. It allowed quite a flexible
approach to the control of the digi-bee outputs (if input 1 is on AND
input 2 is off THEN turn output 2 ON…… that sort of thing). However I
wanted to do my own thing with visual basic software so I used the DLL
(dynamic link library) supplied on the installation disk. As a quick
test I copied the example VB program on the disk into my Microsoft
compiler and fired it up. This was a very simple program that just read
the inputs and set the outputs but it let me make a start without any
headaches. The DLL provided three functions, InitDgb(), ReadInputs() and
SetOutputs(). This was all I needed for basic I/O, the rest was up to
me.
Without going into the fine details of my VB program, the basic strategy
was to set up a loop which repeatedly reads the inputs, determines the
next state of the outputs and then sets the outputs. Where something
needed a timed turn-on (eg the window closing) I simply turned it on for
a counted number of loops. The loop interval I chose was one second.
Running the
System.
When I first ran my program nothing happened. After a moment or two of
pondering, I realised that it was not hot enough for the windows to
open, dry enough for watering or arid enough for spraying a humidifying
mist. I then had to adopt a sensible approach to testing. I selectively
disconnected each of the soil sensors in turn making sure the watering
system came on each time. I got the hairdryer on to the thermistor and,
lo and behold, the windows opened. Each in quick succession. I tried the
hairdryer on the humidity sensor but nothing happened. I assumed that
hot air could still be humid so I needed a better test. Disconnecting
the sensor worked ok (i.e. the atomiser spray turned on) but I really
had to wait for better weather to test it properly. All in all I was
quite pleased with the functionality of the system but I still needed to
fine tune the durations used for watering and humidifying to try and
achieve a reasonable soil moisture balance. I also had to fine tune the
temperature switching points for opening and closing the windows. I
didn’t rush to do this, but rather waited for genuine hot weather
conditions to judge the best settings.
After running it for a few months I discovered one or two
issues, which I hadn’t initially considered. In the very hot weather
even with all of the windows fully open the greenhouse was still too hot
and needed the door open. I considered automation this but decided
instead to fit a fan to increase the airflow. The four independent
dryness sensors were not really enough to represent the varying
conditions within different plant pots. eg some plants like tomatoes,
take large amounts of moisture very quickly out of the soil while others
take much less, even for the same size pots. The solution would be to
either fit more sensors or arrange the plants in common pots. I have
also considered fitting and controlling a space heater for early season
work where the temperature, particularly at night, needs a boost. This
should be straight forward but does mean that I have to use an electric
heating system rather than my old favourite paraffin.
All in all I can’t say the tomatoes taste any better but the greenhouse
is certainly a much more interesting place to spend some time now. I am
constantly trying new ideas for sensors and controls which can be easily
added to the system in the optimistic hope of getting to the stage of a
fully automated greenhouse.
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