(this is something I've had "in production" for many months now, I just
haven't had the time or energy to do a proper write up about what I did)
We have a cat water bowl, it looks like this:
Under "normal" circumstances, it usually lasts about seven days. So when our
weekly routine is happening, we'll refill it on a Saturday whilst doing
house chores, and it'll last until the following Saturday when it gets
refilled.
Unfortunately, sometimes our routine gets disrupted, and we forget.
Sometimes, we travel and have a house-sitter, who may not pay as close
attention to such things as ourselves.
Once, one of our cats was licking the condensation off a chilled bottle of
soft drink that was on the kitchen counter one evening, before we realised
the water bowl needed refilling. Naturally, we felt like terrible pet
owners.
So I think it was some time around the 2008 Maker Faire, that I hatched the
idea of having some sort of water sensor on the cat bowl, which would
communicate to one of the various computers in house. At the Maker Faire, I
bought a copy of Making things
talk, and an Arduino starter kit, which consisted of a Diecimila board
and a make-it-yourself proto
shield. I also bought a little electronics starter kit, which consisted
of a breadboard and various components, and a USB-TTL cable.
I decided to use Bluetooth to communicate with the board, as I already had
my MythTV setup using a Bluetooth keyboard and mouse, and it was within
range of the water bowl. I decided against using Zigbee, because I didn't
know anything about it, and I didn't want to add (or learn about) yet
another wireless infrastructure just for this project.
I should point out that I know very little about electronics. I'd never
owned (or really used) a soldering iron until I embarked on this project. I
took a basic soldering class at the Tech
Shop, but I'd already assembled the proto shield by the time I took the
class, so I'd pretty much figured it out.
I had a very naive vision that I could just basically shove two wires in
some water and it'd close a circuit and that would be my water sensor. Of
course this didn't work, so I started hunting around on the Internet for a
circuit that would do this. I happened upon a circuit (I don't seem to have
retained the URL, so I can't link to it), which just consisted of a couple
of transistors and some resistors. So I headed off to Fry's to try and buy
the transistors I needed.
I quickly discovered that I didn't have sufficient information to identify
the transistors that I wanted, but I did happen to stumble upon a cheap
assemble-it-yourself water alarm. It consisted of a PCB, and a
transistor and some resistors and a buzzer. I bought a couple of these
instead.
Between studying the PCB and the circuit
diagram that came with the alarm, I was able to reproduce it on my
breadboard instead of on the PCB. Sure enough, placing the two probes in
water closed the circuit. I replaced the buzzer with an LED so I could see
what was going on.
I attached the circuit to the Arduino proto shield, and had it feed into one
of the digital I/O ports. I wrote some quick and dirty Wiring code so that
when water not present (i.e. the circuit was open and no current was
detected on that I/O port) the LED was switched on. Really at this point, I
didn't need a microcontroller, I could have presumably achieved the same
thing with a NOT gate.
At this point, I wanted to make the sensor remotely queryable. I bought a BlueSMiRF
Silver Bluetooth modem, which I attached to the TX and RX lines of the
board (I first configured it by attaching the USB-TTL cable to it and using
Minicom on my laptop). I extended the Wiring code to provide a rudimentary
prompt, and accept a command to check if water was present.
I think around this time it also dawned on me that I could use the digital
I/O pins as a switch. When they're "high" they provide power. So rather than
constantly running a current through the water, I only needed to briefly
power up the water detection circuit, see if the circuit closed or not, and
then report if water was present if it did.
I much preferred this, as at the time, I was endeavouring to power the whole
sensor off a 9 volt battery. I figured I'd get much better battery life if I
wasn't running a current through the water the entire time. I should point
out that I did some "tongue tests" in a glass of water while the circuit was
powered up, and couldn't detect a difference between when the circuit was on
or off. The last thing I wanted to be doing was zapping the cats!
At this point, the Wiring and Arduino work was pretty much complete. I setup
ser2net on the MythTV server, so that I could just connect to port
4000, and be connected to the water sensor.
apollock@icarus:~$ telnet teevee 4000
Trying 172.16.0.9...
Connected to teevee.andrew.net.au.
Escape character is '^]'.
ser2net port 4000 device /dev/rfcomm0 [9600 N81] (Debian GNU/Linux)
waterbowl> s
Water is present
waterbowl>
telnet> q
Connection closed.
apollock@icarus:~$
The Wiring code running on the Arduino board is checked in here.
Next, I wanted to monitor this with Nagios. One thing I found with the
Bluetooth connection was that it wasn't all that reliable. Not every
connection to port 4000 resulted in a connection with the water sensor. I
elected to write some standalone code that submitted results to Nagios by
way of a passive check, rather than having Nagios try to actively monitor
it. Again, trying to conserve energy, I decided to only check the sensor
once every 8 hours.
So I wrote a Python daemon, which tried to be as robust as possible. If at
first it doesn't make a connection on its 8 hourly schedule, it keeps trying
until it does. Nagios itself has some freshness detection for this monitor,
so if no passive result is submitted within 8 hours and one minute, Nagios
alerts that something is possibly wrong with the sensor itself. (The
original intent was to deal with the situation where the battery went flat)
This is the Nagios service definition I've got. Some of it may be
unnecessary or redundant:
define service {
host_name teevee
service_description Cat water bowl
check_command check_stale!2!"Check water bowl monitor is on and reachable"
normal_check_interval 480
notification_interval 240
active_checks_enabled 0
check_freshness 1
freshness_threshold 28860
max_check_attempts 1
check_period 24x7
use generic-service
stalking_options o,w,c
contact_groups everyone
}
The code for the Python daemon is checked in here.
So everything was going swimmingly, apart from I couldn't get even 24 hours
of monitoring on my conservative 3 times a day schedule, and the Bluetooth
modem configured for all of its power saving options, and the water sensor
itself only being powered on when it was performing a check. There wasn't
much more I could do to try and reduce power consumption, at least with my
limited electronics knowledge. Perhaps using Zigbee instead of Bluetooth
would have helped, but I still think I'd have been going through 9 volt
batteries more quickly than I'd have liked.
I also had a brief foray into solar powering the board. I thought maybe the
lighting in the house would be sufficient to power the board. Of course this
didn't work out. I'd also have needed to make the code running on the board
more self-sufficient, and have it just provide a water status indication
whenever it had enough power to do so, instead of being a fairly dumb device
like it currently is. This all felt a whole level more complicated, and out
of my league, and I wasn't interested in attempting this sort of
remote-sensing exercise at this time.
The Arduino board can also be powered by USB, so as I already had some long
USB type A to type B cables (that had funky lights in them to boot), I
decided to just get a wall wart that had a USB type A receptacle, and
powered the Arduino board that way. So much for being completely "wireless".
(I could have also gotten a general purpose DC power supply that was capable
of spitting out 9 volts, but I doubted I'd get one with a sufficiently long
cable, which is why I went for the USB-powered option, as I already had a
cable long enough)
Speaking of wires, the most challenging part was the probes for the water
sensor. As they were going to be permanently in the cats' drinking water, I
didn't want to contaminate the water with them. I figured plain untinned
copper wire would be okay, since water pipes are copper. Finding untinned,
unstranded copper wire was a real challenge. I started out using some FM
antenna cable, but that was stranded, and the shielding was a nightmare to
strip. It was also reasonably difficult to make go where I wanted it.
What I really wanted was something more solid, that would flex and stay
where I put it. I cannibalised a spare IEC power cable, but it was also
stranded copper wire. I finally managed to obtain some solid-core CAT-5
cable from a hardware store, and this has worked exactly as I wanted.
I haven't done any further work on the setup since getting the CAT-5 cable
for the probe. Further improvements that I'd like to do at some point:
- transfer the circuitry from the breadboard to a breadboardless proto
shield, and generally try to house the sensor better
- find a better way of connecting the probe wires to the circuit, so it's
easier to remove the monitor when refilling the water tank (currently the
CAT-5 wires are just shoved into the breadboard). The whole thing is barely
spouse-approved as it currently is, let alone being easy enough for an
untrained house-sitter to try and negotiate
- integrate with Asterisk so a house-sitter gets a telephone call when the
water bowl needs refilling
I had a lot of fun with this project. I had a real sense of achievement,
being able to go from concept to completion, and learn a few things about
electronics along the way. I'm normally not a fan of messing around with
hardware.
Some photos of the project are here.
