transistors to be mounted in inconvenient places on the
table. I guess if there was no time or real budget, a cool
over-the-top method would to use a ceiling mounted
Microsoft Kinect sensor to generate a 3D representation of
the table and use software to look for a cat. The optimal
solution within budget was an ultrasonic sensor which
could be set up so it is looking out across the table. Then, it
could measure the distance between it and objects in front
of it. This way, it can determine if the object is close
enough to be concerned with or if it should be ignored.
Alarm Method
Once you have detected that the cat is on the table,
you need to scare it off. There were three motor types that
were considered: a gear motor, a servo motor, and a
stepper motor. All would have done the job, but some
would have taken more work. The gear motor would turn
slow enough and have enough torque to turn something
like a small doll (my cat version of a scare crow); the issue
is that the gear motor can stop at any location and would
need some type of index sensor to allow the actual
position of the doll to be known. This is required since the
arms of the doll could possibly cover a sensor if it came to
rest in the wrong rotational position.
The stepper motor was ruled out since when powered
down, it could be turned out of position. So, even if it
didn’t lose any steps during operation, a true starting
position would still be needed. The servo was selected
because of its simple wiring requirements and that no
external position sensor was required. The servo can easily
be commanded to move to a certain position, and it works
quickly and reliably. Best of all, only three wires are
needed: 5V, ground, and a signal from the microcontroller.
A second alarm method for easily adding different
scaring abilities is a 12 volt alarm output. With this output,
you are no longer limited to the movement (and sound) of
the servo. A loud 12 volt buzzer or vibration motor would
be a simple addition to the system to guarantee results.
Project Box Selection
The design was going to be stuffed in an off-the-shelf
project box, which meant it would have four sides. So, it
would need four ultrasonic sensors. I thought this would be
a bit of overkill since cats usually snoop around for a bit
before they settle down for a rest on the table. In my mind,
I was thinking three areas of detection from a box mounted
38 July 2014
Power button.
Reset button.
Screw nut.
“Twirling
thing.”
