are each held in place with three T connections; once
tightened, they hold everything in place very well. I am not
sure how well it would hold up if I dropped it off the table,
but it has a nice solid feel in the hand. The laser machine
has the capability of etching onto the wood. This was used
to etch in some identification information, as well as the
project name. Getting the etch power just right so that the
results are nice and dark takes some playing around, but
thankfully baltic birch plywood is quite consistent.
There is actually quite a large number of items
mounted to the top of this box. We have the on/off switch,
a reset switch, three LEDs, a servo motor, and three slots
for the T-slot nuts and bolts. I wanted the servo horn to be
centered in the design which dictated where a lot of the
other items needed to go.
The servo motor is held in place very securely using
four nuts and bolts; the power switch and LED bezels are a
simple press-fit design. The reset button was the only item
on the top of the box that would have been better if I had
a panel mount style momentary switch. The switch that I
used is meant to be a replacement switch for computer
reset buttons. These switches are snapped into a housing
and have a button cap that presses it. It is mounted to the
underside of the top panel using some hot glue, and has
the switch shaft poking up through the top about 3 mm,
which isn’t too bad. Thankfully, the switch throw is only
about 2 mm before it activates. The bottom panel doesn’t
have any holes cut into it other than four bolt holes for the
perfboard to get mounted. Turns out a rectangular
perfboard is not that simple to fit into a triangular base
since there are mounting screws as well as the power input
and alarm output jacks in close quarters. The perfboard is
held off the bottom panel using 1/4 inch nylon spacers so
that none of the solder connections are in contact with the
case. It also allows for just enough space for the lower rear
mounting screw to be inserted.
Testing It Out
I tried the project out with the servo disconnected so it
wouldn’t alarm the cats during testing. I wanted to see how
well the ultrasonic sensors worked with a furry cat
compared to the piece of cardboard I was using during
testing. It was a bit of an afterthought that there might be a
problem since I was just dealing with the sensors as magic
boxes. However, in reality, they are sending out a burst of
sound (above the human hearing range) and waiting for it
to bounce off an object and return. One of our cats looks
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42 July 2014
■ Find tons of extra photos of this project at the article link.