Raiders of the Lost Ark. In that scene, everyone’s favorite
archaeologist retrieves a booby-trapped gold statue that is
situated atop a kind of pressure plate. If the weight of the
statue was removed, the trap would trigger and rain
misfortune down upon the hapless pilferer.
Indy outsmarted this trap (or so we thought) by
replacing the weight of the statue with a bag full of sand.
The trap still ended up being triggered, causing a torrent of
arrows to shoot from the walls, ominous stone doors to
close, and the iconic boulder to chase our hero out of the
temple. Guess it just goes to show it can be tough to fool a
good pressure plate.
We figured a similar device would work for our present
defender, sans the arrows and boulders. The presents could
be set on a pressure plate that would be triggered if the
package was disturbed. We should be home free as long as
our pug didn’t have an extra bag of sand lying around (and
the dexterity to pull off the bait and switch).
The littleBits system does include an actual pressure
sensor, but it is not part of the starter kit we have. That,
however, just meant we would have to be a little more
creative. For the pressure plate, we just needed a sensor
that would activate when it became unpressed. That
sounded a lot like the opposite operation of the button
module that was included in our kit. The button module
would read high when it was pressed and low when it was
To flip the operation of the button without any special
programming, we could use the included inverter module.
The inverter module simply inverted the operation of
whatever module came after it. As a quick proof of
concept, we constructed a circuit with the power module,
inverter, button, LED, and buzzer. Just as we had hoped,
turning the circuit on activated the LED and buzzer, and
pressing the button turned them off. Our pug, Paisley did
not like the sound of the littleBits buzzer, so we knew we
were onto something.
With the basic pressure plate circuit working, we
needed to devise a way to have a present trigger it. We
took some cues from the bodging sensibilities on display in
the littleBits manual and crafted a small table for the
present using cardboard, tape, and paper clips. The paper
clips acted as adjustable legs for the table so that we could
get the height just right.
We wanted the table to sit squarely as the weight of
the present depressed the button. The adjustable legs made
quick work of this task, and we had a pressure plate that
reliably activated when the present was knocked from its
perch. Now, we just needed a robot to activate.
A Robot Story
Once again, we were impressed by the ease with which
SERVO 12.2014 71
Twin brothers hack whatever’s put in front of them, then tell you about it.
SUGGESTING A SCIENTIFIC SOLUTION.
MAKING A CIRCUIT, BENCH TESTING IT TWICE.
PRESSURE PLATE SUCCESS.