Servo - December 2014

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 released.

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.