Page 171 - Hacking Roomba

152 Part II — Fun Things to Do

Figure 8-1 shows where it is located on Roomba (in case you want to attach a microphone),
and Figure 8-2 shows what the piezo beeper actually looks like. The beeper sits behind the left
drive wheel. It is a silver-colored disc with a copper colored center disc, held in place by a black
plastic ring.

clean spot
power max
dirt
status detect

Piezo Beeper Location

FIGURE 8-1: Piezo beeper location

How Piezo Beepers Work

The beeper consists of a metal disc glued to a piece of piezoelectric crystal. Piezo crystals are
amazing things. They are naturally found crystals that flex when voltage is applied to them.
Turn off the voltage, and the crystal flexes back to its original shape. Pulse the crystal with
voltage fast enough, and it generates sound. Pulse it 440 times a second, and it will generate
a 440 Hz tone (A440, the musical note A above middle C). The sound is generated just like
with any speaker, by moving air back and forth. Since the mass that a piezo can move is small,
piezos are better at reproducing higher frequency sounds than lower frequency ones. The metal
disc on the piezo beeper in Roomba is the mass used to push air and make noise. Piezo speak-
ers are also used as the tweeters in some audio speakers.
The piezo crystals can also be used in the opposite way. If you flex or squeeze a piezo (from
the greek piezein, “to squeeze or press”), it will generate electricity, up to thousands of volts in
fact. But don’t worry; the amount of current it can generate is tiny, so it can’t harm you. It may