Hardware Hacking - Nicolas Collins

Hardware Hacking 27

Chapter 8: Turn Your Tiny Wall Into a Speaker (or

How to Make a Piezo Driver)

You will need:

• A battery-powered mini amplifier with output jack for external
  speaker.


• You contact mike from the previous chapter.


• Another Piezo-electric disk.


• 2 plugs to match the amp output connector.


• A female jack to match the plugs you are using.


• One small audio output coupling transformer (Radio Shack 273-1380
  or equiv.)


• A few feet of lightweight speaker cable or stranded hookup wire.


• Electrical tape.


• A can of “Plasti-Dip.”


• Small spring clamps or clothespins.


• A sound source, such as a CD player, and cable to connect it to the
  amp input.

Piezo disks are used to make beeps because they do so very efficiently -- which is
to say that they require very little current, and therefore are well-suited to
battery-operated devices. As loudspeakers they display a rather uneven, non-
”hi-fi” response, but they can nonetheless be very useful when coupled to other
objects to make “speaker objects.”

To get the most vibration out of a Piezo disk it is necessary to feed it a very high
voltage signal, albeit at a minuscule (and therefore harmless) current. A
transformer is a kind of audio-lever that allows one to jack up the voltage of an
electrical signal very easily. For this experiment we will wire up an “output
transformer” backwards (see Rule #8) to step up the output voltage of a small
amplifier from around 6 volts to over 200 volts.

1. The transformer will have a “primary” and “secondary” side. The primary
   will be designated as having an impedance of around 1000 ohms (1kOhm), and
   may have two or three wires. We will use the outer two wires -- the center wire,
   as indicated in the drawing or coming out of the package, can be ignored. The
   “secondary” will usually have just two wires, and an impedance of 8 Ohms. In
   the case of the Radio Shack part, the outer primary leads are blue and green and
   the secondaries are red and white.

Strip 1/2” of insulation off the ends of the primary and secondary wires and tin
the ends.

2. Solder one of the secondary wires to the tip of a plug that mates with the
   output jack of your amplifier. Solder the other secondary wire to the sleeve of
   the plug. Polarity is irrelevant here – it does not matter which of the two