Piezoelectric disks can be found in the speakers in some musical
gift cards (see below for how to check
at the store if the card has the right type of speaker without damaging
the card.) Here's an example of one I extracted and tested
by attaching a diode and letting water droplets fall on it.
There seem to be a variety of speaker cases but this should give you
some idea of what to expect.
Speaker from card.
Piezoelectric disk with
diode soldered on.
Use a screw driver and/or knife
to pry the speaker case apart. Then remove the metal plate
and lift the circuit board (they may be wedged in tight) so you can get
at the piezoelectric disk underneath.
Desolder the wires from the
circuit board. I desoldered them at the piezoelectric disk itself but
if you don't want to take a chance on ruining the disk you can desolder
where they connect to the circuit board instead or just cut them.
Select a diode and solder it in
place. Before this step I knew nothing about LEDs (Light Emitting Diodes)
but the fellows at the electronics shop explained to me that the
first value on the packet labels below, 125 mcd and 1,400 mcd, is
the brightness of the diode in millicandelas (thousandths of a
candela.) From testing by connecting my piezoelectric disk to my
oscilloscope I saw that the voltage I got when tapping hard on the
disk was in the 2 to 9 volt range. Nothing on the diode packages told me
if the voltage value was the minimum required or a maximum. If it was
a maximum then I figured I'd be okay since the disk gives it sharp,
short duration spikes. Since the diodes on the right were the brightest,
1,400 mcd, I went with them.
Note that at one point after I'd attached the diode I was messing with
it and pulled off the leg attached to the whitish crystal material.
That included pulling off all the solder below it, exposing a yellowish
material. Luckily I was able to just put the leg and bit of solder
back over the yellowish material and press with my soldering iron and
it worked again.
Test it by tapping with the
fleshy part of your finger or banging on it hard with the blunt end
of a pen or something. It can take quite a bang. You can see in the
photo below that the diode lights up. Note that the light from the
diode is largely unidirectional.
Another test I did was to hold it under the shower while the shower
was just putting out drops of water. I couldn't just hold it under
the sink's tap because the water drops had to fall a long distance
to build up momentum in order to have enough effect to light the
diode. It does work.
Lighting the diode using the pressure of water drops
(sorry, the water kept splashing the camera lens.)
Remove the disk from the base
of the speaker case. This is done easily by scraping around the edge of
the disk with a sharp tipped knife. It's only glued in place around
the circumference. Then cut a hole in the side of the wall of the
case down to the level of the disk so that you can slip the sharp
tip of the knife under the disk to gently wedge it up.
Scraping at the circumference of the disk.
After cutting a hole in the wall, use the knife to wedge the
The resulting disk and attached diode.
Gift cards either have electromagnetic speakers or piezoelectric crystal
speakers. The speaker is usually glued in between layers of cardboard
in the card but you'd have to damage the card to see it. It should be
fairly easy to tell from the thickness of the card where the speaker
might be. There may also be a separate circuit board resulting in
multiple thick areas. See the photos
below. The electromagnetic speaker is of no use for piezoelectric
experiments but how do you tell which type it is without damaging
The electromagnetic speaker has a magnet as part of it.
It's a little difficult and may not work but if you bring something
metal that's attracted to magnets. Some keys are for example but test
with a magnet ahead of time since some keys are not. Then open the
card and look for where the card is thickest. Hold the metal object
loosely by one end and touch the other end to various spots where the
card is thickest (see third photo below.) Do it on both sides of the
cardboard. If the metal
object is attracted to and tries to stick to the cardboard then it's
because it's being attracted to the magnet. That means it's an
electromagnetic speaker and of no use for piezoelectric experiments.
If it's not attracted then it may or may not be an electromagnetic
speaker, since it can still be difficult to tell, but at least with
this method, if it is attracted you know not to get that one.
Where things might be.
The gift card torn open to show the parts.
Testing the type of speaker.
Other Piezoelectricity explorations