Open the 10th door and remove
another resistor. It is 2.2 k
Ω
and has
three red coloured rings. Now build
a circuit with the piezo speaker, the
resistor and two LEDs. The battery is
not connected and may be removed
from the battery clip. The push
button should be pressed for the first
experiment. Now lightly tap the piezo
disc. This will again result in weak red
and green flashes of light. Attention!
You must not use too much force,
otherwise the ceramic disc could
break.
The experiment has shown that the
piezo speaker can not only convert
electrical energy into sound, but
also vibrations into electrical energy.
The same component functions as a
speaker, microphone and electrical
generator. Therefore, it is also called a
“piezoelectric sound transducer”.
Deformation owing to pressure on
the membrane causes charging and
thus generates electrical energy. But
the same also achieves a change in
temperature. It’s easy to try. Open the
switch and hold your warm finger
against the membrane for a few
seconds. Then close the contact. It will
cause a crackling sound and a flash of
light. Then open the contact and wait
a little longer until the pane has cooled
down again. Closing the contact again
generates another crackling sound
and a second flash of the other colour.
Use a cable instead of the push button
to hear soft sounds from the piezo
transducer.
Light flashes without battery
10
