A tone generator
Behind door number 21 you’ll find a
capacitor with 10 nF (overprint 103). Now
build an electronic sound generator. You
will hear a uniform sound from the speaker.
Touch the condenser with your fingers so
that it heats up a little. The tone changes
very slowly. Touching the piezo transducer
also changes the sound.
The frequency, i.e. the pitch, depends on
the components. Replace the small capacitor
21
with 10 nF with the larger one with 100
nF. The tone is now much deeper and more
like a rattling. The sound can be changed
by partially holding the sound hole of the
speaker.
The LED seems to light up evenly. But if you
make a quick movement with your eyes, you
will see that it actually flickers very quickly.
You can see the flickering even
more clearly if you look at the
LED over a small mirror while
moving move at the same time.
This way you can even recognise
the individual vibrations of the
high tone.
The two transistors switch each
other on and off again and again.
This circuit is called a stable flip-
flop, because neither of the two
states is stable. The fast changes
create a movement of the piezo
disc and thus a sound.
