Door number 20 hides a resistor
with 10 k
Ω
(brown, black, orange).
It is used in this circuit as a series
resistor for the LED. Two transistors
form a sensitive amplifier. In the idle
state, the LED lights up only weakly.
The piezo converter is connected to
the input of the amplifier. When the
diaphragm of the piezo transducer is
made to vibrate, it generates a small
voltage which is greatly amplified.
That’s why the LED flickers clearly
visible.
The circuit even reacts to loud sound.
The piezo converter then works like
a microphone. Clap your hands and
watch the LED flicker. Then turn
the transducer over and place a
small weight on the diaphragm.
It could be an eraser or a coin,
for example. Then gently tap
the table. The LED will flicker
brightly. Even the smallest
vibrations of the floor can be
displayed when someone is
walking through the room.
Button switch
Open the 19th door and remove the
resistor with 4.7 k
Ω
(yellow, violet,
red). Now build a flipflop that turns
on the LED for a short time each
time you press on the piezo disc and
then turns off by itself. It is often
enough to tap on the table next to
the sensor. This time there are two
LEDs in series at the output.
A circuit like this is called a
monostable flipflop. This means
that there is only one stable state, in
this case the off state. The on state
is only switched on for a short time,
until the capacitor is fully charged.
19
A vibration sensor
20
