Instructions

112
2.36 Metronome
Circuit 87: Metronome with discreet unijunction transistor
Setup information
Install the two transistors, the four resistors and the two electrolytic capacitors as shown in Fig. 2.87. The
nine jumper wires come next. Last but not least connect the Piezo element. Check the placement on the
board again and only then connect the battery. You can significantly increase the sensitivity of the Piezo ele-
ment by using a sound plate or even a sound box. Read more about it in Chap. 1.2.3
Components required:
1 x 2N3904 transistor,
1 x 2N3906 transistor,
1 x 1 kΩ resistor,
2 x 2.2 kΩ resistors,
1 x 220 kΩ resistor,
1 x resistor, see box Experi-
ment, 1 x 1 μF electrolytic
capacitor, 1 x 10 μF elec-
trolytic capacitor, 1 x Piezo
element, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.87: Metronome with
discreet unijunction transistor
Experiment
If everything is wired correctly, you will not hear anything once the battery is connected. Up to now you still
don’t know the value of resistor R2. This resistance defines the metronome pulse. Details are listed below:
Resistor R2 pulse
47 kΩ 50 bpm
22 kΩ 80 bpm
10 kΩ 180 bpm
If you use a potentiometer (value 50 kΩ or 100 kΩ) instead of a fixed resistor, you can adjust the pulse
continuously. On a side note: The two transistors are wired in a way that they will function like a unijunc-
tion transistor.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.87.