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2.14 Acoustic continuity tester
Circuit 36: Acoustic continuity tester
Setup information
Install the two transistors and the four resistors as shown in Fig. 2.36. Then install the two electrolytic
capacitors and the four resistors followed by the eight jumpers. Finally connect the Piezo element. Check
the placement on the board again and only then connect the battery.
You can increase the sensitivity of the Piezo element significantly by using a sound plate or even a sound
box. Read more about it in Chap. 1.2.3
Experiment
Even if everything is wired correctly, nothing will happen at first. But when you move a resistance between
the probe tips (the ends of the jumper wires which serve as substitute), you will hear a sound. If short-
circuited, the frequency is high; at 100 kΩ , the pitch is low. When you use this circuit to test devices, you
don't need to read the display. Your eyes won't get distracted, and your ears will hear whether a short cir-
cuit or a high-impedance connection is present.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.36.
Components required:
2 x 2N3904 transistors,
2 x 1 kΩ resistors,
2 x 10 kΩ resistors,
2 x 100 nF ceramic capaci-
tors, 1 x 10 μF electrolytic
capacitor, 1 x Piezo ele-
ment, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.36: Handy continuity
tester
61
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