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Circuit 33: Peak VU meter with coil-moving instrument
Setup information
Install the transistor and the diode as shown in Fig. 2.33. Then install the three resistors and the electro-
lytic capacitor. The three jumper wires come next. Check the placement on the board again and only then
connect the battery. Connect the circuit input to the loudspeaker output of an audio amplifier – but only if
you are familiar with the relevant safety guidelines on working with devices that are to be connected to the
mains as per VDE and know how to observe them.
Experiment
Even if everything is wired correctly, nothing will happen at first; the coil-moving instrument has nothing
to show. If the audio amplifier delivers a signal, the instrument needle will swing as per volume. The scale
could be calibrated in watts output power. Return travel is a bit delayed by means of the electrolytic capaci-
tor C1. Short pulses can thus still be displayed.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.33.
58
Components required:
1 x 2N3904 transis-
tor, 1 x 1N4148 silicon
diode, 1 x moving-coil
instrument,
1 x 100 Ω resistor,
1 x 220 Ω resistor,
1 x 100 kΩ resis-
tor, 1 x 10 μF elec-
trolytic capacitor,
1 x breadboard, 1 x 9V
battery with clip con-
nector
Fig. 2.33: Peak VU meter
with coil-moving instru-
ment
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