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2.12 Peak-VU meter
Circuit 32: Simple peak VU meter with LED
Setup information
Install the transistor and the two diodes as shown in Fig. 2.32. Then install the three resistors and the elec-
trolytic capacitor. The two 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.
It is important that you are familiar with the relevant safety guidelines as per VDE and know how to observe
them when working with devices that are to be connected to the mains. If this is the case you can now con-
nect the circuit to the audio amplifier.
Experiment
Even if everything is wired correctly, nothing will happen at first; the red LED remains dark. Once the audio
signal reaches a certain level the LED starts to glow. For example, the LED will flash to the beat of the music
playback. Depending on the setting, the circuit could be used to prevent that a defined volume level is
exceeded. If the volume is too high, the LED will flash to alert you that the volume control must be turned
back.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.32.
Components required:
1 x 2N3904 transistor,
1 x 1N4148 silicon diode,
1 x red LED, 2 x 220 Ω
resistors, 1 x 100 kΩ resis-
tor, 1 x 10 μF electrolytic
capacitor, 1 x breadboard,
1 x 9V battery with clip
connector
Fig. 2.32: Simple peak VU meter
with LED
57
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