This audio meter project for measuring audio levels. If you listen to most MPEG, AVI and WMV files, you will find the audio levels all over the place. This level meter is the tool able to use as audio test equipment in the multimedia audio.
How The Audio Level Meter Circuit Works
The circuit given below is a simple audio-amplifier powered from a 9v battery. The first two transistors are a voltage amplifier, and the right-hand pair form a complementary output pair. The two diodes between the bases of the output transistors give the required bias, but if the output devices ever draw too much current (or get even slightly warm) then increase the 1K0 resistor to another value until they do not get in the slightest bit warm.
The gain of the amplifier is controlled by the negative feedback, which in turn is controlled by the 470 Ohm resistor (R4) and the 4K7 (5K) pot. If the pot is set to about 100 Ohms then the voltage gain of the amplifier is about 5. But, that gain only occurs when the 4-diode bridge rectifier is conducting. If the point of the waveform is less than the forward conducting voltage of any diode, then the gain is probably around 250,000. When the diodes conduct, they rectify the signal and pass the current through the meter. In this way the diode conduction / amplifier transfer characteristic has become more linear. The value of the POT determines how much current actually flows. This circuit works well with a low sensitivity 20mA FSD meter, but works equally well with those cheap 100uA meters you get from most component shops.
If you do not need accuracy, for example, a simple audio level meter application, then a good source of cheap meters is to rob one from a battery tester.
The project needs no special tools, no setting up (other than a single sensitivity adjustment) and almost all components have an extremely wide range. With low sensitivity meters then the 5K Pot could be replaced with a 500-Ohm pot, or replaced with a 25K pot if you used a 30uA meter. The potentiometer (pot) can be any type of device, from a small pre-set pot glued to the board, to a normal 1950's type volume- control mounted in a hole drilled in the board. Download Project Documentation.
Source: Audio Level Meter - Dual Stereo Vu Meter Kit
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