A circuit of measurement of level based on a typical application of National. The circuit round the IC1 makes input adaptation and amplification with the trimmer TR1 [GAIN]. The circuit round the IC2 makes half-wave rectification of acoustic signal. With switch S1, we select the type of indication, that we will have from the LED. With the prices in resistances R6 and R7 that exists in the circuit, the level of signal, in the entry is 7.8V (gain of first stage IC1, is one), and the difference of level between the LED D10-11, should are 3 db. The positive department of supply, should have the possibility of giving more current, one and it is overloaded with the current of LED..
R1-2=10Kohm C1=100uF 25V D1-19=LED 3 or 5mm any colour.
R3-4=10Kohm C2-5=10uF 25V D20-21=1N4148
R5-8-9=1Kohm C3-4=100nF 100V
R6=330Kohm C6=1uF 25V IC1=TL 072
TR1=47Kohm Trimmer S1=mini Switch IC3=LM3916