Description
The circuit below responds to sound pressure levels from about 60 to
70 dB. The sound is picked up by an 8 ohm speaker, amplified by a
transistor stage and one LM324 op-amp section. You can also use a
dynamic microphone but I found the speaker was more sensitive. The
remaining 3 sections of the LM324 quad op-amp are used as voltage
comparators and drive 3 indicator LEDs or incandescents which are spaced
about 3dB apart. An additional transistor is needed for incandescent
lights as shown with the lower lamp. I used 12 volt, 50mA lamps. Each
light represents about a 3dB change in sound level so that when all 3
lights are on, the sound level is about 4 times greater than the level
needed to light one lamp. The sensitivity can be adjusted with the 500K
pot so that one lamp comes on with a reference sound level. The other
two lamps will then indicate about a 2X and 4X increase in volume.
In operation, with no input, the DC voltage at pins 1,2 and 3 of
the op-amp will be about 4 volts, and the voltage on the (+) inputs to
the 3 comparators (pins 5,10,12) will be about a half volt less due to
the 1N914 diode drop. The voltage on the (-) comparator inputs will be
around 5.1 and 6.5 which is set by the 560 and 750 ohm resistors.
When an audio signal is present, the 10uF capacitor connected to
the diode will charge toward the peak audio level at the op-amp output
at pin 1. As the volume increases, the DC voltage on the capacitor and
also (+) comparator inputs will increase and the lamp will turn on when
the (+) input goes above the (-) input. As the volume decreases, the
capacitor discharges through the parallel 100K resistor and the lamps go
out. You can change the response time with a larger or smaller
capacitor.
This circuit requires a well filtered power source, it will
respond to very small changes in supply voltage, so you probably will
need a large filter capacitor connected directly to the 330 ohm
resistor. I managed to get it to work with an unregulated wall
transformer power source, but I had to use 4700uF. It worked well on a
regulated supply with only 1000uF.
Circuit Schematic Diagram 1
Circuit Schematic Diagram 2
