The decibel meter circuit responds to sound from a pressure level of about 60 to 70 dB. The sound pick-up 8 ohm speaker consists of a transistor and an LM324 stage operational amplifier amplification section. You can also use a dynamic microphone, but the speaker is more sensitive. The four operational amplifier parts of the remaining 3 LM324 are used as voltage comparators and driver 3 LED indicators or incandescent lamps with an interval of about 3dB. Another transistor needs to be displayed under the lamp with incandescent lamp. Use a 12 volt, 50 Ma lamp. Each indicator light represents a volume change of about 3 dB, so that when all three lights are on, the volume is about 4 times the level required for a light. Adjustable sensitivity with 500K pot, such a lamp comes with reference sound level. The other two lights will show an increase of about 2 and 4 times the number.

Circuit diagram of decibel meter

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