Decibel meter circuit diagram

A decibel meter circuit responds to sound pressure levels ranging from approximately 60 to 70 decibels. The sound picked up by the 8 ohm speaker is amplified by a transistor and an LM324 stage op amp section. You can also use a dynamic microphone, but speaking is more sensitive. The remaining 3 quad op amp sections of LM324 are used as voltage comparators and drive 3 LED indicators or incandescent lamps spaced about 3dB apart. An additional transistor is required, with the incandescent lamp under the display. Use a 12V, 50mA lamp. Each light represents a change in volume of approximately 3 dB, so that when all 3 lights are on, the volume is approximately 4 times higher than the level of light required for one light. Adjustable sensitivity with 500K pot so a light comes with reference sound level. The other two lights will show approximately a 2x and 4x increase in quantity.