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  • Sound Level Indicator

  • Created: Aug 27, 2014

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Summary

A sound level meter is a device that measures sound pressure level; this device is commonly used for ergonomics in industrial and environmental studies. This can also be applied for other uses like entertainment (lights and music). This project is a sound level indicator, it senses and lights a series of LEDs (Light Emitting Diode) according to the detected sound or noise.

The flow of the project is simple; an input signal (sound) is sense by the electret microphone and converts it to electrical signal. The MC33172’s first stage acts as a pre-amp with a gain of 101 and the MC33172’s second stage is use as a rectifier driver with a gain of 1.8. The LM3915 bar-graph IC drives the series of LEDs for a range of 30dB.

Three 2N3904 transistors are used as emitter-followers; the first 2N3904 is in the negative feedback loop of the 2nd op-amp as a voltage reference for the other two transistors. The second 2N3904 charges C8 and slowly discharges into 330kΩ resistor and is used as a peak detector. The third 2N3904 act as automatic gain control and a peak detector with slower charge and discharge time. Together with the 220kΩ resistor connected from the +5V power supply in a voltage divider configuration, this will drive the “R-HI” pin of the LM3915 to determine its sensitivity. Loud sounds cause the third 2N3904 to drive the “R-HI” pin to 5.1V for reduced sensitivity and 0.51V when there is a very low sound level detected.

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