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Input signal circuit for Sound Level Meter(LM3915).peramp..

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ryeeson

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lm3915 a-weighting

I want to design a input circuit for my Sound Level Meter with LM3915.
this is my first project with an audio signal. LM3915 is bassicly a display driver.
the aim of this project is to display the signal received form microphone to sound level meter. example, let say the Mic receive a noise from sorrounding area(people talk), then the Sound level meter will be able to display(LED light up) the noise signal.


the input should be from computer mic or 8 ohm 0.25W mic.

so the circuit connection should be look like below.

Mic=>peramp=>rectifier=>LM3915.

do i need a rectifier here?
what thing i should consider in my preamp stage?
what is the peak detector for? peak detector=rectifier?

This sound level meter will monitor the noise caused by the student at the library table. That is, the corresponding LED will light up when the noise increased.
so the LM3915 must be cascede in order to display within 60dB range.

Help me. i hope i have a details explaination in the preamp and rectifier stage. what Opamp do i need.. how much power and also the frequency ..

thanx
 

micpic

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lm3915 mic

Does anybody have some hints to this question. I want to design exactly the same.
 

ptmsl

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lm3915

No a peak detector is not a rectifier but you will need also a rectifier and an integrator. You can find the topology for AC/DC converter (rectifier and integrator) and peak detector at page 18 of National Semiconductors application note 31. For a microphone preamp use a low noise opamp like LM833 (dual), op27, or similar and dont use large value resistors. Also be careful with the PCB topology grounding supply etc. If you want to measure the noise level from people talking you dont need to use the entire audio spectrum since most peoples ears have very limited sensitivity above 10KHz. Ideally you should use a Psophometrically or "A" weighting filter after the preamp to adjust the sensitivity of your meter to the sensitivity of the human ear.
 

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