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DIY Microprocessor Sound Level Meter Demoed At MIT
An anonymous reader writes "A Piezoelectric Sound Level Meter was demoed at MIT's Battle of the Bands last month, borrowing its display from the do-it-yourself USB LED marquee that was the subject of a previous Slashdot story. This video tutorial describes in detail both the analog electronics plus the C code that runs the system. If this is your first experience at the intersection of digital and analog systems, don't be scared!"
ok so pretty cool, but can someone explain how hooking a mic up to an ADC is worthy of a mention for MIT? It sounds more like a high school project at face value, what am I missing?
It IS a high school project at face value. The kit can be yours for only $80!
Shouldn't ads like this be paid for?
Just goes to show ya that MIT guys will crack a nut using a bulldozer. There's plenty of dedicated level-meter chips around which cost next to nothing and provide a better, logarithmic response, which is what you want for sound.
The LM3915 is an oldie but a goodie, you can even daisy-chain them.
See http://www.national.com/mpf/LM/LM3915.html
This project is an excellent example of how having a little theoretical knowledge is a bad thing.
They have just enough knowledge to get into complicated and pointless gain calculations, but they miss most of the really important things. Here's a few:
(1) A piezo buzzer is not designed for any kind of flat frequency response. Which is a basic requirement for a sound-level meter. Major fail from the get-go.
(2) We're going on 60 years of having a spec for sound meter weighing curves and envelope filtering characteristics. Yet no mention of that in the article. A randomly designed meter is useless.
(3) They go on and on about calculating the gain of the amplifier stage, and they do it incorrectly. We care not one whit about the DC gain. The AC gain is dependent on the AC impedance of the source and load. Even the DC gain they calculate is useless as those transistors have a huge range of gains. And no analysis of the DC stability, which is harder to get right. Gain just happens, stability has to be designed in.
(4) Biasing the base from a pot in that fashion is never done in practice. A better design would use two resistors and avoid the cost and impedance variations of the one pot "design".
(5) A real design would have the +5 volt line decoupled and filtered to keep microprocessor switching noise out.
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In summary these designers should wait until they get past the first chapter of their transistor class before going out and trying to design anything. Good design requires more than slavish focusing on one small area. An engineer has to have a broad view.