DIY Microprocessor Sound Level Meter Demoed At MIT 81
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!"
A CPU for this? (Score:5, Informative)
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 [national.com]
This is an "article" by timothy...expecting more?? (Score:0, Informative)
Here is some of dem linkies!
Link 1 [makezine.com]
Link 2 [youritronics.com]
Link 3! [elechobby.com]
Yea, building a VU or SPL meter is soooo easy that even an MIT student can build one. Timothy, did your mother know that you posted this story? GO TO YOUR ROOM.
A little knowledge is a dangerous thing (Score:5, Informative)
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.
Re:Ho-hum (Score:5, Informative)
Many of the cheap micro-controllers have ADC's that won't do the job well, at least not well enough to get any kind of dynamic range out of the circuit.
A bigger problem with the MIT design, is that it uses a Piezo-Buzzer for a microphone. This will give a wickedly non-linear frequency response curve. Piezo-Buzzers are designed to have a narrow range of frequencies in which they operate effectively.
The MIT design also uses a single transistor amplifier circuit. It wouldn't surprise me if the harmonics on the output are poor. Specifically, with this circuit, the average sound level can be determined by simply averaging the output of the transistor amplifier. Essentially, the average voltage on both the collector and emitter of the transistor should change if an AC signal is applied to the base. If this average is read with a DC voltmeter, then it should give an approximation of the sound-level, subject to the microphones frequency response curve.
I am not clear why anyone would build a sound level meter without using either a proper microphone or an effective amplifier circuit. A quad op-amp IC, and a few circuits from the web, should give you the average sound level over an extended frequency and amplitude range. It is even possible to do RMS to DC, peak-level to DC, and log-linear conversions in analog. For a retro-look, an old-fashioned voltmeter or amp-meter can be used for a display. For a more modern look, it is possible to use a cheap micro-controller with a slow ADC (or an LM3914) for the analog to digital conversion. Historically, this was the way it was done in many stereos, and the same circuit is probably still in use in many professional recording labs.
The advantage of implementing a proper micro-phone is the much flatter frequency response curve. The advantage of the log-linear conversion, is that most sound meters read in dB, which is a logarithmic scale. It takes a very good linear ADC to implement the same conversion digitally. A 5-bit (32 count) ADC reading a log input has more dynamic range than a 24-bit ADC reading a linear input (2^32 >> 2^24). Although in practice, I wouldn't recommend using less than an 8-bit ADC on an analog circuit.
Nerdkits Ads Strike Slashdot Again! (Score:1, Informative)
Why does slashdot allow garbage like this to be posted time and time again?
Clearly these MIT kids are smart because all they do is get free advertising from slashdot by submitting their stories multiple times...
This is not news. Furthermore, the story isn't even worth of being slashdotted. Just stop already.
Re:A little knowledge is a dangerous thing (Score:3, Informative)
The criticism may be blunt, but if students do a cookbook project, they should at least be smart enough to know the errors in their ways.
This project has quite a few major problems in the analog front end. Especially, given the fact that with a micro-controller, you could at least try to fix some of them. The report makes no mention of the fact the students even noticed them. Also, the calculations that they did do, are considerably more complex than necessary. Specifically, the students missed the fact the piezo transducer is a charge coupled device, so they are essentially dropping current straight into the base of the amplifying transistor. No voltage gain calculation required.
Is this the level of knowledge at MIT? (Score:3, Informative)
I really like the simplicity of the circuit, and the way they try to explain the basics of transistor design. Nowadays, there is an integrated circuit for about anything, but just using that doesn't make you learn anything, and - in my opinion - takes away the fun of creating something from scratch.
But am I the only one to see the huge error in the equations they are using? ;-)
They state
Ic = Ib * beta
Ib = Is exp(Vbe/Vth)
where it should be
Ic = Is exp(Vbe/Vth)
Ib = Ic / beta
or, their equations are off by a factor of beta!
That does not seem too important, it appears you could compensate for this in Is, but in practice, that is not so straightforward.
The exponential relation between Ic and Vbe holds over many decades, whereas beta is not nearly as constant as we sould like.
So, if these are really MIT students, I'd like a word with their professors...
Does this thing even work? (Score:3, Informative)
I built up the circuit as a SPICE model, and while it amplifies, it doesn't filter much. That weird filtering circuit in the emitter leg doesn't seem to accomplish anything. Treating the piezo microphone as a voltage source with a 1K resistance, generating a 1KHz input signal at 0.005V (based on a Murata piezo buzzer data sheet), what comes out is a voltage swing of about 0.6V at 1KHz, with a DC offset of 2.8V. The filtering seems to be insensitive to RM; changing RM from 10 ohms to 10 megohms doesn't do much to the output waveform. The 100K pot was adjusted until the voltage across RE was 3.3V, as specified. (This happens with the top end of the pot at 4.4K).
Why didn't they just put a nice simple low-pass filter on the output, instead of trying to get cute and put it in the emitter lead? And shouldn't there be a diode in there somewhere, to extract the waveform's envelope?
I actually built something like this in my teenage years, and had it hooked up to a surplus chart recorder (mirror galvanometer, phototube, relays, and motors, a mechanized Wheatstone bridge). (This dates me.) Mine worked.