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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?
Sound level meter? Did i miss something particularly difficult or innovative about this thing?
Whats the point? Selling MC Kits?
Been there, done that, have the T-Shirt.
We also build a simplistic computer on a bread board with our own MMU, serial port, clock, ROM, and RAM. We programed in assembly and had to burn our programs onto the ROM.
This stuff is undergrad work at any reasonable college.
Or indeed, a java app for most Symbian phones?
With a full-featured 1/3 octave spectrum analyser as well. Please
Sent from my ASR33 using ASCII
I'm more interested in turning an iPhone / iTouch into a dynomometer for engine performace tuning. Use the accelerometers. You'd need to find some way to enter RPM, probably a passenger hitting the Ks.
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
LM3914 can handle 10 LED's per chip and can be cascaded for more. Add an amp for signal conditioning on the front end, and then hookup lots of LED's per line if you use a transistor to drive a bank of LED's.
They need a micro because their display is too complex for the job. But, make the display simple and you can make the whole design simple. Yet, since this is MIT's it has to be complex for some reason.
Of course it isn't news.
Here is some of dem linkies!
Link 1
Link 2
Link 3!
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.
Lets pretend I'm from MIT. This was the slashdot page quote:
All things are possible, except for skiing through a revolving door.
If the revolving door is ten times larger than normal, you could ski through it. Or shorten the skis and the door only needs to be slightly larger than normal. It also depends on the speed you ski. If you were skiing slowly through a viscous oil at a 35 degree decline (more like sliding than skiing), the doors would need a (roughly) 10ft radius or with skis half as long a 6-8ft radius.
You might be asking why anybody would want to ski through a revolving door. That's a question for a psychologist, not a scientist.
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.
----
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.
If this is your first time at DIY club, you have to DIY.
This is nice for some educational value, but it's not news. I've designed far more sophisticated systems that didn't have cookbook solutions and then brought them to fruition. Haven't we all? You won't see my personal projects on slashdot, no matter how cool they are, because they simply aren't news.
Everyone knows that microprocessors are silent. It's the heatsink fans that make all of the noise.
Take three nails.
Put two on a piece of wood, parallel to each other, and attach wires to them.
Rest the third nail across the other two.
Connect a 1.5v battery and a small resistor across the two wires.
Connect a small value capacitor to one nail, and a new wire to the other.
Connect the other side of the capacitor and the new wire to your soundcard input.
You now have a microphone capable of picking up a fly's footsteps walking on the wood!
This is the third time I've seen a "news story" on slashdot that ended up being yet another advertisement for "nerdkits".
Slashdot, stop putting up these nerdkits stories or at least move them to an advertising section, please.
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.
The article never says the thing was designed my an MIT student, does it? I am a half-baked electrical engineering undergrad here and even I can see the flaws in this design. And supposedly, all the code is in C. Any self respecting MIT student would use assembly for this. I highly doubt it was designed by an MIT graduate. Also, I agree there is nothing substantial in this - this is like thirty minutes worth of work. Any fool can hookup an ADC to a micro. No one here at MIT thinks this is smart or anything. However, we let these things happen here because if we want sponsors for any of the real things we do, we have to give them something in return and most of the times what they want is advertisement of this form. Stop looking at this as something representing MIT. It was just displayed by a kit manufacturing company at a tech school at an event designed for prospective freshmen ( = high school students), clearly a great marketing opportunity for this kind of a thing.
... why do it with 8$ worth of analogue components when you can also do it with 80$ digital components with a fraction of the resolution...
Yes. These are MIT students, remember. Now, if they designed something simple that got the same results as a properly calibrated A-law sound level meter, that would be useful. Or, for example, they could use the microprocessor to do an integrating dosimeter calculation, so you know when you've overdosed on live music. That would be useful to do cheaply, because noise dosimeters are still expensive, over $1000.
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...
How about a DIY Breath-Analyzer? It could at least do a better job than the lousy coded version(s) out there that so far do everything from chop off most of the precision, disable error detection, and can't perform a running average, to giving fake high readings when an NYC cop knowingly keys his radio to harass you with. If I had my own unit I could always compare it to theirs when it came to a court battle -- not that I ever advocate driving while impaired. That kills other people!
"It's the height of ridiculousness to say for those 9 lines you get hundreds of millions."
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.
I would expect something better from MIT. With a marquee like that, They should be doing Spectrum analysis. Here is something more interesting. http://elm-chan.org/works/akilcd/report_e.html They wrote the FFT in assembly. Microcontrollers are very impressive. They use to have to spend millions on FFT chips in the air force for their radar, now you can do it on a $4 chip.
I interpreted the caption that the MIT guys are listening to sounds made by a CPU. They are not, but it reminded me of an interesting phenomenon.
I had a transistor radio pick up signals from the CPU (or some other unidentified hardware component, not sure). I Just tuned the radio to receive static (somewhere in the 80-90Mhz FM range I believe) and depending on how busy the processor was, different squeaky noises could be heard. That was way back in the 386 era, don't know if this trick still works with modern PCs.
assignment != equality != identity
Hey, not so hard. While it may not be news to you nerds and geeks, it is to me. I'm 62 and am teaching myself electronics. I've had a full business career and am looking for something new. If this helps me, so much the better. Yes, I do read the comments.
"There are good ships, and there are wood ships, the ships that sail the sea. But the best ships are friendships, and ma