How this circuit works?

The output of the first op-amp will oscillate between Vdd and Gnd so that the final signal that arrives at the 2nd op-amp input will oscillate too and will make the buzzer to oscillate as long as the final signal is something more than Vdd/2

But it's a comparitor, not an op-amp, so the output "wants" to be either all the way high or low (insofar as a given part can reach either rail).

If the (+) terminal at Vcc/2 is more positive than the (-) terminal then the output will rise towards Vcc. The (-) terminal will then rise towards Vcc at a rate determined by the R3 and C2. At some point it will be greater than the (+) terminal, at which time the output switches to ground and the process reverses.

It's AC coupled to its input. Imagine what would happen if R4 and C4 were connected together.

--
Rich Webb     Norfolk, VA

Right there in the text "The left side of the circuit uses a low-power dual comparator (MAX9022) to form a relaxation oscillator..."

So, (a) you've been _told_ what's going on, and (b) it's not an op-amp, it's a comparator.

On the right side of the circuit, consider what happens when a bunch of AC shows up on the open end of C4.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

OK, this is clear now, shame engineers use the same symbols for different things...

This is still not understood. The text talks about 155KHz, which is more than the human ear can hear, so which AC are we talking about? And even if we could hear 155KHz, D3 and C5 for a rectifier which converts any ac to dc, so how can a dc signal vibrate the piezo?

The piezo device is a self oscillating buzzer or squeaker. It just requires a few volts of DC to make it work.

--
Best Regards:
                     Baron.

When *you* make the drawings, you can use the Burr-Brown symbol:

Your Usenet posting technique needs serious work:

*-*-*-*-*-*+Usenet+go.to.the.BOTTOM+don't-top-post+2006+Summarize.what.you're.following.up+STOP+*.*.*.*.relevant.*+remove-*-*-*-irrelevant+before.the.original.*.*

Look also at the 1st line of this post.

If, *instead* of using an email address for a username, you would use JUST A NAME--without a domain tacked on,

*THAT* won't be obfuscated by Google and it will make it easier for others (especially other Google Groupers).

--
No, the shame lies with idiots like you, who have a document in front
of them which clearly states that a _comparator_ is being used as a
relaxation oscillator and apparently don\'t even bother to read it.

Lovely post.

We are truly impressed by your vast and impeccable knowledge, and your haughty way of sharing it with the low-life.

You are indeed the master of "bottom" posting.

GFYJF

In fact, some time ago I asked on this very newsgroup what is the difference between an op-amp and a comparator, and the replies that I received then where that they are basically the same thing, that comparators are "optimised for switching", so due to those idiots (as you put it) back then I have been missed informed till yesterday. Just goes to show how careful one needs to be about info posted here. In fact, what our designer wanted was a little hysteresis symbol inside the op-amp symbol, which is the agreed symbol for comparators with built in hysteresis (?). And the datasheet of this particular one, does show the correct symbol.

Yes, this has been explained above, the little + sign is the give- away that it is not a "bare" piezo.

Right. Which means that few comparators work at all where op-amps are indicated, and none work well. An op-amp is more likely to kind of work as a comparator, but it'll have its own problems. Review those answers and see if you weren't told this.

It sounds like you're trying to use newsgroups to learn basics. Newsgroups are good for fill-in, but if you have great swaths of knowledge to learn you still need to get a good book on the subject (Like "Art of Electronics") and read it.

If you stand on a street corner railing about people not using the latest 'correct symbol' then you'll never get your work done.

Comparators and op-amps are almost universally represented by the same symbols. If you allow this to confuse you you'll be forever confused.

-- snip --

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

--
Not really, since "optimised for switching" should have been a clue
that the comparator\'s output likes to live in one of two states, "ON"
or "OFF" and is designed to have as little time between those states.

Moreover, you should take everything posted in this newsgroup with a
grain of salt and then go on to definitive sources (TI, NS, ON semi,
LTC, etc., etc,) and peruse their tutorials for the final say. 

>Just goes to show how careful one needs to be about info posted here.

--
Thank you! :-)

FYI:

Comparators really are opamps. There are two main differences I can think of right now. First, they don't have compensation to prevent oscillation, so they are faster, but more likely to oscillate in some situations. Secondly, many if not most comparators are 'open collector', meaning they can only sink current on the output, and not source it. This is useful in many situations, but you need to be aware of it to design the circuit properly.

However, if you look on one of the mfgr's sites for information about these, they keep the comparators separate from the opamps. The usage, and thus the internal optimizations, are quite different. Comparators are always used to detect that a certain input voltage has exceeded some limit. Opamps, on the other hand, are used to perform some kind of calculation with the input voltage, such as scaling, differentiating, integrating, sample-and-holding, etc.

I don't know about the symbol. See this:

Regards, Bob Monsen

I reckon at a pinch a couple of 555's or a 556 could be used to do the same !

--
Best Regards:
                     Baron.

Of course. What else would it mean?