# How this circuit works?

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### u...@slax-he.info

Jun 25, 2008, 9:20:43 AM6/25/08
to
Hello
I have the following questions, can anyone explain to me how the
following circuit works, how does the first op-amp oscillate, and how
does the buzzer sound?

The first op-amp is configured with negative feedback, which normally
means that the negative input will try to equal the positive input,
which is always at half vdd, therefore it is unclear why it would
oscillate.

Secondly, the 2nd op-amp is connected to a dc level, so how would we
hear the piezo, which needs an ac signal to be heard?

The circuit is here

### GM

Jun 25, 2008, 9:59:40 AM6/25/08
to
u...@slax-he.info wrote:
> Hello
> I have the following questions, can anyone explain to me how the
> following circuit works, how does the first op-amp oscillate, and how
> does the buzzer sound?
>
> The first op-amp is configured with negative feedback, which normally
> means that the negative input will try to equal the positive input,
> which is always at half vdd, therefore it is unclear why it would
> oscillate.
>

http://en.wikipedia.org/wiki/Relaxation_oscillator

> Secondly, the 2nd op-amp is connected to a dc level, so how would we
> hear the piezo, which needs an ac signal to be heard?
>

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

### Rich Webb

Jun 25, 2008, 10:17:35 AM6/25/08
to
On Wed, 25 Jun 2008 06:20:43 -0700 (PDT), u...@slax-he.info wrote:

>Hello
>I have the following questions, can anyone explain to me how the
>following circuit works, how does the first op-amp oscillate, and how
>does the buzzer sound?
>
>The first op-amp is configured with negative feedback,

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).

> which normally
>means that the negative input will try to equal the positive input,
>which is always at half vdd, therefore it is unclear why it would
>oscillate.

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.

>Secondly, the 2nd op-amp is connected to a dc level, so how would we
>hear the piezo, which needs an ac signal to be heard?

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

--
Rich Webb Norfolk, VA

### Tim Wescott

Jun 25, 2008, 11:04:22 AM6/25/08
to

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

### u...@slax-he.info

Jun 25, 2008, 1:42:54 PM6/25/08
to

> 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.

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

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

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?

### Baron

Jun 25, 2008, 3:38:44 PM6/25/08
to
u...@slax-he.info wrote:

> 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.

### JeffM

Jun 25, 2008, 4:35:29 PM6/25/08
to
u...@slax-he.info (aka use@ slax-he.info) wrote:

>Tim Wescott wrote:
>>and (b) it's not an op-amp, it's a comparator.
>>
>OK, this is clear now,
>shame engineers use the same symbols for different things...
>
When *you* make the drawings, you can use the Burr-Brown symbol:

Look also at the 1st line of this post.

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

### John Fields

Jun 25, 2008, 7:03:41 PM6/25/08
to
On Wed, 25 Jun 2008 10:42:54 -0700 (PDT), u...@slax-he.info wrote:

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

---
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.
---

>> On the right side of the circuit, consider what happens when a bunch of
>> AC shows up on the open end of C4.
>
>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 155kHz signal from the oscillator is being rectified and smoothed,
and then if the resultant DC rises to Vcc/2, the output of the second
comparator will go high and supply DC to the piezo buzzer, which has a
built in oscillator used to drive the piezo transducer.

And bottom post.

JF

### Charles

Jun 25, 2008, 8:08:51 PM6/25/08
to

"John Fields" <jfi...@austininstruments.com> wrote in message
news:e8j564l8a11gpm6hb...@4ax.com...

> 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.

> And bottom post.

You are indeed the master of "bottom" posting.

> JF

GFYJF

### u...@slax-he.info

Jun 26, 2008, 7:37:50 AM6/26/08
to

John Fields wrote:

> On Wed, 25 Jun 2008 10:42:54 -0700 (PDT), u...@slax-he.info wrote:
>
> >
> >> 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.
> >
> >OK, this is clear now, shame engineers use the same symbols for
> >different things...
>
> ---
> 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.
> ---

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.

>
>
> >> On the right side of the circuit, consider what happens when a bunch of
> >> AC shows up on the open end of C4.
> >
> >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 155kHz signal from the oscillator is being rectified and smoothed,
> and then if the resultant DC rises to Vcc/2, the output of the second
> comparator will go high and supply DC to the piezo buzzer, which has a
> built in oscillator used to drive the piezo transducer.

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

>
> And bottom post.
>
> JF

### Tim Wescott

Jun 26, 2008, 10:35:56 AM6/26/08
to
u...@slax-he.info wrote:
> John Fields wrote:
>
>> On Wed, 25 Jun 2008 10:42:54 -0700 (PDT), u...@slax-he.info wrote:
>>
>>>> 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.
>>> OK, this is clear now, shame engineers use the same symbols for
>>> different things...
>> ---
>> 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.
>> ---
>
> 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",

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.

> 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.

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.

> 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.

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 --

### John Fields

Jun 26, 2008, 10:58:25 AM6/26/08
to
On Thu, 26 Jun 2008 04:37:50 -0700 (PDT), u...@slax-he.info wrote:

>
>John Fields wrote:
>
>> On Wed, 25 Jun 2008 10:42:54 -0700 (PDT), u...@slax-he.info wrote:
>>
>> >
>> >> 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.
>> >
>> >OK, this is clear now, shame engineers use the same symbols for
>> >different things...
>>
>> ---
>> 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.
>> ---
>
>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.

---
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.

---
Well, this _is_ USENET, where any number of idiots are free to spread
as much misinformation as they choose and your job, knowing that,
should be learning to separate the wheat from the chaff.
---

>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.

---
But so what?

There are myriad comparators _without_ internal hysteresis which don't
sport the symbol.
---

>> >> On the right side of the circuit, consider what happens when a bunch of
>> >> AC shows up on the open end of C4.
>> >
>> >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 155kHz signal from the oscillator is being rectified and smoothed,
>> and then if the resultant DC rises to Vcc/2, the output of the second
>> comparator will go high and supply DC to the piezo buzzer, which has a
>> built in oscillator used to drive the piezo transducer.
>
>Yes, this has been explained above, the little + sign is the give-
>away that it is not a "bare" piezo.

---
So are you finally happy?

JF

### John Fields

Jun 26, 2008, 11:06:10 AM6/26/08
to
On Wed, 25 Jun 2008 20:08:51 -0400, "Charles"
<charles...@comcast.net> wrote:

>
>
>"John Fields" <jfi...@austininstruments.com> wrote in message
>news:e8j564l8a11gpm6hb...@4ax.com...
>
>> 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.

---
Thank you! :-)
---

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

---
Thank you again, and I do agree with you that haught is the proper way
to deal with some folks.

However, I disagree with your assessment of the OP's social status
since he seems, basically, to be lazy and somewhat confused.
---

>> And bottom post.
>
>You are indeed the master of "bottom" posting.

---
Thank you once again; you're too kind! :-)
---

>> JF
>
>GFYJF

---
Good For You, John Fields?

JF

### Bob Monsen

Jun 26, 2008, 11:51:36 AM6/26/08
to
<u...@slax-he.info> wrote in message

>
> 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.
>

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:
http://encon.fke.utm.my/nikd/latest/sloa067.pdf

Regards,
Bob Monsen

### Baron

Jun 26, 2008, 4:44:04 PM6/26/08
to
John Fields wrote:

> The 155kHz signal from the oscillator is being rectified and smoothed,
> and then if the resultant DC rises to Vcc/2, the output of the second
> comparator will go high and supply DC to the piezo buzzer, which has a
> built in oscillator used to drive the piezo transducer.
>
>

> JF

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

--
Best Regards:
Baron.

### Charles

Jun 26, 2008, 5:04:07 PM6/26/08
to

"John Fields" <jfi...@austininstruments.com> wrote in message

news:8lb764d5jvker5uqh...@4ax.com...

>>> JF
>>
>>GFYJF
>
> ---
> Good For You, John Fields?

Of course. What else would it mean?