How do they do it? Question about an electronic buzzer

I found an electronic buzzer I have. This one. http://www.designnotes.com/Merchant...uct_Code=SV2&Category_Code=bu&Product_Count=2

I attached a 6 volt DC source to it and heard a high pitch beep. Now this is exactly what I'm trying to make with my oscillator project. So I opened the thing up (obviously broke it in the process) and saw that all it has is a BJT, 2 resistors, and 2 diodes.

What the heck? How do you create oscillations with that? Is there some microscopic op amp or crystal oscillator embedded in it somewhere?

This just does not make sense. If anyone knows how such buzzers are made, I would really appreciate the help. No I don't have a schematic of it. Sadly.

Hello,

Could you see what the "speaker" looks like?
Is there a coil or is it a copper plate with a white disk on it (piezo)?

Greetings,
Bertus

It has a coil. There is some metal disc and then a coil inside.



That is the speaker itself. There is a coil inside. Other than that, what you see is what there is.

Hello,

Does the "speaker" have more than 2 connections?
I am puzzeled that there are no capacitors.

Greetings,
Bertus

Yea it's possible...
...but usually there are three connections to the transducer (for an added feedback signal) as others have questioned.
But no diodes are required, just an NPN and 3 resistors.
It's based on a Hartley oscillator.

If you look at this project...

555 Hysteretic Oscillator

you will see one of the simpliest RC oscillators out there. Feed that into a pizeo speaker or regular speaker and you have sound. Buzzers or alarms take it one step further, using the pizeo sounder as a crystal (as in crystal oscillator). The two devices are very similar, you use something like a single transistor to make it oscillate (as a crystal oscillator). Since the pizeo sounder is at resonance it is also at its loudest.

BTW, the pizeo sounder I keep refering to is also found in watches. It is a piece of precision cut quartz crystal.

I've been known to make a decent buzzer with a simple relay circuit. Making sound is one of the most basic forms of electronics (or electromechanical).

Cool thanks Bill. I will still stick to my Colpitts oscillator as in the other thread. By the end of this summer I will make it work, and then come to next semester with infinitely more knowledge.

So the crystal in the buzzer is most likely microscopic so I couldn't see it, or something. The photo above is not the best quality. What are the diodes for? This puzzles and interests me. The company that made it would never reveal their secret of course.

From the sounds of it is a primitive type buzzer, more like the relay version I was talking about. Basically the coil caused the magnet to vibrate, were there any solid state devices?

Think of a simple relay. Wire where in the relaxed mode the coil is energized, causing the amature to start moving, which breaks the circuit, which allows the amature to make contact and makes the circuit. Repeat infinately.

Interestingly, this surge of current through the coil creates high voltage spikes, which also means it is a tingler.

A lot of custom made buzzers in the older days were basically relays, but the armature was a large flat sheet of metal to increase surface area, a better sounding board.

It can't be that hard to design an oscillator using 1 BJT, 2 resistors, 2 diodes and a speaker.

Can you post a better photo of the insides of the buzzer?

Reminds me of something I built for fun a few years ago. I took a 555 and a little speaker with a pot to control the frequency. I tuned it to a high pitch just barely above the highest audible frequency I could detect. You could hide it someplace and people would say they have a ringing in their ears, but couldn't figure out where it was coming from. It would also cause headaches after prolonged exposure. Extremely annoying, but fun.

It is a piezoelectric buzzer, that exhibits the "piezoelectric effect." When voltage is applied to it, a crystal inside oscillates to produce a high pitch beep. They have AC buzzers and DC buzzers. I have AC ones so you have to produce the sine wave. But with the DC ones, you dont. It is also the same for the inverse. If you push on a piezoelectric buzzer you'll get voltage on the output.

The peizo crystal wafer is a transducer that move a little when a voltage is applied or it generates a voltage when it is flexed. The single transistor has a feedback connection from the piezo transducer that creates an oscillator.
A piezo transducer with only two wires is a high pitched speaker that makes sound when a signal is fed to it.

Here is the oscillator circuit:

It is probably a mechanical one, using a coil/transformer as the OP has mentioned.

The one I have uses one NPN, one 56K resistor and two diodes. It has two set of coils on the former and there is also a small magnet on the vibrating reed.

One set of coil to use to pull the reed towards the former and the other set of coil is used to turn off the transistor.