three pole buzzer

bomber

When taking anything apart, you have really almost no idea of whaty you will find. when taking apart a microwave I found a VFD display and also a wierd buzzer. it is a piezo buzzer with three leads! When checking the circuitry around the buzzer, I got only more confused. One lead went to ground (as indicated by the silkscreen), one lead went to the emitter of a transistor, and the third went to the base of that same transistor! Can somebody please tell me how to hook this buxxer to a BS2 please.

Franklin

A part number, picture, schematic or model of the microwave might help. Do you know the voltage that should be feeding it?

bomber

The buzzer was on the low voltage circuit board. My guess is between 3 and 12V feeding the buzzer. As far as pictures..

LoopyByteloose

The truth about hacking for parts is that you have a high risk that nothing will come of it. At times, all you can do is apply limited power to something and see if it works. The buzzer may work fine at 3 volt with two of the leads. It is hard to guess what the third wire that is attached to the base might be doing as this configuration is rather non-standard for generic parts.

One rule of thumb is to apply as low a voltage as you can to get results. And another is to get the polarity right. Reverse polarity and over-voltage are the two foremost reasons for destruction of any device - known or unknown.

wass

Here's an example of just such a device with a small sample schematic showing its use.

http://www.puiaudio.com/pdf/AT-2428-TWT-R.pdf

bomber

wass, the schematic is almost identical to the board layout. thank you.

Beau Schwabe

Back in my day ... piezo's only had three wires.. and we had to build our own self resonant piezo transistor drivers to make them work. :-)

Seriously though, Forrest Mims provided a driver that was generically accepted as a standard. (See attached)

erco

Two wires, three wires, it really doesn't matter. The main thing is to say piezo properly. It's PEE-YAY-ZOH, not PEE-ZEE-OH.

And along the same lines, it's NU-CLEE-UHR, not NU-KYEW-LUR !

ps: Forrest Mims still ROCKS! http://www.forrestmims.com

HollyMinkowski

I say piezo like

pie as in apple pie

and Zo as in Zoey

Hey, that's what it looks like pie-zo :-)

erco

OMG.

Go Directly To Jail, Do Not Pass GO, Do Not Collect $200!

Beau Schwabe

Apparently today I'm easily entertained ... :-) ... Erco is correct (PEE-YAY-ZOH)

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

'p' ... 'p' in pie
'i' ... 'y' in happy
'ie' ... long 'a' in base
'z' ... 'z' in Zion
'o' ... 'o' in Bode

localroger

I was telling one of our younger techs (early 20's, worked in a hardware store for a few years so quite sharp on mechanical stuff) how a Barantec touch switch worked, and when I said the word "piezoelectric" he had no idea what I was talking about.

Then I quite properly blew his mind telling him that the same principle powers those spring-loaded sparkers that start your propane barbeque grill.

bomber

self resonant piezo transistor drivers

By this do you mean that the output frequency is a constant frequency? Is there a circuit where a Micro (like a BS2) could control the tone of the buzzer?

Beau Schwabe

"self resonant piezo transistor drivers" ... "By this do you mean that the output frequency is a constant frequency" ... yes, one of the three wires is part of a mechanical feedback loop which maintains a constant frequency determined by the crystal itself. Size, temperature and mechanical load characteristics can alter the resonant effects. The largest plate (bottom plate) is usually the ground. The middle sized plate is what you drive the piezo with, the smallest plate is the feedback and is on the same plane and polarity as the middle sized plate. Because of the nature of the crystal and how the polarities interact, the smallest plate and the middle plate are electrically isolated as if they are two back to back series diodes only coupled by a small amount of fringe capacitance.

Middle plate ---->|---- Ground Plate ----|<---- smallest plate

"Is there a circuit where a Micro (like a BS2) could control the tone of the buzzer?" - yes, just use the ground plate and the Middle plate as if it were a two wire element. If you want you can merge the small and medium plate as if it's a single plate.

bomber

Thank you everybody who anwsered!! I have offically titled this solved!