Why this strange piezo-buz circuit?

[flat5]

What is the reason for doing the circuit this way?

 

[ataul]

buzzer use as a simple feedback for oscillation.

 

[flat5]

That occurred to me but I thought the feedback was negative.

 

[Ubergeek63]

That occurred to me but I thought the feedback was negative.

the piezo is a reactive element that introduces phase shift.

 

[Hero999]

I can't see it working, not without something to bias the transistor on slightly.

 

[Roff]

I can't see it working, not without something to bias the transistor on slightly.

That's what I thought. Either the piezo element has a shunt resistive component, or bias current is provided through the jumper.

EDIT: Figure 9 in **broken link removed** from Murata has a similar circuit.

 

[flat5]

The circuit was taken from the Proteus v7 samples.
Here is an excerpt from the Murata PDF.
Thanks Roff and et al

 

[audioguru]

Murata's schematic of the oscillator for a piezo beeper uses a piezo with a 3rd electrode, the feedback electrode. The feedback electrode is out-of-phase to the main electrode (it is a Hartley oscillator) so the transistor has positive feedback.
I think it automatically oscillates at the loudest frequency of the piezo/helmholtz cavity.