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Question for the analogue boffs

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Ian Rogers

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I bought ( in error ) 40 piezo buzzers.. 86 db they turned up but!!! No internal circuitry... So no worky!!

I made this
upload_2017-8-23_8-23-23.png

and the buzzer buzzed!! Yeah! But just audible... I went to change the cap ( as its the wrong size ) and low and behold nearly 2.5Khz on the buzzer and quite loud... So 12v in.. 10k resistor.. common TTL NPN and piezo??? How is it oscilating?
 

Nigel Goodwin

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Presumably zener type action in the reverse biased transistor?, the capacitor charges until the transistor 'zeners', which then partly discharges the capacitor.

It's essentially a 'modern' version of the neon oscillator used many decades ago.
 

ericgibbs

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hi,
Most of the common piezo sounders have a resonant frequency of around 2.5khz thru 3.5khz, which is the most sensitive range for the human ear.
E
human-hearing-sensitivity.png
 

Les Jones

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I think 12 volts exceeds the base emitter junction breakdown voltage (Note the transistor is connected with the polarity reversed.) When this happens I think this causes the current through the base collector junction (Which is behaving now as a base emitter junction.) causes a large current to flow between collector and emittor and this continues until the capacitor discharges below the base emitter reverse breakdown voltage. The capacitor then starts to charge again ant the process repeates. It is very similar to a neon relaxation oscillator.

Les.
 

Ian Rogers

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Thanks all of you but!!! the capacitor was removed, so there must be stray capacitance somewhere.

The capacitance must be coming from the piezo as the thing doesn't oscillate when its removed..
Now you have mentioned the breakdown voltage, it may be purely thermal!!
 

OBW0549

Active Member
Thanks all of you but!!! the capacitor was removed, so there must be stray capacitance somewhere.
Piezo sounders are nothing more than capacitors with a highly microphonic ceramic dielectric-- that is, applying voltage causes the ceramic to change its shape, and vice versa. I tested a handful of piezo discs with my capacitance meter, and got anywhere from 10 nF to 60 nF.
 

ci139

Active Member
here's another one for pickups
(i have adjustable welder ??? soldering thing - so i turn it to it's min. and still lightly tap the weld not to overheat it on piezos and light batteries - but i'm not doing it too often so i quite can't suggest anything here :arghh: this guy is so cute :confused:)
 

dr pepper

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I'm not a boff.
There are 2 methods I have seen for increasing volume in beeper tone generators, one is to use a blocking oscillator transformer circuit to increase the voltage, and the other is if the element is driven by a switching transistor to place an inductor across the element, so when the tranny switches off the flyback gives the piezo a kicking.
 
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