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Question for the analogue boffs

Discussion in 'General Electronics Chat' started by Ian Rogers, Aug 23, 2017.

  1. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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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?
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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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.
     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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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
     
  4. dave

    Dave New Member

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  5. Les Jones

    Les Jones Well-Known Member

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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.
     
  6. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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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!!
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Of course, a piezo is a capacitive device.
     
  8. OBW0549

    OBW0549 Member

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    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.
     
  9. ci139

    ci139 Active Member

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    http://www.vk2zay.net/article/157 by search

    some while ago i modded with compasses carpet cutter the non-segmented piezo to a feedback one (in a sense it can be done - perhaps with coating mask and ?right? chemicals)

     
    Last edited: Aug 23, 2017
  10. ci139

    ci139 Active Member

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    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:)
     
  11. dr pepper

    dr pepper Well-Known Member

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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.
     
  12. ci139

    ci139 Active Member

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