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    Piezo preamplifier missing cap values

    Dear All,

    I would like to build this preamp which is from a Weishi Timegrapher, see attached pics.
    I managed to figure out the circuit, but since I have only the picture of it I stuck with the smd capacitor values.

    Watches are under 10Hz so the aim is to amplify the vibrations from the piezo in this freq range.

    Any help would be nice.
    thanks.
    Zedman

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  2. #2
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    Re: Piezo preamplifier missing cap values

    Hi,

    In most cases fc = 1 / ( 2 × Pi × R x C) applies.
    R3, R4 act as a voltage divider, thus for C2 2.55k applies.
    Choose fc to be very low, much lower than 10Hz.

    C3 should be tge same as C7, with 470k as resistor for fc calculation.
    Fc should be lower than 10Hz.

    C4 and R8 determine the upper cutoff freqyency of the signal.
    The same applies for C5 and R6.

    R5, C6 determine the lower cutoff frequency of the signal,
    Fc should be much smaller than 10Hz.

    What frequencies to choose depends on the application...

    Klaus
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    Re: Piezo preamplifier missing cap values

    You forgot to tell us what is and post a link to a Timegrapher and how it is used.
    You also forgot to tell us that a piezo transducer, not a piezo beeper is used and it is used as a microphone.
    I looked in Google to see that an old Seiko watch produces 21600 clicks per hour which is 6Hz.

    The circuit might work if the reactance of C7 equals the resistance of R1, if the reactance of C6 equals the resistance of R5, if the reactance of C5 equals the resistance of R6 and if the reactance of C4 equals the reactance of R8. Capacitance is easy to calculate using the frequency and the reactance.

    C2 is an interference filter, use 100nF and C3 can be 4.7nF.



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    Re: Piezo preamplifier missing cap values

    Not sure if the piezo sensor pick up the fundamental frequency (1 - 10 Hz) acceleration or the click impact sound (kHz range). Lower and upper amplifier cut-off frequencies would be respectively different. We can easily agree that C2 and also C3 are simply filter capacitors, 100 nF or even larger does no harm.



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    Re: Piezo preamplifier missing cap values

    Thanks,

    timegrapher is a device to measure / graph the operation of mechanical watches, it measures time between tick and tock and deviation in tick-tocks' length etc, and tells if the watch gains or looses time.
    From the measured data you can figure out what to set or modify to get more precise time keeping.
    The Weishi even shows a graph on the LCD. Attached a pic of it.

    It's an expensive device, so for a hobbyist like me there are programs for the PC (WatchOScope, TG) to do this job with the PC's mic input, but it needs a good amplifier.

    I'd like to use a piezo beeper as vibration pickup.

    In the original there is a solid approx 3x12mm piezo(?) bar you can see in the attached pic, I don't really know much about it.
    Could it be completely different than a piezo disc from a beeper?
    Last edited by zedman2; 22nd February 2020 at 15:56.



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    Re: Piezo preamplifier missing cap values

    A piezo transducer is used as the microphone on an e-drum. It picks up taps from the drummer hitting it.
    A piezo beeper has a transistor oscillator circuit in it that you do not want. Instead you need a piezo transducer that can be used as a high pitched speaker or used as a microphone.
    I have never seen and you probably cannot by a piezo "bar". A piezo transducer is shaped like a thin coin.

    Why not use the "app" available for your phone to set the watch timing?
    62 years ago when I was in school I set my mechanical watch a little each day until its accuracy was awesome. Then each day later I got up from my desk, said to the teacher, "have a nice day" and the moment my hand touched the door handle the bell rang.



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    Re: Piezo preamplifier missing cap values

    :)

    It's easier and quicker to set the watch with this than to wait a day to check if it looses or gains...

    Life a bit accelerated in the last 60 years :)

    Iam going to use the pieso disc from a piezo beeper.

    BTW I'm experimenting with the caps but noticed that despite the 9V Vdd the output is only 1V p-p, what causes this?
    I pushed the amplification up a bit but the signal gets capped at 1V.

    Thanks,
    Zedman



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