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I am using a TC4427EPA to drive a piezo buzzer, with two bypass capacitors on the supply voltage, as recommended by datasheet. I have used the same setup to drive a 40 kHz ultrasonic speaker (with a 40 kHz square wave), and it seems to work fine, with the ultrasonic speaker giving off 40 kHz sound.

Now, I change the buzzer to an audible one (resonant frequency 2300 Hz +/- 500 Hz according to seller), and change the signal to a 2 kHz square wave, hoping to generate 2 kHz sound ...

schematic

simulate this circuit – Schematic created using CircuitLab

And something has puzzled me. I use a (very cheap) oscilloscope to inspect the voltage coming out of the TC4427EPA.

Without the buzzer, the waveform looks like this, which is expected:

enter image description here

Re-insert the buzzer, the waveform looks like this. Note the little "knife edges".

enter image description here

Then, I use a microphone and an amplifier circuit and the same (very cheap) oscilloscope to inspect the emitted sound. It looks like this. Note the frequency at 4 kHz.

enter image description here

My questions are: Why the difference between 40 kHz and 2 kHz? What can I do to make it emit 2 kHz properly? Is it the bypass capacitors? Is TC4427 supposed to work at such low frequencies? Is it the buzzer itself (can't generate precise frequencies)?

You can see I am an electronics novice. Any help is appreciated. Thanks.

More Info in response to some comments:

  1. Yes, I am using a 9V battery. I am working in a very low-tech environment.

  2. This is the waveform on the 9V supply:

enter image description here

  1. The buzzer's model is TMB12A05

And I realize that a piezo buzzer may be driven by a simple circuit with a transistor and a resistor. My main puzzlement here is why TC4427EPA works on a 40 kHz buzzer but not a 2k Hz one.

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  • \$\begingroup\$ Is the 9V coming from a single 9V battery? Have you looked at the power supply pin with scope? Which piezo is it, please link to a datasheet? Have you verified with other equipment what the frequency or waveform looks like, such as guitar tuner or mobile phone with oscilloscope/spectrogram app? \$\endgroup\$
    – Justme
    Nov 9, 2021 at 7:35
  • \$\begingroup\$ I have given some additional info above. For the one question I have not answered: No, I have not verified the situations with another piece of equipment. It's a puzzlement, but not (yet) a big enough problem for me to go that length =) \$\endgroup\$
    – Nick Lee
    Nov 9, 2021 at 8:10
  • \$\begingroup\$ You need to find a buzzer data sheet that has the full spectral response. The data sheet you linked is barely worth the paper its written on. Also post a link to the data sheet for the 40 kHz device. \$\endgroup\$
    – Andy aka
    Nov 9, 2021 at 8:11
  • \$\begingroup\$ That datasheet is the best I can find. I thought the buzzer's resonant frequency (2300 Hz +/- 500 Hz) is the most relevant, and I was hoping that there is a general explanation not too specific to this buzzer. \$\endgroup\$
    – Nick Lee
    Nov 9, 2021 at 8:36

1 Answer 1

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The buzzer data sheet is useless. But these piezo buzzers have high capacitance so it takes a high surge of current to charge it. The 9V battery is an extremely weak source of current. At 2 kHz the duration and magnitude of current pulse to charge the piezo capacitance is so large that the bypass capacitors and battery can't provide it so supply voltage drops. As the supply voltage drops and has ripple, so will the drive waveform to piezo.

Why it seems to work at 40 kHz means that the pulse duration is faster so you can't see the voltage dropping so much, it might just be lower on average with less ripple.

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  • \$\begingroup\$ Thanks. I will find a power supply and see if it remedies the issue =) \$\endgroup\$
    – Nick Lee
    Nov 10, 2021 at 3:11

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