Circuit for LCD Watch Buzzer

Using Arduino Audio
1

Hey all!
I am working a project where I would really like to use a piezo disc to make sound just like my old LCD watches from the 70's and 80's. I am using an esp32 MCU. I have purchased some small piezo devices, but they are not very loud.

Does anyone know what kind of circuit was used for the LCD watches to drive the piezo disks? I mean those are very loud. I have seen schematics where people have suggested using various specialty voltage booster ICs but I'm like" Tony Stark did it in a cave in the 70's with 3V battery in a friggen wrist watch"

So here is the drill:
I want to drive a piezo disk
I am using +3.3V DC source
What is the circuit?

2

Arduino_VibromotorTransistorCircuit_AbstractPictorialDiagramPlusCircuit
Arduino_VibromotorTransistorCircuit_AbstractPictorialDiagramPlusCircuit2464×797 169 KB

3

Man, I don't know if you are for real, or you are trying to get me to build a sex toy (never heard of a "vibromotor").

Seriously, I have seen similar circuits. Here's one:

I think we are close.

Fish

4

The circuit in post #3 above works well if the inductor and the piezo capacitance are resonant at the driving frequency, which greatly increases the driving voltage (10X or more). I've seen L values on the order of 30 mH. Design considerations

5

Well done, @jremington! That is the one. I actually found this manual on my bookshelf (I forgot I had it):

image
image1532×1884 260 KB

This was published back in 1983. There was actually a section on the alarms and here is the circuit:
image
image1459×1053 143 KB

You can see that same circuit on the right.
I guess we can say for sure that this is the circuit.
I am going to try to build one. I'm not an electrical engineer and my electronics background isn't the best - but the reference you gave me looks pretty good. I'll start there and do as much homework as I can. Any chance you could look at my final design and provide feedback?

Appreciate the help tracking this down!

Fish

6

O.K. So looking at this reference: Increase piezoelectric transducer acoustic output with a simple circuit - EDN Asia

I already have some questions:

  1. The author states, "The electrical resonance frequency need not be the same as that of the acoustic resonance. In fact, if it is roughly 2× the acoustic resonance, the peak voltage across the transducer can be greatly increased.". I think I can take that at face value even though he doesn't reference a source. So according to his example:
  2. Power supply = 5VDC
  3. L1 = 3.2mHy
  4. C(piezo) = 2nF
  5. Signal source frequency = PZ1, resonant frequency = 40KHz
  6. Signal source duty cycle is adjusted to eliminate a large current spike on turn-on

If I interpret this correctly, The mechanical (acoustic) resonance frequency is 40kHz so we would want to target an electrical resonance frequency of 80kHz. But if you do the math
image
so using his values of L=3.2mH and C=2nF
f=62850 Hz. - this isn't 80kHz.
But a few sentences down he says his oscilloscope shows, " * Violet = electrical resonance voltage across the transducer, 92Vppk. at 80KHz"

am I missing something?

7

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