Using Arduino Uno to Drive full H-Bridge at high frequencies

[John_F26]

As more of a proof of concept rather than anything else, I'd like to make a DIY aroma diffuser using a piezo element with a 1.7mHz resonant frequency.

I tried generating a 1.7mHz square wave with the Uno and am running into 2 problems:

1.) The waveform is rather messy when looking at it through the oscilloscope - I can use a capacitor to smooth this out but the result is more of a triangle wave than a square wave.

2.) More importantly, this waveform could not possibly power the piezo element, as it requires a much higher Vpp to diffuse water. Note: While I can't find the specifications anywhere, I have been able to find some YouTube videos of people using these elements with success around +/- 24V.

So: My idea of a solution was to make a full H-bridge out of TIP127's and use an external 24Vdc power supply - which hopefully in theory would give me the +/- 24V I'm looking for.

Is this a viable solution? If so, how would I control the H-bridge with the Arduino Uno, and would I require any additional IC's?

Or, should I try going a different route?

Thank you!
John

[sterretje]

Can you show your code that generates the messy waveform? Can you show an image of the messy waveform?

Hardware is not my strong point but why a H-bridge and not simply a logic level mosfet?

[6v6gt]

Post a link to the piezo element for the aroma diffuser you intend to use.
Is it similar to this application of a fluid atomizer:

Ultrasonic Mist Maker - ElectroSchematics.com

In this case a simple mosfet seemed to be enough.

Post crossed with @sterretje

[John_F26]

I've attached a picture of the waveform before and adding a capacitor to smooth it out.

sterretje:
Can you show your code that generates the messy waveform? Can you show an image of the messy waveform?

Hardware is not my strong point but why a H-bridge and not simply a logic level mosfet?

Here's the code I'm using - I found this on another forum and it seems to produce the correct frequency...

const uint8_t OUTPUT_PIN = 3;  // = OC2B
const uint8_t PERIOD = 9;      // 9 CPU cycles ~ 1.778 MHz

void setup()
{
    pinMode(OUTPUT_PIN, OUTPUT);
    TCCR2B = 0;           // stop timer
    TCNT2  = 0;           // reset timer
    TCCR2A = _BV(COM2B1)  // non-inverting PWM on OC2B
           | _BV(WGM20)   // fast PWM mode, TOP = OCR2A
           | _BV(WGM21);  // ...ditto
    TCCR2B = _BV(WGM22);  // ...ditto
    OCR2A = PERIOD - 1;
    OCR2B = PERIOD/2 - 1;
}

void loop()
{
  soundBuzzer();
}

void soundBuzzer() 
{
    TCCR2B |= _BV(CS20);  // F_CPU / 1
}

void silenceBuzzer() 
{
    TCCR2B &= ~_BV(CS20);
    TCNT2 = 0;
}

[John_F26]

6v6gt:
Post a link to the piezo element for the aroma diffuser you intend to use.
Is it similar to this application of a fluid atomizer:

https://www.electroschematics.com/ultrasonic-mist-maker/

In this case a simple mosfet seemed to be enough.

Post crossed with @sterretje

Yes - that application does look quite similar to what I'm trying to accomplish.

Here's the link to where I bought the piezo element: https://www.amazon.com/gp/product/B0794MW9FQ/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

The main differences between the two I notice are that the one from your link has a 113kHz resonant frequency rather than a 1.7mHz frequency like the one I'm using. I read somewhere that higher resonant frequencies produce a finer mist, which was the reason I decided to use that one vs the 113kHz.

I also see that it seems as if the included circuit diagram uses a 555 timer to produce the frequency rather than an Arduino uC - is this observation correct?

I'm having a bit of trouble understanding how that circuit produces an AC waveform, but based on the output waveform I understand why it works successfully - they achieved a very clean AC waveform with ~60Vpp at the desired resonant point. I basically need to do exactly the same, just at a much higher frequency.

If the 555 timer is responsible for producing the correct frequency, do you think a 555 would be capable of producing the higher frequency I need at 1.7mHz?

[cattledog]

OP's waveform images.

How is your scope tip grounded? Are you using a ground spring? If not, try shorten the probe ground lead.