use of LED control for buzzer?

Archibald
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Re: use of LED control for buzzer?

Postby Archibald » Fri Jun 01, 2018 10:37 pm

mikemoy wrote:
Archibald wrote:
mikemoy wrote:Just an FYI, it's not a good idea to drive a piezo electric device via a GPIO pin. Piezo devices can generate negative voltages.
I'm getting only -40mV.
Is that measured with a scope or meter.
That was measured with a scope with 4kHz signal. There's no negative voltage if I drop the GPIO drive strength to minimum. I would not expect a piezo transducer to work well if it's just in series with the drain of a transistor but that should be OK for the electro-magnetic transducer.

I would recommend a kick-back diode (reference) with that electro-magnetic transducer to protect the transistor. I see its coil resistance is only about 16Ω.

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mzimmers
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Re: use of LED control for buzzer?

Postby mzimmers » Fri Jun 01, 2018 10:57 pm

Yes, we'll add that diode. Thanks for the recommendation.

mikemoy
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Re: use of LED control for buzzer?

Postby mikemoy » Fri Jun 01, 2018 11:59 pm

That was measured with a scope with 4kHz signal. There's no negative voltage if I drop the GPIO drive strength to minimum. I would not expect a piezo transducer to work well if it's just in series with the drain of a transistor but that should be OK for the electro-magnetic transducer.
Cool, at least you have a scope. Just because you are not seeing any negative voltage does not mean its not gernerating any. You could be using the body diode in the chip. The best way to be sure is to just hook up a spare piezo to the scope and tap on it. If you see a negative voltage then your relying on the body diode for protection.

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