use of LED control for buzzer?

[Archibald]

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.

[mzimmers]

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

[mikemoy]

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.