Hello,
I am building a project in which I am using esp32 (please excuse if this forum does not deal with esp). I have the need to power a simple peizzo buzzer from it.
If I power the buzzer with 3.3v it is not very audible. So I need 9v for clarity. Esp digital pin will only output 3.3v.
Can I use a dc to dc step up converter for this ? I understand peizzo buzzers don't really use lot of current. Am I right to use a step up here then?
Please guide.
Regards
No. An ESP pin doesn't have enough current to drive a step up convertor. You just use the ESP output to gate a 9V power supply to the buzzer. A FET circuit can do that.
Ok thanks 
Generally, a transistor will do such as the the ubiquitous 2N2222 with a 2k2 base resistor. Of coruse a FET is better but you have to pick a logic-level one - I was about to suggest the 2N7000 for up to 150 mA but it is marginal on 3.3 V.
Yeah, it'll be a borderline solution - may or may not work well enough. However, if 2 of them are cascaded, no problem.
Interesting. How would you propose that?
Well of course, the maximum specified threshold voltage is 3 V, so you are using the marginal conduction of the first to switch against the 10k.
Sort of workable, but marginal. 
Not "sort of" workable. It works.
In simulation...
You show it running on 9V. The 2N7002 has a VGSth of 2.5V max, so the pre-driver just barely makes it. That will rise as the temperature drops, so it would be sensitive to cold environments. Mathematically, it's okay, but you are supposed to leave a noise margin...
True, but the current it has to switch is negligible so technically, it would - just - work. 
Two FETs to do the job of one BJT.
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