Good morning,
I'm opening this post because I'm not so good in electronics, and I would like to learn more. In particular, I've read that arduino nano 33 ble (sense) has an intrinsic voltage regulator but it's connected only to Vin. This means that it's not "connected" to the other pins, and its function is not to delivery a constant voltage of 3.3V to all other units? Do I have to buy another voltage regulator to do that?
I'm sorry if the question could be stupid.
Thank you in advance! 
That Arduino has two ways to feed by 5v from usb or by (7 to 20)V by the VIN pin.
The vin input is connected directly to the internal controller that gives a 5V output, between the latter and the received by the USB is selected, or mixed, (I don't know exactly), by a diode that I don't know how it works,in ord er that we have a stable feed of 5v to feed the plate and other components strenuous at the outlet 5V pin.
We could also feed the plate through this 5V pin, but we'd have to regulate it extermamemte exactly 5V.
Then we have the output of the 3.3v pin to power other external circuits, but I don't know exactly how the performs I suppose you'll have another internal regulator, whether chip or zener diode.
Best regards.
There is a +3V3 pin that you can use to power other parts of your system with 3.3V. However you should only power small sensors, buttons and leds with the build in voltage regulator. If you want to power anything with larger current requirements you should always use an additional power supply.
In the Arduino store under DOCUMENTATION you will find a pin diagram, schematics and more information about the power options under FAQ.
https://store.arduino.cc/arduino-nano-33-ble-sense
Remember converting energy creates heat that needs to be transferred away. There is very little room for that on the Arduino Nano PCBs.
Also the traces on the PCB are rather small and will not carry a lot of current.
In my case I would like to connect a heart rate / SPO2 sensor (max86141) which works at 1.8V. so I shouldn't worry about that thanks to the built-in voltage regulator, or I have to use a step down converter?
You have to use a 1.8V voltage regulator for the sensor and level converter for the signals between your Arduino and the sensor.
Commercial products would likely just use 1.8V for the entire system. The NINA-B3 module on the Arduino Nano 33 BLE can run at 1.8V as well. You can disable the onboard voltage regulator by cutting a bridge at the bottom of the PCB.
But normally you don't usually buy the chip directly, (or is that what you're going to do?),
Normally you buy a module that includes all the circuitry needed for easily connect it to the Arduino. Between that circuitry includes the necessary step down 3.3 or 5v to 1.8V. If you're going to connect the chip directly if you're going to have to lower the voltage to 1.8 either 5 or 3.3v.
Best regards.
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You need a voltage regulator of 1.8V and level shifters for all the signals.
I'm sorry, I thought they were two different questions, but at the end they are connected! Sorry!
If I use something like this https://datasheets.maximintegrated.com/en/ds/MAX38640-MAX38643.pdf , the level shifter is required the same? Because I have very strict size constraint since I have to put the final device inside a mouth...
Yes I'm going to buy the chip directly...
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