The Voltage divider tutorial is VERY helpful and straight forward. But as I plan to use one to divide 5 volts down to 3.3 as done in your example, I am concerned that my calculations do not include the resistance (impedance) of the device being connected across R2. In the case of the GPIO of a Raspberry Pi or in the accelerometer case, when I connect the input of this across R2, might if significantly change the effective resistance of R2 and thus the amount of voltage across it? How can I know the impedance of devices such as what I mentioned? Many thanks.