I'm designing a board on which I'll connect a microcontroller with the SIM900 module. The recommended circuit for powering on the module (from the manufacturer datasheet) is given below.
The module is powered on/down by setting the power key to low logical level. I guess that transistor should saturate in order to do that. But, last time I used transistors was long ago and now this circuit looks completely strange to me. So I have a few questions.
Will the transistor saturate if my microcontroller has 3.3v logic level? My guess is that it won't since 3.3v is higher than 3v for 0.3v, not 0.6v. But I'm not sure about that at all.
For what is the second resistor (47k) used? How are the resistor values calculated? I could probably figure it out for resistor 1, but would like to hear opinion of others. I still can't remember all of the equations I learned at high school.
On the right you can see that that the project will be battery powered. A typical li-po battery voltage is 3.7 v. On the board, there will be other devices (the screen) that will require 3.3v logic level. So I decided to wire the battery to a 3.3v voltage regulator, so it can power the mcu on the left. The SIM900 module is powered directly from the battery without any voltage regulators, as SIM900 can take up to 2A of current. The MCP73831T is there for battery charging, and the ch340g for serial communication with a computer. Hope this clarifies things a bit, and sorry for me not mentioning it earlier.