There is a great deal you don’t understand. You don’t understand you need resistors in the base of a transistor. You don’t understand how to draw a schematic. and you don’t understand what happens when you remove the power from a device yet still have active signals going into it.
Okay, so I’ve understand why it’s important to put a resistor in the base of the transistor (Ohm’s Law). In fact, the GSM module does not work properly now, so the reason can be due to I didn’t put this base resistor.
You are also wasting power by having a pull down resistor on the switch rather than using the external pull up resistors
I don’t understand what you refer to external pull up resistor. Do you refer to connect the switch to the Vcc instead of a digital pin?
Stop it, they are not wasting power. When not in use these devices will be in standby using ( not wasting ) very little power.
When you send a signal into a device that is not powered the inputs look like a diode short to the power and ground rails.
This project needs to use the lowest quantity of power as possible. So SD card can’t be in standby mode. As well as, the project uses the SD card few times.
It makes no odds because the SPI pins are active if any one device is active.
It’s logical, but when the transistor is in cut-off mode the current of SD card pins is not 0! Why? I don’t know.
I was thinking again the importance of the base resistor. In any case, is it possible that the A6 module contains an initial resistor to protect the circuit from crazy people like me? Thanks to it, base resistor wouldn’t be necessary, or am I wrong again?