How to choose a transistor for PWRKEY on SIM808 design?

Question

I am using a 3.3V digital output signal from a microcontroller.

The design documentation suggests using this circuit. How do I determine the transistor that I need?

Answer 1

An ordinary general-purpose switch such as an NPN transistor or n-FET can work. 2N3904 or similar for the NPN; 2N7001 or similar for the FET.

Using this kind of open-collector/open-drain device avoids two issues:

• GPIO I/O voltage vs. the SIM800H VBAT pull-up voltage
• Host power state vs. the SIM800H power state

If you don't use a transistor, you'll need to carefully consider these issues in your design to ensure that there will be no leakage currents in any of the power states (including VBAT being not present.)

Answer 2

The intention of the circuit is to simply pull down the input to a digital pin that has a pullup. Use whatever small NPN/nMOS transistor you already have on hand/in inventory which is compatible with the external 3v3 turn-off. A BJT with a base resistor, or a small logic-level FET which can be turned on at 3v3 can be used (in the case of the FET, the 4.7k series resistor is not needed).

Additionally, depending on your choice of SIM800H supply voltage, you may be able to get away with an open collector output on your microcontroller and a series resistor without exceeding the maximum ratings of the controller. It depends on the datasheet, which we don't have since you haven't given the model. Note that in this case, the logic signals must be inverted compared to those provided to a transistor.