Hello friend,

I have one problem with my GSM base project.
In my project, i use P89v51Rd2 means 5V microcontroller and my GSM module (SIM900) is working at 4.2V, so how can i connect my Micro RX, TX which is at 5V to SIM900 RX, TX which is 4.2V support.

Please help me to design my circuit.
Thanks in advance.

Regards,
Ghanshyam

 

SIm900 tx/rx can work with 5v microcontrollers. You can directly connect microcontroller tx/rx to sim rx/tx.

For sim900 power supply just place a 1n4007 diode which drops 0.7V from 5v.