SIM900 Circuit Design. Where to connect PWRKEY of SIM900.?
LM317 is not a good candidate for this project,use LM2576 &amp; LM2596
adjustable voltage regulators
check UART level between your host controller(MCU or ...) &amp;
Module,if they operate in the different level you need a level
shifter buffer.
PWR KEY used for turning module ON/OFF,for doing this you must pulled it down for a moment(less than 500 ms or around)
Read simcom's app notes and reference manual &amp; datasheet,all above described there in detail.