hello, i rather new on arduino and electrical. I want to operate sim800l on arduino. And i look some tutorial on internet but itsnt working right. Sim800l led keep blinking 3 time on short period. So i put off my sim800l from dc to dc stepdown ( ineed step because arduino have 5volt on vcc and the sim800l need 3,7-4,2 volt). And i measure the stepdown input and out put and it have right voltage. But when i attacched my sim800l and measure the stepdown input, output, the voltage went drop. So my deduction it must be the lack of current. So i need your help to solve it. Any suggestion will helpfull. Thanks 
According to comments in this thread: SIM800L issue - Project Guidance - Arduino Forum
the Arduino does NOT provide enough current to start this module. You will Need to attach your drop down power supply to another source and not the Arduinos 5V pin.
You need an external power supply of 5V to be supplied ,
or your can get a 5V dropped down with 7405 from a 9V battary
thanks, i alr did that, but there is another problem, when i send "AT" on serial , it just respond with “ÿÿÿÿÿ”. i dont know how to fix it 