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I am designing a circuit for SIM900 with a MCU. I have never designed the circuit for SIM900 so thought of posting question so that someone can review it. As per the datasheet, SIM900 works on power between 3.2v-4.8v. For this I have used LM317 with 150 & 370 resistors. So it will give output voltage of 4.33v. Below is the schematic for it:

enter image description here

VBAT is the output voltage of 4.33v.

Now in SIM900 I have connected TX RX pin to the MCU and have made connection for status and netlight, SIM Card connector. I have connected VBAT and GND. But I am confused with PWRKEY connections. Below is the schematic :

enter image description here

I don't have an idea about PWRKEY. I have a GSM Module and they are using a switch button for it which when pressed connect it to ground. But I do not want this. Is there any alternative for this. Also please review my circuit. Thanks.!

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    \$\begingroup\$ The ability of an LM317 to to provide the required current during transmit is questionable. Try looking at published designs for the SIM900 (or failing that, seeing what you can learn from SIM800 or SIM808 designs). \$\endgroup\$ – Chris Stratton Jul 5 '16 at 6:25
  • \$\begingroup\$ -1 For not reading the datasheet. The answer can be found in it for more scenarios. \$\endgroup\$ – Bence Kaulics Jul 5 '16 at 8:42
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  1. LM317 is not a good candidate for this project,use LM2576 & LM2596 adjustable voltage regulators
  2. check UART level between your host controller(MCU or ...) & Module,if they operate in the different level you need a level shifter buffer.
  3. 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)
  4. Read simcom's app notes and reference manual & datasheet,all above described there in detail.
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