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I am designing a SIM900 schematic so before going further I thought of verifying the schematic first. I will using SIM900 with PIC18F2520. So to power the SIM900 and Microcontroller, I am using LM7805 for MCU and LM2576 for SIM900. Following is the circuit:

enter image description here

LM2576 o/p 4.1v is for SIM900 & LM7805 o/p 5v is for MCU

SIM900 Circuit:

enter image description here

Explanation:

PWRKEY: PWRKEY is connected with a push button which when pressed will connect this pin to ground, in order to start the power on scenario of module.

UART: TX RX lines are connected to RX TX of the MCU.

NRESET: connected to a push button to reset the module.

VRTC: I am not using any RTC function so just connected a 4.7uF cap.

NET & STATUS: Connected led's to these pins to show the status and network of the module.

Apart from the above schematic, is there any recommended circuit which I am missing in the circuit.?

Do I need a voltage level shifter between TX RX of SIM900 and TX RX of MCU.? I initially used a breakout board of SIM900 and tested it with the MCU, it was working fine without any external circuitry for voltage level shifter.

Is there anything which I am missing in the circuit. Any suggestions would be very helpful.

Breakout board: enter image description here

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SIM900 Serial interface runs on 2.8 V logic levels, PIC runs on 5 V so you are definitely need level shifting. You did not provide a link to that break-out board but I am sure that there must be some level conversion circuit on that.

From application note.

Please note that the UART level is 2.8V, if the level is not matching, a level shift circuit is needed.

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  • \$\begingroup\$ Can you give me a link for that circuit \$\endgroup\$ Commented Dec 20, 2016 at 14:13
  • \$\begingroup\$ There are plenty of them on this site, do a search please. \$\endgroup\$ Commented Dec 20, 2016 at 14:29
  • \$\begingroup\$ @SAndrew I have used dedicated ICs but simple circuits with some MOSFET will do the work. \$\endgroup\$ Commented Dec 20, 2016 at 16:12
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    \$\begingroup\$ You can try this IC: ti.com/lit/ds/symlink/txb0104.pdf \$\endgroup\$ Commented Dec 21, 2016 at 6:09
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    \$\begingroup\$ This alternative is also fine and simple: electronics.stackexchange.com/a/196466/123923 \$\endgroup\$ Commented Dec 21, 2016 at 6:12

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