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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

S Andrew
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1 Answers1

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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.

Bence Kaulics
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  • Can you give me a link for that circuit – S Andrew Dec 20 '16 at 14:13
  • There are plenty of them on this site, do a search please. – Bence Kaulics Dec 20 '16 at 14:29
  • @SAndrew I have used dedicated ICs but simple circuits with some MOSFET will do the work. – Bence Kaulics Dec 20 '16 at 16:12
  • Thanks. I will definitely use level shifting. But actually when I did this on breadboard using breakout, I didnt used any level shifting and thats why I got confused. I have added a link to that breakout in question. – S Andrew Dec 21 '16 at 04:32
  • Hey I studied about the level shifting and I found out that its not that much complex. I found this document page 7 where they have described level shifting using transistor which looks best for this problem. Also found this link where they have discussed that using voltage divider circuit is also enough. Which one should I go with.? – S Andrew Dec 21 '16 at 05:13
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    You can try this IC: http://www.ti.com/lit/ds/symlink/txb0104.pdf – Adam Calvet Bohl Dec 21 '16 at 06:09
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    This alternative is also fine and simple: http://electronics.stackexchange.com/a/196466/123923 – Adam Calvet Bohl Dec 21 '16 at 06:12
  • @SAndrew Yes, it is the cheapest and easiest way to go with transistors. Also unfortunately the supplier of the breakout board does not include schematic so I could not check the level shifter. But I am sure that there is one, probably they go with transistors as well. – Bence Kaulics Dec 21 '16 at 08:17
  • @AdamCalvetBohl Thanks that IC is also best for this. But unfortunately its not available near by me so I'll go with transistor solution. – S Andrew Dec 21 '16 at 08:43
  • @BenceKaulics Apart from the level shifter, is my power circuit design good for this purpose as I am using two regulators, one for MCU and one for SIM900. What other things I need to keep in mind while designing its board layout. – S Andrew Dec 21 '16 at 08:47