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SIM900A shield

Discussion in 'General Electronics Discussion' started by Karthik rajagopal, Jun 12, 2020.

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  1. Karthik rajagopal

    Karthik rajagopal

    May 9, 2016
    Hi all,
    I started working with my project which requires the use of SIM900A. My code gets executed with arduino and GSM without any problem. But I will be using a 3.3V microcontroller whose I/O pins are not 5v tolerant. I am using a SIM900A GSM shield from Astar. I did some continuity test on the board to find out two MOSFETs connected to the Rx and the Tx pins in the configuration I have shown below(the same was for Rx also).
    IMG-20200612-WA0029 - Copy.jpeg

    I thought this could be a level shifter but none of the circuits for a level shifter that I found on internet matched with this...all had their souce and gate connected. Will it be safe to connect my GSM directly to the MCU?
    I also tried connecting the Tx pin of the GSM to the MCU through a voltage divider,but it did not work.
    Please help.
    Thanks in advance.

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Are you really certain you didn't mix up source and drain when extracting the hand drawn circuit?

    It looks like this module has a RS-232 interface, not TTL, on the D-SUB connector.
  3. Karthik rajagopal

    Karthik rajagopal

    May 9, 2016
    Thanks for the reply.
    Even I was confused with that..I checked it several times ,don't know why they have connected it that way . I have attached the part no. along with its image. IMG_20200612_222654.JPG IMG_20200612_222253.JPG The GSM board also has TTL ports... IMG_20200612_221332.JPG
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