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I have a "Raspberry Pi (2 Model B rev 1.1)" wired with "SIM900A using a "USB to TTL module". Send/recv SMS, make/accept/hang calls functionalities are working as expected using "AT" commands. Further, I want to record the incoming call audio on "Raspberry Pi". I also want to playback a pre-recorded audio from RPi when an incoming call is received.

  1. I searched web and also asked ChatGPT, which seem to suggest connecting "SP+", "SP-", "MIC+", "MIC-" directly to GPIO pins of "Raspberry Pi". But, I am unsure if connecting "SIM900A" pinouts directly to RPi GPIO is a good idea, because of different operating voltages.

  2. RPi doesn't even seem to support a microphone input either, otherwise a 3.5mm audio jack cable could have done the trick.

  3. Another option that comes to mind is, use an "A2D module", which can help interface SIM900A digitally to RPi GPIO pins.

  4. I am wondering if swapping RPi for an Audrino or upgrading to another microcontroller or even another GSM module will help my cause.

Has anyone tried and successfully achieved such a feat using a GSM module along with a microcontroller. Pointers, help, tips are much appreciated. Thank you!!!

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    The Pi does NOT have any analog capability so is a poor choice for audio. GPIO pins are all digital.
    – Milliways
    Mar 11 at 7:27
  • Thank you. Are there any existing microcontrollers where I could possibly get away by directly connecting SIM900A SP+, SP-. With RPi, I guess my best bet is to use a A2D.
    – Stupid
    Mar 11 at 9:19
  • Frankly I have no idea what a "SIM900A SP+, SP-" might be or do. A Pico has analog capability, GPIO (similar to Pi) so it may be suitable. Pico are also readily available, cheap, more capable than Arduino (but possibly harder to program). Suggesting hardware is off-topic for this site - you need to do your own research.
    – Milliways
    Mar 11 at 10:58
  • stop thinking Raspberry Pi and start thinking Linux computer
    – jsotola
    Mar 11 at 22:52
  • "SP+" and "SP-" are just speaker positive/negative out. SIM900A drives out analog signal through these pins. These pins are essentially providing speech audio as received from the other end. IMHO, an ADC is the only option (16-bit depth preferred). For providing audio input into SIM900A via its MIC+ and MIC- pin, I might still be able to hack it by connecting RPi 3.5mm audio jack directly into these pins on SIM900A.
    – Stupid
    Mar 12 at 10:14

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