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I'm using a SIM800L GSM module for my project. In this module there is a electret mic input pin but I want to give audio to this module from headphone output from a mobile (There is no line in pin available). If I directly input from headphone out the module make too much noise. Now what is the best idea to implement this?

Here is module instruction diagram:

SIM800L mic integration

Mic input/Audio output characteristics

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  • \$\begingroup\$ Welcome to EE.SE. Not sure what you are asking of us. To use a speaker as a microphone? Using a speaker as a mic will overload the mic input. The mic input may have a small bias voltage as well. \$\endgroup\$ – Sparky256 Sep 28 '18 at 19:57
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    \$\begingroup\$ See electronics.stackexchange.com/questions/239708/… Not a dupe but likely does solve it. \$\endgroup\$ – Passerby Sep 28 '18 at 21:49
  • \$\begingroup\$ Decrease the volume on the source and turn it up on the target device. This sounds like it could just be typical behaviour. \$\endgroup\$ – K H Sep 29 '18 at 7:50
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I just did this, so I can answer by experience. I have a SIM800L module that's answering calls via a PHP script on a Raspberry Pi 3, and I wanted to send PicoTTS voice on the line using the AUX jack audio output. I followed this youtube tutorial using "mostly" the correct values components. I got the resistances pretty much right by playing with 4.7k and 1k resistors, and just the capacitor is shooting up with 47uF instead of 1uF.

I only connected the left side of the jack output leaving the other lead unconnected, the level I get on the phone is alright, the voice quality is alright, I just get some vibrating noise as soon as the phonecall is connected, which disappears slowly and after a while of mostly clean audio comes back, as if a big fly was briefly flying by and then going away again. But still, I can testify it works, an the noise is OK with me anyway.

Line level converter

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The 90mW into 32 ohms from the headphones output of your mobile is 1.7V (RMS?). The output of your voice from an electret mic (conversation level at a distance of 10cm) is only 10mV. So you must attenuate the headphones signal 1.7V/10mV= to 1/170.

90mW into 32 ohm headphones is VERY loud. 100mV from an electret mic is also VERY loud. Then attenuate to 1.7V/100mV= 1/17th.

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    \$\begingroup\$ This would be a lot more useful as an answer if you explained how to do that. If the asker knew how to design an attenuator they probably would not have asked the question. \$\endgroup\$ – Chris Stratton Sep 29 '18 at 4:38

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