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My aim is to connect Mobile phone's Mic to Laptop headset out for injecting audio into mobile phone calls. So that Mobile phone calls should get answered automatically and system should play the recorded audio. This is not possible by programming Android or iOS. Hence i am looking for hardware approach. Searching online i got two different circuit diagrams. Not having much electronics exp, i am unable to decide which one would work. Need help to achieve this

Circuit 1:

Circuit 1

Circuit 2 :

Circuit 2

Note : I have tried circuit 2 and it worked well on iPhone but not on Android (Samsung). On Android audio was very garbled. Not sure whats the difference ?

Any suggestions?

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  • \$\begingroup\$ The second one contains a frequency filter. Probably explains the garbling. \$\endgroup\$ – Bradman175 May 23 '16 at 4:23
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    \$\begingroup\$ The schematic of the second is quite hard to read, could you please redraw it? \$\endgroup\$ – clabacchio May 23 '16 at 8:06
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(pad) This seems to work nicely:

enter image description here

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Phones supply a small DC voltage to power the microphone. If you connect an external signal to the mic input you should use a DC-blocking capacitor as shown in your second circuit. 1uF is more than enough for excellent low frequency response.

However, the first circuit shows a full resistive divider attenuator to reduce the speaker level down to microphone level. The second circuit uses quite low value series resistors (330 ohms) which provide very little attenuation. Ideally, you would use a "series resistor" (like the 10K in your first diagram) and a "shunt resistor" (like the 1K).

Ideally, you would provide a 10K resistor from each channel (presumed Left and Right), and a single shunt resistor in the values shown, followed by a 1uF capacitor, and going into the mic input.

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Your first circuit will attenuate the audio but does not block (dangerous?) voltage differences between the laptop and phone. The second circuit contains capacitors to block this voltage. However, most phones detect external devices through testing their resistance. The capacitors likely present an infinite resistance at some point after being plugged into your phone. So the phone might eventually think there is nothing plugged in and will switch back to using the normal microphone. Try adding the first circuit AFTER the second circuit. If the 1K ohm resistor is low enough, the phone may continue using the external sound source.

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