0
\$\begingroup\$

This is my first question on this forum, and I would like to say that I am learning electronics, and I am not an expert by any means.

I am currently trying to realize a small project using a LM386 to accomplish the following objectives:

  • Amplify sound from a capacitive microphone and listen to the amplified sound through a small 30ohms speaker
  • Finally, use a jack to connect a mobile phone and be able to use the capacitive microphone in my circuit as a microphone for the mobile phone.

I have achieved the first objective. Here is the schematic:

enter image description here

I have tried to make some connection, but I can't manage to use the microphone that I have connected in the LM386 also in the phone.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Do you want the signal from the microphone to go to the speaker and the phone at the same time, or just to the phone when you plug it in? \$\endgroup\$
    – JRE
    Commented Dec 22, 2022 at 15:58
  • \$\begingroup\$ Yes, I need the microphone signal to go to the speaker and to the mobile phone at the same time. \$\endgroup\$
    – Miguel Hernández
    Commented Dec 22, 2022 at 16:09
  • \$\begingroup\$ Why can't you use the internal microphone of the phone? \$\endgroup\$
    – Andy aka
    Commented Dec 22, 2022 at 16:12
  • 1
    \$\begingroup\$ I want to use this circuit on a motorcycle. The microphone of the circuit is in the helmet and the mobile phone is stored but connected with a cable via the jack connector. I can not use the microphone of the mobile phone. \$\endgroup\$
    – Miguel Hernández
    Commented Dec 22, 2022 at 16:16

4 Answers 4

Reset to default
2
\$\begingroup\$

You can't just connect a microphone to an Android headset and expect it to work. An Android phone expects a certain circuit inside the headphone so that it can detect the headset.

enter image description here

The important thing here is the 2k resistor from the microphone pin to "Ring 2." The phone expects that 2k resistor from the microphone pin to ground - that's "Ring 2."

You have to provide that 2k resistor without disturbing your existing circuit.

You'll need to do something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

That should make it work, but you might find that the sound is rather weak. You can try using a larger value for C50, but that will only help a little. You'll probably need to use a lower value for R21 as well. R21 should usually be something around 2k.

If that is still not loud enough, then you will have to move the microphone input to the speaker output.

schematic

simulate this circuit

The difficulty here is that how loud you sound on the telephone will depend on how loud you've set the speaker volume.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ Thank you very much JRE! It works!! Thank you very much for your time! Best regards!! \$\endgroup\$
    – Miguel Hernández
    Commented Dec 22, 2022 at 17:35
  • \$\begingroup\$ Which circuit did you use? \$\endgroup\$
    – JRE
    Commented Dec 22, 2022 at 17:35
  • \$\begingroup\$ The first one works very well. The volume is perfect and I have tried making a call and they hear me perfectly. I am very grateful! \$\endgroup\$
    – Miguel Hernández
    Commented Dec 22, 2022 at 17:47
2
\$\begingroup\$

You are not using the manufacturer's power supply voltage, resistors and capacitors for the LM386 amplifier. With a supply that is only 5V then the maximum output power into the very high impedance speaker of 30 ohms is only 0.06W. Usually a 9V supply and an 8 ohms speaker is used for an output power of 0.56W. The datasheet shows correct resistors and capacitors: LM386

\$\endgroup\$
2
  • \$\begingroup\$ Dear Audioguru. Thank you very much for your reply. Indeed, you are right. My values do not correspond to those indicated by the manufacturer. I have modified them according to your indications and I have added the JRE contribution to the schematic. Finally, I am not sure if it is ok, I have connected the sound of the mobile phone to be able to listen to it in the speaker. Attached schematic: i.sstatic.net/25qrr.png Thank you very much, I am very grateful. \$\endgroup\$
    – Miguel Hernández
    Commented Dec 22, 2022 at 20:45
  • 1
    \$\begingroup\$ In your new circuit, the mic signal is weak and the phone volume is too loud. When you turn down the volume control for the phone then there is no audio mixer circuit so it kills the mic signal. \$\endgroup\$
    – Audioguru
    Commented Dec 23, 2022 at 1:25
1
\$\begingroup\$

Your problems were to simply amplify the mic signal, to simply attenuate the phone level and to mix them together. Here is an audio mixer circuit:

mixer

\$\endgroup\$
1
  • \$\begingroup\$ I'll give it a try and see how it goes. Thank you very much for your attention and time. Regards. \$\endgroup\$
    – Miguel Hernández
    Commented Dec 23, 2022 at 17:13
1
\$\begingroup\$

I improved the mixer by adding a coupling capacitor: mixer

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.