Discussion in 'Homework Help' started by D Jans, Sep 6, 2011.

1. ### D Jans Thread Starter New Member

Sep 2, 2011
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0
I need to make any mobile phone believe there is a microphone headset inserted in the 3.5mm audio jack. Does anyone know where I can find a circuit diagram for a phone headset microphone?

At the moment I have experimented with resistors from mic to gnd, like this:
mic(3.5jack)------------
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xxxxxxxxxxxxxxxx[R]
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gnd(3.5jack)------------

I have had some success with the phone recognizing microphone when R has been above 2kOhm, but I want to understand why and thought I might get some better knowledge if I had a diagram of a real phone headset.

2. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Actually, I had repaired a headset once, replaced its speaker, and if I remember correctly each speaker had a measured resistance of 32Ω. 2kΩ sounds a bit much.

Is your phone output mono or stereo? If it is stereo it might need to "see" a load in each channel before switching to the according profile.

Shorting both channels, and then driving them both to ground through one resistor, might work or might not.

Please tell us what you have found.

3. ### D Jans Thread Starter New Member

Sep 2, 2011
5
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You are correct that the speaker resistance is low, that part I have solved already. It is the microphone part that is a bit more tricky and from what I have read it seems to be from around 1kOhm and upwards.

I am however not sure if I have put the resistor correctly, or if I need more resistors.
When the "fake" cable is done it is supposed to be connected to sound output that the phone should record using the 3.5mm jack.

4. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Well, the first approach would be to measure the resistance of each channel of an actual headset, from each pin to the "sleeve" (ground). So, in essense I would first use three resistors, one for each channel.

I guess your jack has four pins, three for signal and one for ground, right?

5. ### D Jans Thread Starter New Member

Sep 2, 2011
5
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I think the gnd is on pin 3, and I have measured the resistance between pin3 and pin4 on an actual working headset and it was 38kOhms. The L/R wiring works already.

1 2 3 4
L R GND MIC

Also I need to get sound into the cable and I have no clue if a resistance or two will do something to the sound?

6. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Maybe it's time you explained exactly what you want to do with your project. The details are becoming important and in order to help you we must know more.

7. ### D Jans Thread Starter New Member

Sep 2, 2011
5
0
This is what I want to do:

I have a homemade 3.5mm 4-pin cord that I want to be able to plug into a mobile phone. I want the phone to recognize that there is a headset microphone inserted. After that I want to connect the other end of the cord to a sound source, probably mic or line level.
I want the phone to be able to record this sound(as if I was speaking in a real headset microphone)without distortions.

8. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
In that case, I would do the following, which requires you to ruin a headset, but I think it is important, so you could buy a cheap one:

While the headset is connected, strip the ground and the mic wire of the headset at a point in the wire where you have access to. Use a multimeter set to AC function to measure the amplitude of the voice signal that goes to your cell when you speak. That should be your maximum amplitude from any signal source you connect later to your cell phone. Any higher and you run the risk of burning your phone.

If your multimeter is sensitive enough, I would try to measure the output current too, that way finding the mic's output resistance too.

After that, you can simply connect your signal source to the phone, taking care that you provide the necessary signal volume with the required output resistance.

If someone has an idea that will save you a headset, I 'd love to hear it.