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Good day,

I'm currently getting into DIY electronics and Netduino programming and there's this one personal project that I want to do but I'm hesitant because of lack of information. Basically this is what I want to do:

  • Use a mobile phone, in my case an Android tablet (Galaxy Tab) and maybe a WP7 handset in the future, to communicate with a Netduino via a makeshift serial connection using the audio (headphone) port and a custom app. I have found a circuit that I could use and modify for my needs so this part can be considered solved... apart from the app, that is, but that's a different story.

  • So with that, I also wish to be able to transmit information to the Galaxy Tab via the tablet's microphone input by designing a DAC circuit to emulate an electret mic. Is this even possible? I am no electrical engineer but I do have electronics know-how (although that's with regards to high-voltages) because of my line of work but I have little experience working with digital/analog conversion circuits. I don't want to fry my Galaxy Tab's innards because of assumptions so I've been searching for information around the net to no avail. I hope someone can point me to the right direction. Will really appreciate it!

Thanks a bunch,

Dan

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  • \$\begingroup\$ I assume that stevenvh edited the title of my question. I'd like to say thanks. I noticed I made a mistake with the title just recently but my net connection broke so I wasn't able to change it. Thanks again stevenvh. :) \$\endgroup\$ – DeVilFisCh May 27 '11 at 20:13
  • \$\begingroup\$ You're welcome. If you want to know who made which changes to a question or answer click on the date next to "edited". \$\endgroup\$ – stevenvh May 30 '11 at 9:14
  • \$\begingroup\$ Oh, I didn't know such a feature existed. Thanks for the heads up. \$\endgroup\$ – DeVilFisCh Jun 4 '11 at 9:18
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It should be possible, but you need to match the power and impedance expected. An electret mic will have power running to it. You might find it easiest to use a 1:1 audio transformer to isolate each sides and allow the signal to carry the voltage of the electret.

You will most likely need to run something to give attenuation. This could be either just a divider or an Op Amp with a gain less than one.

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  • \$\begingroup\$ Thanks for the enlightenment. I'll try to whip up a circuit using your advice. If only I had an oscilloscope to check the actual range of voltages it would have been a lot easier. \$\endgroup\$ – DeVilFisCh May 27 '11 at 20:11
  • \$\begingroup\$ I picked up a $400 Rigol 1052E DSO (which you can modify to 100 MHz with a software hack.) This is the first scope I've been able to afford for home. It is really great to have access to for this sort of thing. I would err on the side of too much attenuation and work from there. You could easily damage a sensitive input with too much power. \$\endgroup\$ – Joe May 27 '11 at 20:59
  • \$\begingroup\$ Thanks for the heads up. I'll try looking for that model in the local market. Oscilloscopes tend to be prohibitively expensive on this part of the world so I hope I can find a good deal. I'm also concerned about possible damage on my device's inputs so I'll play safe and acquire an oscilloscope first. Thanks again. \$\endgroup\$ – DeVilFisCh May 29 '11 at 0:09
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I suppose there is some risk involved, but basically I think you want to wire up an audio-taper potentiometer from the output of the dac to ground, and then tap off the wiper with a series capacitor to inject the signal into the mic input.

I suppose it is possible you might need a resistor between the audio input and ground as well, but my guess is you will not.

Start with the potentiometer turned all the way down, and turn it up slowly until you get the cleanest signal (get a spectral fft type of app for your testing, and run your test modulation).

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I did more research and learned that my original plan was a lot more complicated than it needed to be. I found that I could use frequency shift keying (FSK) like the one I saw here and here which are both for the iPhone and should be usable for other devices with little or no modification. I'm currently researching how to do it directly using C# code on the Netduino which isn't in the scope of this site anymore so I have to find help elsewhere.

Anyway, thanks to both Joe and Chris Stratton for providing answers. I'm sorry if I had to unset Joe's answer for my question as it does not apply anymore. Both answers are still very informative on my side though so thanks a lot. I'd probably make use of them in another project.

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