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How to build a digital FM receiver for recording audio in MP3 format? Preferably directly to Android phone? Answered


I have this thing:

Which I use during lectures so that I could properly hear the lecturer, and it works very very well - I can hear him perfectly when he "wears" it so that microphones are close to him.
I also have the FM receiver part which works in conjunction with a hearing aid - no issues.

However, I also would like to RECORD everything he says, a technician told me the FM frequency used by the Phonak system is Channel 16 frequency: 174.1200 MHz (174120.0 HZ)

I tried a few Google searches, but I did not come up with any leads or ideas, how to record this transmission - best would be to an MP3 file or something, I could store on Evernote or something along with the lecture notes I take.

I should also point out, I have an Arduino Uno at my disposal (with Prototyping shield).

I am open for any and all ideas, and suggestions how to get this done.
I might require some help as electronics is unfortunately not my strong suite (software development is, however).



So how does this link into your hearing aid ? By the loop input ?

Yes, using the loop which has built in FM receiver and is creating the magnetic resonance picked up by the Telecoil built into the hearing aid.

In that case, just pick up the loop signal from the receiver straight into your recorder. No need for an arduino. Your standard plugin earphone-mic might well be all you need.

What do you mean by plugin earphone-mic? can you give an example?

The thing that plugs into your phone for headphones and microphone

The Loop cable is integrated with the receiver, so took me a while to figure out what you (possibly) meant:
According to: http://www.phonak.com/content/dam/phonak/b2b/C_M_tools/FM/Receivers/UserGuides/MyLinkPlus-User-Guide.PDF
On page 9, it has a Headphone socket (2.5mm) - did you mean to use this output, as microphone input for the phone? if so, how?

Googling a bit further, I found this: http://forum.xda-developers.com/showthread.php?t=1548401

Apparently it is a 4 contact miniature jack - so I am not sure what to look for, that would work bridging the two together...

That helps enormously, just get the 4 pin jack off a mic/headphone cable and wire a 2.5mm jack by removing the microphone from the cable.

I don't guarantee it will work without a bit of extra circuitry, but it might. Try it.

My original idea was to make a pickup coil and pick up the loop signal through that.

Interesting idea, I will be looking for such a cable (along with 2.5mm male jack) that I could cut and solder instead of the microphone :)

BUT, this method has one BIG drawback: according to the above linked manual, once I plug in the external "output" (originally designed for say, headphones) - it will disable the Loop to save battery power!!

So, your idea of building a a pickup coil is still preferred - could you please give me instructions as to how to accomplish this? if its not much more complex, I would rather try that first.

And thanks so much for the help so far! I have hope :)

Have you anyway of laying your hands on a "dead" hearing-aid receiver ? I'm not sure how the pickup is achieved: I know, of course, what it needs - basically a coil wrapped on a former.

I muse idly on the idea of using a tape-recorder pickup head as the coil... cheap and simple.

Try it. Its hard to suggest much else without experiments.

I do have some older hearing aids which are no longer in use, but it is past midnight now, I will look into this tomorrow :)

PS: any idea how to recognize the telecoil? everything is miniaturized in that little thing :)

We may need to make a tiny amp for the signal, we may not. This is getting interesting.

Hi Again Steve :)
I have opened a very old hearing aid I had (of the generation before the digital ones) - based on the picture we saw online of the telecoil component, I MAY have identified it, but it is sooo small!
Like, really small! the connectors are as tiny as hairs almost... I am not sure how to solder that.

In addition, one connector got torn off while I opened the case :(

I am attaching two pictures - one of them taken using a magnifying glass :)

What do you think?

I think you've nailed the identifcation, but I think soldering it will be tricky !

That is an understatement, Steve!
I tried to get the wire off with some spare, but it tore off without spare :(

Any suggestions on how to solder this? perhaps there are special strategies for this scale?
Do I just drop a small drop of the solder over each contact, with the hope they will keep the touch to the tiny contacts remaining? The drop would pretty much cover the whole hole...

You really need a little piece of prototyping board, solder the tracks to the board, solder the wires to the board - then there is no strain on the wires....or get the telephone pickup ;-)

damn, instructables filters out HTML and image links.. I attached them here.


All I can suggest is some GOOD photos ! Let's see if its detectable.

ALSO, you might be able to find a "telephone pickup coil" from somewhere. Like this http://www.maplin.co.uk/telephone-pick-up-coil-3519


...or this http://compare.ebay.co.uk/like/321077321479?var=gv&ltyp=AllFixedPriceItemTypes&var=sbar&_lwgsi=y&cbt=y&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=65&ff19=0

Are you certain the above two are telecoil based?
I tend to think they are mic based...
Anyway this evening I will update with pictures once I take apart old hearing aid :)

Meanwhile, I found this:

In the shop: http://www.grupopremo.com/in/shop/

But I am not sure - is that the telecoil generator, or the receiver we are looking for?
Looking forward for your answer, because apparently I can order the one on the Shop online, and no extra shipping charges :)'


That looks like the receiver coil to me. Its VERY small though ;-)

Do you mean one edge to the ground, and the other to the Microphone?
Because as I understand it, the 4 jack receiver goes this way:
A headphone 4 pins (sound + mic) have 4 contact, beginning for the point are : Sound Left, Sound Right, Ground, Microphone.