HS-47 equivalent circuit

[gordon 229]

Appears I aimed a bit too high with my first project, and am in need of a bit of a help.

 

The project involves (indirectly) interfacing a Nokia cell phone to the Launchpad, "listening" to one of the audio out channels (don't need the other one) and simulating control button presses. For this I need the phone to reliably recognize the interfacing circuitry as a "basic headset" (if using a non-conforming naive straight-through cable, my two test subjects, an E51 and an E90 are reliably unreliable at working with the manual accessory-type selection, and keep asking "what is this plugged in my @ss?" after every call).

 

The actual circuit, when fitted with the speaker(s), the microphone and the control button is quite simple, the extensive specification of the detection process (Nokia AV hardware interface specification, with coinciding information here and here, among some other pinout sites) however suggests to me that the implementation must be quite strict, and everything matters.

 

What I need basically is circuitry that has no speaker(s) or a microphone, but passes the detection as a "basic headset".

 

Cannibalizing a HS-47 and using it for parts was also considered, but the result is very fragile both physically and in reliability (working with those litz wires is completely out of my league, also with the speakers and/or the mic cut off, the rest apparently doesn't pass detection).

 

I have sort of been stamping in one spot with this for the past couple of days, so a couple of well-directed pushes are appreciated.

[oPossum 1,083]

The spec says the mic bias voltage must be above 700 mV and less than 1750 mV for a headset.

 

Try a shunt made from two 1N4148 or similar diodes in series connected to the mic line.

[RobG 1,892]

After quick look at your links I think all you need to do is put 4k7 or bit smaller (2k2) resistor between mic and gnd.

[gordon 229]

Both of these approaches are working fine, tested with a number of handsets (various Nokia models) I was able to pry from colleagues' hands during the day. Sometimes there is a slight hesitation in reacting to control button presses, but this seems more tied to a particular device than anything else (coincidentally, it is my E90, but i don't often use the button on the headphone, so I can't really tell; a different E90 reacts just fine).

 

Thank you guys.

[gordon 229]

working with those litz wires is completely out of my league

I found this nice trick on the intartubes, and it works pretty well.

 

There are many tips and tricks floating around, from burning the lacquer off with a lighter or torch (with it's accompanying problem of oxidizing the wires, which is then hard to scrape off) to dipping the wires in various acidic substances (where you probably don't readily have any suitable kind at home, they usually can't be shipped, and anyway come in way too much quantities than needed for a lifetime) and waiting a couple of ten minutes.

 

The trick I found makes use the best of these worlds, and, somewhat surprisingly, involves common everyday aspirin pills.

 

The active ingredient of Aspirin is acetylsalicylic acid, and getting it activated is just a matter of heating. Put your litz wire on a pill, and with a tinned iron heat both until you see the wire taking the solder. The pill will melt, and you will basically bathe the wire (and your iron) in boiling hot acid. How long it takes obviously depends on many things, for me it was in the 15-30 second range for thin headphone wires.

 

Important thing: do not breathe the fumes or let them in your eyes. It's acid, and it's not good for you. Use of at least one of these PC fans for forced ventillation is recommended. Maybe mask and goggles too.

 

You may want to do this on a bit of a glass or ceramic surface (ashtray or a leftover floor tile), as the molten carrier of the pill sticks to everything. Also, you probably don't want to use your regular iron, but some old/cheap one (again, you are bathing it in hot acid).

[zeke 693]

:shock: :?

 

This kinda sounds dangerous and risky.

Have you done this procedure yet?

 

I would just gently scrape the wire with a knife.

[gordon 229]

:shock: :?

 

This kinda sounds dangerous and risky.

Have you done this procedure yet?

It isn't, really (just don't breathe the fumes). It's about as dangerous as when rosin meets the iron. I did this twice already for practice, and there's nothing particularly scary or risky about it. I did it with an iron a bit under 300 degC, and it's not like it squirts or splashes or something, just boils a bit, and releases some fumes.

 

I would just gently scrape the wire with a knife.

Tried that too. On the amount of work needed vs. convenience vs. risk scale, for me, this method wins by a huge margin.