~ 23 Nov 2016, 06:37

As it is well known, technologies can be helpful, but can also be just as annoying, as today's rant will show.

In the old days, PC and laptops provided 2 audio ports: an output one (for headphones), and an input one (for a mic). You can hardly make this simpler, but more importantly it worked well:


Well, in the last 4 years the trend is to combine these two ports into a single, four-pin (TRRS) one:


I can't for life understand why the laptop manufacturers thought this is a good idea - but the fashion came out from the smartphone world, where the space budget is tight and that sort of tricks actually make a lot of sense[1]. Anyway, the result of combining those two ports into one is that questions like this or this arise, and people are wondering how to connect their good ol' microphones to the laptops. The problem is that the combo port works for normal headphones, or for headphone+mic combos (and only if your specific combo has the correct pinout at the jack, as there are two competing standards for that). But just a plain mic doesn't work, as its pinout is completely incompatible.
The standard solution to this problem, as far as these threads on the web go anyway, smells of the characteristic 21st-century obtuseness: just buy a new USB microphone! It connects so easy and just works, etc.

That's great, but in my case I'm not connecting a mic. I'm connected a DIY light-to-sound converter, which I'm using to download the data from my Battery level meter. It connects via a 3-pin audio jack, but that's where the similarities with a mic end.

Of course, you can buy splitters like this one:


I had the necessary jacks so I crafted my own splitter, but it didn't work. Turns out, the laptop requires to sense that you have headphones connected, otherwise it'll assume the audio port is not occupied.

This was a bit too much. Out of spite, and led by a bout of OCD, I crafted the following scheme:
The 15-ohm resistors simulate the headphones :)

It fits on a very small perfboard:

The cable from the 4-pin jack arrives here; the other side of the board houses the resistors and the female 3-pin receptacle. The latter accepts a regular mic jack, whereas the 4-pin is for the stubborn laptop, which now "sees" a headphone+mic combo.

As I wanted the whole adapter to be a bit more rugged and safe, I crafted a box. The picture above was actually taken to provide a convenient way to sketch out the required sizes. After some CAD-ing on Blender:


Then I 3D-printed it and forced the board inside:


These thin wires would be a fragile spot in the future, so I fixed innards with a hot glue gun:


Which completed the adapter - the microphone circuitry in the laptop finally yielded and was happy to record. I still wonder if it was worth knocking up this contraption; it probably isn't, but I was furious about the shortsightedness of the Lenovo engineers. So this eccentric response serves them some poetic justice :)

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