Recently while browsing a Goodwill I came across these. They caught my eye because while they looked at first glance like a normal pair of headphones, the design was kind of retro, and the material looked odd.
On closer inspection, I realized they were really old (well, as wireless headphones go). It turns out they're from 1990 or so, use infrared (as opposed to Bluetooth, which of course didn't exist in 1990) and were intended to be sold alongside a big base station with a weird infrared transmitter thingy. Of course, they were not being sold with said base station, so I knew I'd have to either get one (which sounded more trouble than it was worth) or have them as a display piece.
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| They have that future-retro aesthetic. |
But I got to thinking - they take AA batteries, and have a slot on each half of the headphones for them. I realized that I'd probably be able to find a bluetooth reciever module and a small lipo battery to fit in those spaces, so...
$9.99 later, I'm the proud owner of a pair of Sony MDR-IF510 "Cordless" headphones. I love how they used the term "Cordless" as opposed to "Wireless" - it feels like a little tell-tale of the era they came from, before many of the technology standards we take for granted today were even in conception.
Some days later I got around to taking them apart. Each side has its own infrared demodulator circuit, paired with - to my delight - a little op-amp circuit.
I could just use the bluetooth reciever's built in amplifier - but the headphones have a little volume dial built into the circuit board on each side and I'd really like to use those if I can.
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| The circuit board for one half. |
One neat thing - the circuit boards are identical on both halves. They come with two places to solder in the volume adjustment potentiometer, and the PCB on the other side had the potentiometer soldered into the other spot. They're even labelled "RCH" and "LCH," for Right Channel and Left Channel, presumably.
After looking at the datasheet and probing around, I was able to figure out the audio path (in orange) and rig up a little test setup to verify that my work was correct.
I've gone ahead and ordered the lipo battery and bluetooth module - but I think I still need to figure out a battery charging solution (likely another cheap amazon module and a USB C connector) and possible some way to integrate an external audio jack for wired listening.
Oh, and I need to replace the foam ear padding that's completely disintegrated into 90's foam dust.
