Adding Bluetooth to Vintage Headphones – Part 1

A few years ago, I rescued a pair of nice headphones from the trash and replaced the original speakers with better sounding Sennheisers.

Then, I changed the jack cable for something fancier (and I must admit, less practical).

Now, it is time to put the headphones on the operation table once again to cut the wires..

Because I wanted to keep the headphones as much in their original state as possible, without adding holes, I decided to use the original potentiometer for control and volume. This means I had to find a way to detect a press on the potentiometer. For the bluetooth part I will use the Microchip BM62 Bluetooth module. It’s really cheap and has everything already built-in (Bluetooth LE, a serial interface for control, pins for LEDs, a battery charger and an audio amplifier).

The potentiometer button

The potentiometer is original, 220 Ohms and audio-grade, from Dralowid, a now defunct german (DDR) company, from Berlin..

It was attached by two rivets and a bunch of washers to the side of the left ear cup:

After drilling the rivets out:

The back side:

The idea was to use the rivet holes in the earcup to attach a piece of PCB with a pushbutton. Another piece of PCB will hold the potentiometer and have an actuator for the button and an axis in common with the first PCB.

For this, and also to refresh my Autodesk Inventor skills, I decided to model the whole assembly in 3D.

First, the potentiometer:

Then, the earcup. It’s not perfect, but the essential part (the recessed half-round volume with the two rivet holes on the side, the potentiometer slot and the two openings on the “bottom” part are accurate):

From this point, we can try to make the first PCB with the axis and pushbutton:

The slot below the pushbutton will limit the range of the potentiometer push and pull course. I chose an SMD pushbutton hard enough not to need a spring. A piece of foam glued on the ear cup speaker will help to hold the second PCB in the “off” position.

I started with a M2.5 axis screw, but switched to a smaller M2 size because the screw head is smaller and makes a smaller countersink in the 1.6mm PCB. The two screws holding the first PCB to the ear cup are M2 with nuts and spring lock washers on the outside.

I designed the second PCB to hold the potentiometer and actuate the pushbutton:

I exported the drawings to Altium and designed the “real” PCBs:

I made two versions of each PCB: one with a M2.5 axis screw and one with a M2. The small PCB rings are to be soldered of the inner sides of the axis holes, to increase the inner axis length and to constrain the rotation.

Then an other export of the PCB step models into the Autodesk 3D model. Finally I printed the various parts on paper and cut them to check if they were accurate.

Signaling and charging

The left ear cup has two additional holes for the audio cable: a round one for the cable itself and a slot, for the cable restrain. These will do nicely for a RGB LED and a mini USB socket:

The USB socket and the LED are on their own little PCB, which is attached by two pieces of acrylic hand-cut to fit the curve of the ear cup (easier to adjust for the ear cup curves and to tweak the USB plug angle). I will create a custom light pipe for the LED, made of acrylic too.

The main PCB

The BM62 module has a serial interface, but can be used standalone with a button interface too. Because we need a way to read the position of the potentiometer, it would be best to interface the Bluetooth module to a small microcontroller. A PIC of the 18 range will do nicely, especially from the LF voltage range (2V … 5.5V). It will save the hassle and BOM lines of a DC/DC converter – the BM62 has an integrated battery charger.

An other plus for the microcontroller: the headset battery will be recharged via the mini USB socket. The Bluetooth module’s configuration is stored in its onboard EEPROM, programmed via the serial interface, the PIC will act as a USB-RS232 bridge, allowing me to easily reconfigure the module.

I tried to keep the things simple and, because at the time when I designed the PCB, I didn’t have the module or the microcontroller, I routed all the pins of the BM62 to the PIC. I also included options with 0 Ohm resistors to select the source of the PIC power supply (the BM62 has several pins tapping its internal voltage regulators) and to select the source of the LED drive (Bluetooth module or microcontroller).

The PCB with the USB socket and the RGB LED is connected to the main PCB with a flat flex ribbon (connector P5 on the schematics).

(sorry, my Altium version seems to have problems with 3D and bottom soldermask).

So, let’s see if we can get everything into the ear cup:

So far, so good. There might even be some room for a small LiPo battery.

I ordered the PCBs and the components. The 3D vs. reality and the final assembly will be for the second part of this article. Stay tuned!

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