Sennheiser PX250 is a noise canceling headphone which shares the similar appearance as PX200. I got it from a friend. He was just about to get a new one since he lost the button of it. I designed and printed the switch to repair it. Later on, I found it was annoying to get AAA x 2 for it all the time. So I decided to modify it a little bit, to replace the battery compartment with a lithium battery and a circuit with the capability of charging, voltage measurement, and regulator.
Basically it suppose to be something like this:

So here was the list of things need to be done:

  1. Print a new cover to replace the old one (with extra holes for LEDs, the button, and the MicroUSB port).
  2. Electrodes need to be removed to get more space for circuit and battery.
  3. The On&Off switch which PX250 uses is EG6201 (made by E switch), which contains 6 switches, one is used for power. You can cut the trace off and place your LDO in between topologically.
  4. There are two screw holes, which will help to determine the size of the circuit board and the battery.
  5. Of course, get a battery that just fits in. Design a circuit as well.

Break down:

Circuit design:

Assemble mockup:

New cover 3D modeling

Final assemble. (printed by Makerbot Replicator 2, with helpdisk).