B

D C

EF

Fig. B: The telephone handset, disassembled. Fig. C: The headset’s PCB, showing the wiring for the push button (center) and LED (bottom right). Fig. D: The reverse side of the PCB, showing the

battery and wires for the charger (upper right),
microphone (lower left), and speaker (lower right).
Fig. E: The charging jack and flat washer spacer installed.
Fig. F: The modified charger with coaxial power plug.

between each pin and the negative battery terminal on the PCB. Mount the coaxial power jack on the handset, taking advantage of the round hole left behind by the cord. I added a washer so the jack would fill the relatively large opening (Figure E). Solder wires from the power jack to the charging terminals on the PCB, making the center pin the positive wire.

6. Modify the charger. The charger used a proprietary 3-pin connector to connect to the headset, so I cut it off. Only 2 of the pins are actually used for charging, so I replaced the connector with a more common coaxial plug (Figure F) to match the jack on the phone. Plug in the charger and use the multimeter to determine the polarity of the wires. Then connect the positive wire to the center of the plug, and the negative wire to the outside terminal.

7. Assemble the handset. Reinstall the headset battery, making sure to solder the wires with the right polarity. Use hot glue to secure the battery to the PCB. Put a dab of hot glue over each wire’s soldered connection to the PCB. This gives the wires some strain relief and keeps

them from ripping the traces off the PCB.

Insert the headset PCB into the handset behind the microphone holder. Install the speaker, microphone holder, and microphone. I had to remove a plastic tab from the back of the microphone holder to keep it from hitting the charging jack inside the handset. Screw the caps back onto the handset.

Follow the headset manufacturer’s instructions to turn on your new retro wireless handset and pair it with your cellphone. Call a friend and test it out. If everything works, admire your finished handset.

You’re done!

Jeff Keyzer is an electrical engineer who lives in San Francisco and hacks cars and microcontrollers. Visit his blog to see more projects like this one at mightyohm.com.

Drilling on a Round Object Next time you need to drill a hole in something round, file a little flat spot so the drill can get a good start without skating off to the side. —Frank Ford, frets.com/homeshoptech Find more tools-n-tips at makezine.com/tnt.

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