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
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.
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.