modded handset, charging

This modified handset is charging and connects to any Bluetooth device (phone, computer, etc.) using the headset protocol.

A while back, I saw Hulger and their Bluetooth handsets. Unfortunately, they didn’t have what I thought was the classic desk-phone handset: the one that was featured on the Bell Model 500 phones. So, I decided to make my own.

Update: So, apparently Thinkgeek sells similar handsets. Their handset is modeled after the Bell F1 phone’s handset (amongst others), which I don’t find to be as comfortable as the Model 500 handset. It is, however, rather inexpensive compared to Hulger’s offerings (although Hulger’s are much nicer in quality).

Ingredients

special tools

Construction

The
handset

Note the convenient button on the handle.

I’m not sure what device originally connected to this particular handset, but it was unique in that it put a big button on the handle. When I saw this, I realized that it would be perfect for doing a Bluetooth headset mod, as most offer a 1-button interface for the most important things.

Prepare the Handset

Rip its guts out. Although I don’t know for sure, I imagine that the speakers/microphones of these old handsets require too much juice to use with a headset. In addition, they are rather heavy will make this mod much lighter than the original - a bonus!

Prepare the Headset

For the brains of the handset, I used a cheap Motorola Bluetooth headset (model HS801), but almost any headset will do. It’s probably best to optimize for quality of audio, ease of hacking (not too funky in shape, small enough to fit in the handset), cost and battery life. Aesthetics don’t matter much when you’re just going to rip it apart :-)

Using some careful prying, crack open the case. Watch out as there are sometimes tiny wires connecting various parts that could break with too much force (in this case, it was the speaker’s wire).

I attempted to see how I could fit the headset into the handset and discovered that it would fit perfectly in the handle if I slipped it in from one of the open ends (with some slight modification of the handle). This convenient location would also make it so I wouldn’t have to rewire the main button, which would have required some very careful soldering that I didn’t wish to attempt.

By carving out the interior with a Dremel, I was able to make room enough for both the headset and its battery. This was tedious, due to the cramped space (I was working through the button’s opening) and the delicate main-board. Patience paid off and I was able to slide it in without trouble.

The headset installed in the handset

Note the convenient location of the button. (this shot was taken after all the handset modifications were made, but the headset initially fit in when its battery was removed).

The mod requires a bit of hacking of the headset to fit it into the form factor of the handset. The four main components that need to be taken into account are:

  1. The button(s) (solved, due to careful positioning in the handle)
  2. The microphone and speaker
  3. The battery (solved, after the handset Dremel modifications)
  4. Charging it

Mostly, these components need to be stretched out so that they fit the full length of the handset. This will require a little soldering and some fine wire (as the circuit boards on the handsets can have very small drill holes for their original wires).

Extending the Microphone and Speaker

I removed the microphone and speaker by carefully desoldering them from the main board with the help of a little desolder wick. I ended up not using them (as I accidentally broke their tiny, fragile wires) and didn’t regret throwing them away. You can get equivalent parts from the wired headsets that you get with every new cellphone.

I used some ribbon cable for my fine wire, carefully tearing it into paired strands. That got soldered to the main board and to the extra set of microphone / speaker that I scavenged from the wired headset. I had originally attempted to solder the new wire to the original microphone and speaker, but due to their small size/weight, I found it much easier to solder to the not- so-small ones.

The handset, opened

The tiny microphone is on the left and the speaker is on the right.

As you can see above, the extended microphone and speaker were attached to the case with foam double-stick tape. Hot glue might be a better option.

Charging

I wanted to make sure that the handset could be charged without disassembly. I also didn’t want to solder any components of the power system of the main- board, as it charges a lithium ion battery (and it’s very bad to short those). My design goal was to use the existing socket on the board (in this case, a standard Motorola charging socket) and extend it to the hole that previously contained the handset’s cord.

This turned out to be easier than I imagined, as I happen to have a spare Motorola charger and plug. I also had an extra power socket laying around which was just the right size for the cord hole (always buy two of a part like that for a project: you never know when you’ll need it).

I chopped up the extra Motorola charging plug so that it would fit in the cramped space and cut of the original wires. This one conveniently had the required 33k resistor required for the Motorola charger conveniently connected to ground. Make sure that any such quirks are accounted for when building the extended charger. I wired the plug to my spare socket and superglued the socket into the cord hole (after a little trimming of the socket with a file).

the charger end

On the left is the microphone, on the top is the power socket glued into the cord hole.

For the actual charger, I found a wall wart in my collection that had the right plug and voltage (6VDC). The original charger might be a smarter idea if you can manage to get a spare socket for it. I’m a big fan of not modifying parts when they aren’t needed, so I opted to not chop mine.

Power Button

the handset, emphasis on the small hole

Note small hole in the handle.

This particular headset turns on/off by holding down the volume down button and the action button at the same time. I wanted to make sure I could still do this, so I drilled a small hole in the handle to get at the volume down button. It’s not the most elegant solution, but I figure that the device doesn’t need to be turned off all that often and a paper clip isn’t much to carry.

Done!

the handset in hand

The perfect fit.

the handset, charging