By Bill Reeve (firstname.lastname@example.org)
Adapted for instructables by Mouse (email@example.com)
Disclaimer: The procedure described here may not work for you - that's a risk you have to take. If it doesn't work, or if you break something, it's not my fault - it's part of the adventure's risk. I assume you know how to solder. If you don't, please learn before attempting this procedure.
Introduction: Hooking up an old telephone handset to work with a cell phone is simply a matter of connecting together the correct wires. The goal is to substitute the microphone and speaker in the old telephone's handset for the microphone and speaker in the cell phone's handsfree headset. We will do this by attaching the handsfree headset plug (the metal end that plugs into the cell phone) to the end of the coiled cord attached the old handset. The trick is to identify, and connect together, the correct wires.
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Step 1: Materials
You will need:
1. the handset from an old, broken and late-model telephone (please don't destroy an antique rotary phone),
2. the coiled telephone cord that connected the handset to the body of the old phone, and
3. a wired handsfree headset that works with your cell phone.
You will also need a soldering iron and some shrink sleeving.
If your handsfree headset is significantly different from the one used in this example, you might need a method of measuring electrical continuity, like a digital volt meter (DVM) that can measure electrical resistance. If you don't own, or can't borrow and DVM, you can still make this work with a different headset, but you will need to identify the matching wires by inspection or trial-and error.
Step 2: Opening Up the Coiled Cord
First, plug the coiled cord into the handset.
Now that the coiled cord is plugged into the old handset, cut the plastic connector off the exposed end of the coiled cord (the end that would have plugged into the old telephone body). Using a craft knife, remove about an inch of the outer insulation from the cut end of the cord. This should expose four wires.
Two of these wires (usually red and black) connect to the microphone (in the "mouth" and of the handset), and two other wires (usually green and white in the phone handset, but sometimes they are both white in the cord) connect to the speaker (in the "ear" end of the handset).
Step 3: Opening Up the Wired Headset
Cell phone handsfree headsets also contain a microphone and one (or sometimes two) speakers. We are simply going to exchange the microphone and speaker in the old telephone handset for the microphone and one of the speakers in the modern handsfree headset. We are going to do this by cutting the plug off the modern handsfree headset and connecting it to the end of the coiled telephone cord. The trick is to connect together the correct wires.
What we need to do now is identify the wires in the handsfree headset cord. Since each headset is different, there is no one best approach. If your headset is similar to the one Im using in this example, youre in luck. If not, you will have to use this procedure as a guide and figure yours out.
If you can, the next step is to open the microphone enclosure on your handsfree headset. This will show you the wires and their functions. Figure 3 shows a microphone enclosure both "as built" and opened up.
The second image is a close-up of the "opened up" microphone enclosure, showing the identified wires. As in the first image, the speakers are on the left, and the connection to the cell phone is on the right.
Note how the red and green wires that go to the ear bud speakers pass directly through the microphone printed circuit board (PCB). The bare return wire, which is shared by both speakers and the microphone, is connected to a ground plane on the small PCB and passes up to the speakers. The white microphone wire comes in from the right (the side toward the cell phone) and dead ends at the microphone PCB.
So, now we know which wires to use.
Step 4: Identifying Wires: Alternate Method
If you cant open your microphone enclosure, youre going to have to identify the wires using a different method. To help you, the attached image shows what is connected to the separate sections of a typical handsfree headset plug. You can use a DVM to check continuity between the plug sections and the wires.
Step 5: Prepare the Wires for Soldering
Once the handsfree headset wires are identified, cut the handsfree handset cord about 6 inches up from the plug that usually connects to the cell phone. Strip about an inch of the outer insulation away, and you will find the same color-coded wires inside that we saw at the microphone enclosure.
These wires will probably be fine gage, multi-strand and lacquer-insulated. To remove the insulation on these wires, pass the end of each wire through molten solder on the soldering iron tip. Do this a few times for each wire, discarding the solder each time. This will burn off and remove the lacquer and tin the multi-strand conductors.
Before soldering any wires together, you might want to clip them together and test that the system works.
Step 6: Solder It All Together
Now that you have tested and verified that the handset works, all that is left to do is solder the correct wires together. The attached table shows the solder connections for this example. Be sure to put your shrink sleeve on the wires before soldering them together, and be sure to put a larger overall (probably black) shrink sleeve on the cord before you solder any of the individual wires together.
Note that we will only use one of the handsfree speaker wires, and that both the speaker and microphone share the same (bare) return wire.
Step 7: Final Words
You will probably have noticed that there is a button on the microphone enclosure that can be used to pick up, or mute, a call. This normally-open pushbutton shorts between the microphone wire and return. If you want to incorporate a button with this functionality onto your handset, be sure to test it out before drilling any holes in your handset. Some phones need a very low resistance short between the microphone wire and return when the button is pushed, and many coiled cords are too resistive.
In any event, have fun!