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Note - You will need basic electronic skills to build the project in this tutorial. You will also need to do some research to verify whether or not it will work with your phone and what connectors you will need. Use this information at your own risk; do not complain if it doesn't work for you. Only use as prescribed. Your mileage will certainly vary.
Let me state it again since some folks have ignored it in the comments. I am NOT providing a specific schematic, details on the connector for any particular phone, or more details on how to hook this up. It is an extremely simple circuit if you've got the right skills and knowledge and the ability to use a search engine to look up the details for your particular phone and verify that it'll work with whatever handset you can get your hands on. DO NOT ASK!
Here’s an interesting retrohack which is actually more practical than it seems at first glance – connecting an old telephone handset to your cell phone.
It’s been done before, primarily with Bluetooth. Many of the mods just repackage a Bluetooth headset by removing the handset’s original microphone and speaker and putting the Bluetooth mike and speaker into their locations with whatever extra wiring is required. A few use the original handset’s mike and speaker and connect the wires to a Bluetooth module. But this version is non-invasive (doesn’t change the handset) and is corded.
Step 1: Why a Corded Handset
Why corded? Partially because it’s cheaper, partially because it’s interesting looking, and also because, at least to my ears, the sound quality is better.
Bluetooth and cell phones are a marvel of miniaturization. Remember what cell phones looked like a decade ago? They were almost as large as the communicators from the original “Star Trek” series (which ironically was set in the 23rd century.) Miniaturization results in compromises. The current technology for tiny speakers and microphones is pretty amazing, but the quality sometimes leaves a lot to be desired. When I use my cell phone at home I usually hook it up to a repurposed pair of computer speakers to make it easier to understand conversations. So using an old corded handset with your cell phone may result in better voice quality.
In addition Bell Labs put a lot of research into design ergonomics for telephones. They’re designed to be comfortable (within the technology limitations of the time and given that they had to be produced for a reasonable price). Some people actually prefer using a standard phone handset instead of holding a cell phone up to the ear, having a Bluetooth unit stuck inside the ear, or wearing a headset. Unfortunately ergonomics design seems to have been forgotten in today’s cell phones. (What in the heck was Samsung thinking when they placed the far too low volume speaker on the back of my phone facing away from my ear?)
The specific description in this tutorial is for a phone with a four conductor (TRRS) 3.5 mm. mini jack which is used for stereo audio output and mono mike input, commonly found on many iPhones and Androids. If you want to build this adapter it’s your responsibility to make sure that this is the correct connector for your phone, or what plug is required and its pinouts and wiring diagram for your particular phone.
For example, here’s how somebody wired an older cell phone which uses a 2.5 mm (micro mini plug) with three conductors instead of four.
In addition telephone handsets have evolved over time. The original carbon microphones have been replaced with electret mikes. In some cases telephone handsets are not interchangable because the manufacturer used a special microphone in the handset and the circuit in the phone which encodes the voice is designed to work with that microphone.
Step 2: Parts
You will need a plain ordinary telephone handset. On most phones after the 1970s it will have a detachable curly cord with a 4P4C (a/k/a RJ-9) modular connectors which are slightly smaller than the RJ-11 jacks used to plug the phone into the wall. On even older phones the handset cords are permanently attached and you’ll have to slice off the cord at the end which connects into the phone. A phone where all of the electronics are in the handset will not work – it must be a “dumb” handset which either has a detachable cord or a permanent handset cord which can be sliced off. I’m only familiar with American telephone handsets, there may be differences in different countries.
Optional, but I like it, is a female headset jack. This is a modular jack (slightly smaller version than the RJ-11), which the headset plugs into on the phone. This makes it possible to make this circuit a non-invasive modification, no need to damage the handset cord. If you want to use this on a regular basis I would recommend getting a handset cord (they’re cheap), slicing off one connector and wiring the handset cord directly to the plug which will plug into your cell phone.
If you’re using a modern cell phone with a 3.5 mm jack for a stereo headset you will need a 3.5 mm. TRRS 4 conductor solderable mini-plug. Every word in that sentence after “will need a” is extremely important.
“3.5 mm.” is the diameter of the plug.
“TRRS 4 conductor” indicates that this isn’t the ordinary mono or stereo plugs you grew up with, it has an additional conductor. T = Tip, R = Ring, S = Sleeve. A mono plug is designated TS (Tip sleeve), a stereo plug is TRS (Tip Ring Sleeve). The reason a modern cell phone needs four wires is you have two wires for stereo audio out, one for microphone into the phone, and a ground wire. Earlier cell phones with mono sound used TRS 3 conductor jacks which *look* like ordinary stereo plugs, but they didn’t have a second sound channel coming out of the phone but did have a microphone signal going into the phone.
“solderable” just means it’s designed so you can solder wires (as opposed to crimp which is a mechanical method of squeezing wires into a connector).
“mini-plug” is another way of saying “3.5 mm. plug”. Plug indicates it’s the male end that inserts into the female jack.
This table shows the pinouts for my phone and headset. Note that the mikes and speakers in telephone handsets don’t care about polarity. Make sure they correspond to yours, or make the appropriate changes before warming up your soldering iron.
T 1 Left
R 2 Right
R 3 Ground
S 4 Mike
Black 1 Mike
Red 2 Speaker
Green 3 Speaker
Yellow 4 Mike
With these two connectors and my phone I need to wire the black and green wires from the headset (ground) to pin 3 (the second ring) on the TRRS plug.
The red wire goes to pin 1 (the tip) of the TRRS plug for left audio (since the handset’s mono we’re just going to ignore the right audio channel).
Finally the yellow wire goes to pin 4 (the sleeve) of the TRRS plug for the microphone.
There is no connection to the first ring on the TRRS plug (right audio).
Step 4: Finishing Up and Using It
As always check all of your connections and use a voltmeter to verify that everything’s hooked up correctly.
When you plug the cable into your cell phone you should get an indication that a headset has been attached. Obviously there’s no hook to put the handset on to end your call; that’s still controlled by your cell phone.
When I tested this setup in public my cell phone was concealed in my pocket. The only thing visible was the coiled headset cord coming out of my pocket and my handset. I was walking through a busy shopping mall and getting the strangest looks from folks wondering how I was using an old fashioned phone handset which appeared to be attached to nothing. At my destination I handed the headset to a friend and said “It’s for you” and he was astonished to hear the mutual friend I was talking to before I entered the shop.