Headphone to Cell Phone Headset Converter Cable
I built a cable that takes any headphone level output and converts it to a microphone level output suitable for a cellphone headset input. Basically, it allows you to play music or sound from anything with a normal headphone jack through a cell phone. Think of it as holding a cell phone next to a speaker, but slightly less crappy. (Only slightly.)
Essentially all it is is a resistor network to convert headphone outputs (500mV, very approximate) to cell phone mic level inputs (1mV-10mV, very approximate). There are some extra resistors to bridge the stereo headphone output down to mono without frying the output device (unlikely to happen, but possible if you just bridge the two audio channels.) There is another resistor to attempt to match the resistance of a typical electret mic that would be found in a headset. I also added an audio taper pot for fine tuning the levels.
Close up of pot and resistor network:
It's pretty ugly because I mostly don't know what I'm doing.
Originally, the idea was the set up a sort of cell phone radio station. Because the phone thinks the cable is a real headset, it allowed me to turn on the "auto answer" feature so that it answered any received call automatically. With that enabled, and the ipod on loop, I was able to create an on demand dial-up cell phone radio station.
Entire cable/cell phone set up in operation:
Here is a schematic (click for larger version):
A few caveats: the 2.5mm headset plug pinout may differ depending on the cell phone (mic and speaker may be reversed, or the cell phone may use four pins instead of three.) However, I tested this cable on several phones and it worked on all of them.
Also, be sure to put the pot in the correct way. Two of the pins will have about a 5k resistance across them no matter what position the knob is in. These are the two outside connections that go to ground and the bridged inputs from the headphones. If you connect these outside pins backwards, it will still work fine, but the quietest volume setting will be turned all the way to the right, not left, which is generally considered backwards.
First and foremost, anything transmitted over the cell network is heavily, heavily compressed and ends up sounding like absolute crap. So this is basically unusable for a music radio, as per my original plan. I am 99% sure this is the cell network's fault and not the fault of the cable. However, spoken word sounds just as good as speaking in to a cell phone does (which is to say, not very good, but at least vaguely usable.) It also might be possible to preprocess the audio in such a way as to prepare it better for the compression so that it would come out the other side more listenable, for example with a high pass filter. Calling the cell phone from a land line instead of another cell phone also ought to sound a tiny bit better.
All the resistor values are mostly just guesses. The pot allows a fairly wide adjustment, so it works, but the resistors could easily be incorrect and damage something. I was also informed that 22 Ohm resistors for bridging stereo to mono may be too low of a resistance.
Most cell headsets probably use an electret mic (a type of condensor mic), which requires voltage to work, so a cell phone may output a small dc voltage to the mic. This cable does not filter any DC voltage, so it will run through to whatever is plugged into the other side. It is almost certainly way too low of a voltage to cause damage. But, if DC runs to the pot, it should make a loud scratchy noise when you adjust it. However, since in general you will just set it once and leave it, this isn't a huge deal. Also, in my tests, I had no issue with scratchy sounds.
Currently, only one person can be connected to the phone at a time (you know, like normal phone calls) so this is impractical for anything requiring multiple simultaneous users.
Dial-up, on demand radio or any other audio information. You could connect this to a laptop and stream any kind of audio, such as weather reports, stock prices, news, whatever, and then get access to that information from anywhere in the country/world from any phone. This would especially useful for reporting data from a local physical data source, like the temperature of a specific location from a local sensor, since the equipment is all quite portable.
You could hook a particularly high powered mic/preamp up to the input and then hide the whole mess in a corner of a room. 24 hour realtime eavesdropping/audio surveillance that you can dial up and listen in to from any phone. Except the cell phone compression with probably make things difficult.
It would also be extremely easy to add a headphone jack on to this passed through from the cell phone, so that you could communicate in both directions. You could then have full conversations with other devices, for example a computer set up with custom voicemail recording software, or a computer set up with text to speech/speech to text for mute or deaf people. You could set up voice recognition software to allow remote control of things. In the radio station example, you could have it recognize voice commands (or easier, touchtone input!) to skip to the next song.
Here is an audio example recorded from a cellphone connected to another cellphone hooked to the cable hooked to an ipod. You can clearly hear the resulting audio quality.
"Devil Devine" by Dudestack