What are the steps required to join the microphone and audio lines from a typical cell phone headset into one line so that the audio can be recorded by a standard voice recorder using the microphone and/or line-in input?
Practical use
The practical use for this is for the purpose of recording cell phone conversations with the use of a headset.
In Depth
Intuitively, I would just splice all the lines together. However, this approach seems to have the problem of creating an echo from the mic line to the audio line. I've also heard that it could blow some circuits in the phone, audio recorder or both.
It seems that the job of "splicing: these lines together without interfering with the call is more complex, requiring the use of some components to make it work correctly.
Here is a device that I found on Ebay that does exactly this:
I purchased a similar device off Ebay that has its own headset attached and cracked it open:
Here is a close up of the cracked open part. It does not look very complex, but I'm not sure exactly what it is doing:
(Other side:)
There may be more on the other side of the board. At a guess it's a single transistor similar to Fairchild SemiconductorFDMC6688P preamp with some filters around it.
