Once again, I'm here posting for help on a strange idea.
I have an old Nokia bagphone, C15 handset, which uses the C250-MKII
transceiver unit. I use this phone both in the car, and on the
sailboat, and simply put, 3 watts of power can't be beat! It also has
the advantage of locating the antenna unit away from the user.
However, I have one problem with the phone. I don't have the handsfree
I know there was a handsfree conversion kit for the bagphone, and from
what I can gather, this involved a "craddle unit" which was plugged
into the RJxx connection coming out of the handset and had a connection
of the same nature which went to the transceiver, and a lavalier
microphone with a 3.5mm male miniplug on it (I assume this plugged into
the craddle). Having taken the handset apart, I found the following;
that the mic output seems to go directly into the handset cable. That
the speaker goes through a bunch of resistors on the board, and then (I
assume) to the coiled handset cable--I had some trouble tracing this
circuit around the board. That there is a magnetic reed switch on the
board, which was actuated by a magnet in the craddle unit. This would
have told the phone that it was "hung up" and that it should use the
external speaker instead of the internal one. Picking up the handset
would probably have switched out of handsfree mode, and into "privacy
handset mode." A software setting configured the phone for "H/F
Present" or "H/F Not Present."
So, it would seem to me that by splicing two 3.5mm jacks into the
thing, one for the mic, and one for the speaker, and by using jacks
such as those used in computers, which shutoff the internal speaker
when headphones/external speakers are connected, I could implement the
same thing as the craddle unit used to. And by simply putting a small
magnet on the handset, I could activate the reed switch--making the
unit think it is on a craddle, which also seems to change it over to
DTMF tones via speaker, instead of simple key press beeps via ringer.
In my case, I plan to use a casette adapter for the "speaker" unit.
I don't care about not having a "privacy handset mode." Nor do I care
that this isn't exactly a "new" phone. I want handsfree, I don't need
"voice activated" dialing, or whatever. I can pull over to dial, and I
usually pull over to talk too.
However, before I get too far into this, I would like to confirm theory
and wire tracing with actual pin assignment information for the handset
cable, and impedance information too (if possible). Does anyone have
this information, or know of any source besides Nokia?
Thanks for any suggestions/help. I know this idea is not terribly far
fetched, it is just a question of which wires to use.