Hello fellow engineers,
I have a project where I'm trying to create a "virtual exchange" for these ancient Ericsson wall mounted telephones.
The virtual exchange is based around a RasPi that can take in the L1 and L2 lines which are output from this phone, and respond to the ringer signal - then using a DPDT relay to connect the L1/L2 lines to the appropriate other phone on the local network so both participants can talk.
The logic of how to know which phone to transfer to will be worked out later (using an AI voice assistant), for now I need to work out how to get the ringer detected (via a opto or relay coil), and then subsequently connect a voice circuit in the virtual exchange to an ADC and into the RasPi, so that the AI voice assistance can "talk" to the caller. I'm ignoring for now the outgoing call the exchange will make to ring the remote phone as well, detecting when that handset is picked up at the remote end and activating the DPDT relay to hook up to the two phones. At the moment, I just want to detect the ring, and switch to a voice circuit in the exchange in a way that is safe for the electronics.
My question is, what is the minimum circuitry that I will need in the virtual exchange box to get the connection working between the phone and the RasPi? The L1/L2 lines carry both the 90VAC ringer, but also a voice circuit which features a DC bias, current modulated (via the carbon granule microphones), voice signal that is put through an impedance matching induction coil so that the signal integrity is maintain across the line over a significant distance. I understand that another coil is wound around the same induction coil, which goes to the receiver earpiece which modulates a earpiece diaphragm, so the caller can listen to voice of the other person on the line - of course the hardware is replicated on both side in a complementary pair. It's fascinating to me that this is full duplex communications over 2 lines.
However, in the receiving phone, when the handset cradle is picked up, it swaps the circuit from the ringer coil (which rings the bells) over to the voice circuit - which has the induction coil, mic pickup and receiver. This enables both parties to talk, and takes the high AC voltage out of the lines which could potentially damage the voice circuit.
I'm having some trouble visualizing how this will work in the RasPi, and need some guidance over how such a circuit can be implemented in the virtual exchange. Can anybody advice how I might approach this? I probably haven't understood everything correctly either, so correct me if I'm wrong.
Here is a schematic that is in the phone:
