Workshop to House Telephone.  An intercom using two telephones.

Power Supply.
A range of supply voltages are needed, 50V for the speech circuit, around 75V AC to ring the telephone bell, 12V for the relays and 5V for the ring tone generator circuit.
The mains transformer chosen had windings giving 2x 26V and 2x 9V at suitable current ratings and was small enough for pcb mounting as I wanted all the components to fit on one pcb.

The 9V windings feed a full wave rectifier, giving around 13V, this is used to operate the relays in the line interface section, it also goes to supply the 5V regulator that feeds the ring tone generator circuit.

The 26V windings are placed in series, half wave rectified and "stood" on top of the 13 V supply rail to "Jack" up these supplies to a higher level, the resulting outputs are, around 100V used to ring the bell in the telephone being called and 55V for the speech circuit, the speech supply should be hum free so an active smoothing filter is used to give a clean DC supply.

Bell Ring Generator.
To ring the bell in the old telephones around 75V RMS is needed, I did some testing and found that a 100V square wave signal gave just as good sounding bell ring as a sine wave and was easier to generate, leading to a design that used a few transistors switching fully on and off with little power dissipation in them.

I wanted the bell ring to sound like the real GPO/BT ring, so a little bit of research provided the required timings, after some testing with various IC's a convenient counter, the CD4060BE was chosen, it has an internal oscillator saving a few components that would be otherwise needed.

It was possible to pick of the bell ring signal at 25Hz and a 400Hz tone for use as ringing confidence signal along with the slower, ring ring........ring ring cadence signal to give that old world GPO sound.

Selected outputs from the 4060 counter are gated together in a triple input NAND gate then used to drive the bell ringing switching transistors, giving a 25Hz 100V square wave, current limiting is included in the output stage for protection against accidental short circuits on the line.

Another NAND gate is used drive a push pull emitter follower stage for the 400Hz ringing confidence tone of approx 4V PP, this is provided so that the person making the call gets to know that something
is happening on the telephone at the other end.

Line Interface.
The final part of this mini exchange has the task of detecting the lifting of a handset, connecting the telephone being called to the bell ring signal and latching the answering of the call.
I chose to do this with relays, although transistors could have been used, relays are immune to spikes, interference and accidental short circuits.

Each telephone is connected to the 50V speech supply via the coil of a 24V "Off Hook" detect relay
and a couple of resistors and to the OV line by some contacts of another relay.
These contacts are used to insert the bell ring signal (100V PP) and the confidence tone (4V PP) to
each telephone, depending on which is making the call.

When the called telephone is answered, the "Answer" relay gets latched, remaining in the latched
condition until both telephones are back on their rests, ready for the next request for tea and biscuits!.

This telephone intercom is installed in a workshop in Swindon and is in constant use.

The Telephones used with this project were GPO types 300 and 246, both without a dial.

Have a look at this site for more about old Telephones... Telephones Pre 1960.