My take on the spare phone intercom

There are numerous tutorials online outlining methods of using landline telephones to make simple intercom systems.  While all that's really necessary to provide a talk current is a 9v battery and a resistor, ringing or otherwise signaling a call prompt to a remote station is often beyond the scope of simple designs.  About a decade ago, I put together an intercom system between the shop and the house; since I had to do some service to it recently, I figured I'll write a thing about it. 

The design is similar to the second circuit described here.  Picking up one phone causes the buzzer at the opposite station to sound.  Once both phones are off-hook, the buzzer stops.  My design changes are based around a lower voltage power supply, demonstrating that the design is certainly flexible.  The station phones can be any sort of landline phone: cordless, electronic, or just an old dumb rotary made 80 years ago.  Use twisted pair for the link cable.  Length is not likely limited in your application.  I'm using about 200 yards of cable between stations.  Any attenuation can be compensated by adjusting R3 to compensate (trim to about 25mA off-hook).

The buzzers are self-oscillating piezo buzzers.  The ones I'm using begin oscillating at about 1.5v, and as with many buzzers of this type, they can be tuned by placing a reflector of some sort at an adjustable distance in front of the aperture (I just used a tab of thin aluminum sheet).  This improves buzzer volumes at low voltages.  With higher supply voltages (24v), this may be less necessary.

Resistors R2 and R4 are used to reduce sensitivity to leakage currents or induced voltage in a long or heavily weathered link cable.  Without these bleeder resistors, a waterlogged outdoor cable or a long run of untwisted cable may result in the local station occasionally beeping weakly when both phones are on-hook.  To see why, note that the two station networks form a series voltage divider.  Any shunt conductance in the line tends to bias this voltage divider such that the local station is closer to its zener voltage than the remote station.  Since a telephone is essentially open circuit when on-hook, the divider network is very sensitive to even small shunt conductances.  Adding bleeder resistors diminishes the influence of such system defects.  For short indoor runs, they are likely unnecessary.

Switches S1 and S2 allow the station phones to function as regular landline phones.  Leaving the phone switched to the phone line does not prevent an intercom call prompt from being received.  This connection can be omitted if no landline service is desired.

Capacitors C1 and C3 allow the remote buzzer tone to be heard from the calling station. C2 is for supply bypass.  The LED can be omitted if undesired.  Use whatever power supply is handy; talk current is only 20-30 mA.  I simply used the power supply that was already in the equipment I stripped for the enclosure.

The local station in a repurposed QINGEN enclosure (and a 1941 Uniphone)

Again, this article is not unique.  Perhaps the one thing I can offer is the fact that this as-built design does function over long distances, with various phones, and for over a decade.  That's more than a 9v battery design will provide.