Arduino Whole House Telephone Intercom

Design Considerations

Telephone System Background:  Old-fashioned telephone infrastructure was elegantly simple.  It got a lot of performance from a simple 2-wire line.  Modern features have been layered atop the old infrastructure, but the basics of POTS (Plain Old Telephone Service) are unchanged.

While most modern phones use household electrical power, earlier designs operate entirely from Telco-supplied power.  We keep a couple of older phones connected to be sure we will still have telephone service when household power fails.

The telephone company usually applies about 50 VDC, either polarity, through 1 or 2 thousand ohms resistance.  In olden times, the voltage came from a 48 V "talk battery".  A telephone hook operates a switch, commonly called a hook switch.  When the phone is taken off hook, the hook switch closes.  Phone company current then flows from the  phone company voltage source, through phone company resistance, and then through the telephone earpiece and mouthpiece.  The phone company detects current flow, and modulates the current with a dial tone or voice sounds.  The modulation is heard in the earpiece.  If  sound is present, the mouthpiece modulates the current passing through it in proportion to sound pressure.  Thus, sound at the mouthpiece is simultaneously heard in the earpiece.  Telephone instrument resistance is a few hundred ohms.  Telephone company voltage and resistance primarily determine the amount of current flow, known as "loop current".  Loop current is usually around 20 - 50 mA.  When you are connected to a remote phone, the current modulation generated by your phone is applied to the current flowing through the remote phone, and vice versa.  Thus, both earpieces reproduce sound generated at either end.

When a telephone is on hook, the hook switch is open so direct current cannot flow through the earpiece and mouthpiece.  However, the phone's ringer remains connected through a capacitor of about 0.5 uF.  The telephone company superimposes an AC "ringing voltage" on the phone line to activate the ringer.  Ringing voltage in the U.S. is typically a sinusoid of about 75 V rms at 20 Hz.  All of these parameters are local to the U.S., and may vary in other countries.  Some U.S. phone companies may use different standards.

I have heard ringing voltage was once generated by rapidly reversing the polarity of the talk battery.  This would have produced a square wave of about 96 V peak-to-peak.  Notice that a 75 V rms sinusoid is about 210 V peak-to-peak.  One researcher reports that a "standard ringer," that of a Bell model 2500 phone, draws 8-9 mA AC when ringing.

For our purposes, we need to know that modern DSL service rides on high frequency carriers superimposed on the phone line.  Thus, high frequencies should always be coupled to the Local line, independent of intercom mode.

It is also important to know that on hook flashes activate telephone company features in some areas.  Our phone company uses on hook flashes to transfer between a current and waiting call, or to establish a three-way call.

Ringing Voltage:  I once thought about 90 V peak-to-peak square wave through 3.6K ohms would be fine for ringing all our phones.  I thought wrong.  While it seems to work for ringing one phone, additional phones on the line draw additional current and reduce the voltage seen by each phone.  In our case, I found that only two electronic phones rang at all, and neither of the "real" phones.  The two that did ring did not follow the intended ringing cadence, though it was easy to distinguish the intercom ring from the Telco ring.

I understand Telco ringing voltage is sized to drive 5 standard ringers (REN = 5).  I recently counted 11 instruments on our line.  Fortunately, most modern electronic phones draw very little ringing current.  Many are labeled with Ringer Equivalency Numbers (REN) of 0.0.

The present design theoretically delivers less than 5 ringer equivalencies, but seems to work OK for us.  We are aided by household power toward the high end of the 105 - 125 V. range.  Altogether, our phones present a total load of about 4 REN.  If you have problem, you will have to either beef up the ringing supply, or remove high REN phones from your line.  I expect the present design will drive a couple of REN even if household power is at the low end.  However, I have not tested this.

In retrospect, a step-up switching supply, targeted for 210 V peak-to-peak ringing voltage and a more standard 50 VDC for talk, would be a better design approach.  It could draw power from an isolated "wall wart" supply, eliminating the isolation transformer.  The present design evolved to achieve sufficient ringing amplitude while avoiding excessive power dissipation while supplying talk current.  It is a compromise solution that works, but I consider it less than ideal in both respects.

Ground Isolation:  Household power and the telephone line may reference considerably different "ground" levels, typically with a lot of 60 Hz "hum" between them.  My design arbitrarily considers one side of the telephone line to be "ground".  An isolation transformer should therefore drive the intercom power supply.  Omit it at your peril.