Telephone Intercom

Home Intercom Using Wired Telephones

and Existing House Wiring

Highlighted features

  • The intercom function enables the phone system to work as before.
  • All phones that are connected in parallel act also as intercom terminals.
  • The phone line is not occupied during intercom use, incoming calls are "announced". They can be answered or ignored.
  • Compatible with both pulse and tone (DTMF) dialing systems.
  • Falls back to normal phone operation if power is removed or system turned off.
  • The system is 100% galvanically isolated from the phone-office line.
  • Uses only 30 easily available low-cost components.
  • Low power consumption.
  • No adjustments.


The power supply in this project carries lethal mains voltage. I cannot be kept responsible for any injuries or deaths caused by ignoring proper safety procedures or precautions. You must secure the mains wiring properly so there is no risk of the mains voltage entering the circuit - or the phone line. The intercom must be built in a plastic enclosure. If you decide to use a metallic box, protective earth grounding of the box is essential!


This project was my contribution to a project competition in Elektor Magazine in 1996. This idea won the 6th prize. This DIY project is probably not anymore that relevant when everyone has gone wireless - even at homes. Nevertheless, this idea might be useful for someone, somewhere. There is a chance that you still have the old telephone wiring intact in your house and some old telephones lying around somewhere...

Additionally - I want to present a small challenge! Can you further simplify this design and keep the same functionality - using less than the 30 used components? Let me know! Please note that the resistor network (DIP-16) is already counted as one component...

The problem to be solved (back in 1996!)

Many people who are fortunate to live in big houses, with either plenty of rooms or many floors (and maybe a garage, workshop or basement) - can find themselves installing phones all over the place. The reason is simple; no one wants to run - nor can hear a phone - far away. Since phones are cheap and load only lightly the office line, it is quite common to connect several phones to the same line. Wireless home phones have brought some kind of relief to this problem, but nobody likes to carry them around. [Editors note: mobile phones were at their infancy stages, it was hard to believe that POTS phones will one day completely disappear from homes].

If you live in such a house, it is most likely that there is more than one person in your family. The high number of phones only solves the problem of communication with the external world, not with people WITHIN the house. However, even if you only have two phones but they are far apart, an intercom could be useful.

There is on the market a myriad of different intercom systems. These must be installed parallel with the existing phone system and require their own wiring. To cover the same range of reach - as you do with your phones - also requires as many intercom terminals.

The solution

This design solves the problem (of having two parallel systems) by adding to the existing phones and house wiring an intercom function! The described intercom system is fully transparent, not affecting in any way the normal operation of the phones. Think about the beauty to be able to use the already installed existing phones and wiring as an intercom system, and for example talk (not walk) to your wife when you are in your electronics lab and she needs to talk to you from somewhere in the house...

Here is the schematic (sorry for the quality - will redraw it some day):

Design background

The competition for which I originally designed this circuit required that the maximum parts count was 30 components, the PCB excluded. The circuits should also fulfill some real-world useful tasks. It had also to utilize plain vanilla components.

The heart of this circuit is one HEX Schmitt-trigger IC, and one ULN2003 Darlington array. Two DPDT relays, one optocoupler, a 12V 3-pin regulator, and some passives. Very plain indeed. But quite impressive what a complex function you can achieve with so few parts.

I had earlier thought of an intercom system and had drawn some sketches. The design contained originally over 100 components, and the circuit diagram was easy to follow.

I spent almost 4 months honing and fine-tuning the circuit, removing components - one by one. After many sleepless nights, I ended up with a design using only 37 components - without losing the original functionality. I must have redesigned everything hundreds of times. Then I read the competition rules more thoroughly - a DIP-resistor network (with 8 resistors) could be counted as one part! If I could recalculate 8 resistors to share the same value, then my problem would be solved. And so it was! As a downside, the schematic is now quite hard to follow. Therefore I try here to explain it in some detail.

I dare to claim that there is not a single component that can be removed from this circuit without making it dysfunctional. I challenge you to try - if you have the time and interest. There is actually one Darlington driver free that I did not find any use for...

Functional description of the schematic
The power supply for the intercom consists of a mains transformer having a 2 x 15VAC output at a minimum of 3.5 VA per winding. Using a ready-made AC/DC PSU was not an option since I needed an AC voltage of 30VAC (for making the phones ring) - at reasonable power and low frequency. One of the secondaries together with D3, C1, and regulator IC1 forms a regulated +12VDC supply for the electronics and also doubles as a hum-less DC-bias voltage for the phones during intercom use.

The arrangement (half-wave rectification only) is a little awkward at first look. There is a good reason for it (also other than saving components). This arrangement makes it possible to connect the secondaries in series and an AC ring-voltage can be generated with a high enough amplitude. The ring voltage (or actually current) return path is through the filter capacitor C1. Since this is an AC current and a DC offset has no relevance (the phones have always AC-connected ringers) - this is fully OK.  The other 15VAC winding is in series with the first one producing a  30...40VAC ring-voltage for the phones. Note that these small transformers tend to have quite high idle voltages, which is of benefit here. The normal ring voltage frequency is usually lower than 50Hz, but only the very oldest phones (with massive mechanical ringers) can have a ringing problem.

R3, R5, C9, D6, and IC4 form a galvanically isolated phone line pulse detector. C6 and R1C act as an integrator suppressing pulse-dialing pulses and ring signals, or pulses on the phone line with a higher frequency than about 5Hz. Therefore the output of IC2C is low only during incoming ring signals or pulse dialing, and provides a rising edge after each positive voltage transient on the phone line, assuming that these transients are more than about 200mS apart.

The telephone company's phone line is connected to the phone line in the house via the relay - when the interceom is not activated. When activated, the phones are connected to the intercom and the phone company's line is left idle. However, the incoming line is monitored for ring signals by the optocoupler. This provides complete galvanic isolation between the intercom and the phone company - this is a safety requirement that cannot be ignored! I speak from experience, but that's another story.

C7, D7, D8, C5, R1D, and R1A form a pulse frequency discriminator that together with the inverter IC2F triggers a set-reset (or SR) flip-flop, consisting of IC2D and IC3A. This is achieved by pulling momentarily the input of IC2D low through R2 when the pulse frequency from IC2C is higher than about 1.5 Hz. The flip-flop's state determines if the intercom is active or not.

When the SR flip-flop is triggered (set) three things happen;

      1. RE1 is energized through driver IC3D, disabling immediately all phones from the telephone office, and connecting DC power (voltage) to all phones connected.

      2. D2 is no anymore supplying +12V to C2 so it starts slowly discharging through R1B. After about 25 seconds the input to IC2A is below the lower threshold so that its output goes high pulling the output of low resetting the SR flip-flop. C2 is charged fast through R2 and D2. The input of gate IC2A is protected with the help of R1E and R1F, this is essential because of the high capacitance of C2. C2 also acts as the power-on reset capacitor, ensuring that the intercom system stays initially OFF when power is applied to it.

      3. D4 is no anymore supplying +12V to the low-frequency oscillator consisting of C3 R1G and IC2E. After a short delay, the oscillators start at about 1 Hz, pulsing the relay RE2 through buffer IC3B. This relay is the intercom "ring" relay, supplying the phones alternatively with +15VDC, or 30VAC riding on a +22VDC level (both current limited by R4). This asymmetric AC level is essential for detecting the off-hook condition, as described later. D1 prevents the ringing circuit in the phones to kick back current into the +15V line when the ring relay turns off.

The SR flip-flop can be reset because of three reasons;

      1. If a ring signal is detected on the office line, or if there is a >10V DC transient on the incoming phone line. Whenever the IC2C goes low, then  the 300 Hz oscillator consisting of C4, R1H and IC2B starts, and the first pulse turns off the SR flip-flop with the help of IC3G.

      2. After a 25 second time delay, as described above.

      3. If a handset is lifted when the RE1 is energized (SR flip-flop is set). Whenever RE1 is energized the phones are fed with a voltage with an average DC voltage that is higher than 10V. Since phones are AC-coupled internally as long as the handset(s) is on-hook, the average signal across the shunt-resistor R4 is zero, even if there is an AC-component across it during the ring signal (ring current to all phones). This AC is effectively filtered by R7 and C8, but at the moment a handset is lifted, the DC current through R4 is high enough to turn on drivers IC3E and IC3F. IC3E resets the SR flip-flop and IC3F continues to keep RE1 energized until all phones are again with the handsets back on-hook.

The 300Hz oscillator (IC2B) signal is also coupled through R6 to R4. Therefore, if there is a ring signal on the office line, this tone can be heard in the handsets during intercom use. The purpose of R4 is also to limit the DC-current through the phones to a suitable level.


Click HERE XXXXX to download the parts list. Start with assembling all resistors and diodes, next do the capacitors (except for C1), the IC's and the two relays. The board has no jumpers. Mount C1 normally, or horizontally but close above R1 and R2 if the board is going to be built into a low-profile box. As always, double-check the polarity on all polarized components.

Connection and wiring

The intercom must be connected BETWEEN the incoming phone line (from the office) and the existing phones. Locate the place where the phone line enters the house and wired further to the house phones. Cut this connection. Connect the incoming phone line two wires to the board to the terminal with the text "OFFICE". To guarantee proper operation the polarity is important. Find by measuring with a DC-voltmeter (range 100VDC) the negative wire and connect it to pin 1 (minus). Connect the positive wire to pin 2 (plus).

Connect the 2-wire cable that goes to the phones to the terminal "PHONES". The polarity is not important.

Connect the two 15VAC secondaries of a suitable mains adaptor (or transformer) to the terminal "POWER", the other secondary to terminals 1 and 2, the other to terminals 3 and 4. The proper phase is important so that the voltage on the secondaries adds. The plus and minus signs indicate the phase relationship. The voltage between pins 1 and 4 on the "POWER" terminal should be about 30VAC. If the reading is < 5VAC swap the wires in either secondary.

Setup and testing
There are no adjustments so there's no setup procedure. Just a functional test is required.

Useful and important hints

  • This intercom is tested with many different kinds of phones without any issues. Even old-fashion phones (with mechanical bell and pulse-dialing) work OK. Only very old (the bakelite-era phones) have very power-hungry ringers and will load the system too much. Also, the ringing frequency used (50Hz) might be too high for their massive ringers.

  • This system might not be compatible with telephone office systems that produce dual ringing signals (two fast rings and a pause, then repeating). However, this was never tested.

  • If you have a telefax or answering machine (or any other auto-answer equipment) there must be connected directly to the incoming telephone line, not via the intercom system. This ensures correct behavior since such equipment might answer also during intercom rings. That's certainly something you don't want to happen. These devices can be used and work normally if connected as described above. A fax could be received or sent during simultaneous use of the intercom.

  • If the intercom ring signals become weak with an increasing number of parallel-connected phones, the 15VAC between terminals 3 and 4 is could be increased up to 30VAC by feeding it from another dedicated "ring" transformer's secondary.

  • Wireless (walkie-talkie) phones can also be to answer the intercom, but it can be difficult to initiate an intercom call with some of these phone models. It is simply not possible to turn on and off (on off hook) the line fast enough from the wireless handpiece buttons to initiate an intecom call.

Last but not least, enjoy your even long conversations. Best of all - having a chat over the intercom costs nothing!!

Intercom user instructions

  1. To initiate the intercom and call another person in the house; lift and put back the handset quickly about 3 times during one second (2..4 Hz on/off-hook frequency). Like knocking the phone 3 times but holding the handset in your hand. Leave the handset in its holder.

  2. All phones will now start ringing using a fast, specific repeated ringing pattern (different from the phone company's ringing cadence). The ringing will stop automatically after about 25 seconds - if nobody in the house lifts a handset - to answer the intercom call. If there would be an incoming call during this ringing time - then the intercom ringing will stop immediately and the phones will continue to ring normally - to indicate the incoming call. With this functionality, you cannot miss any incoming calls. 

  3. If the intercom ringing stops (earlier than 25 seconds from the initiation) it indicates that somebody has answered your intercom call by lifting a handset somewhere in the house. Now lift also your handset and start the intercom conversation with that person.

  4. A possible incoming call is signaled (during the use of the intercom) by an clear audible tone in the handset speakers. Simply put back all handsets to accept the incoming call, or continue your conversation and ignore the call - and pretend you are not at home.

  5. The intercom function turns off when all handsets are returned back on-hook, and the phone operation switches back to normal.

  6. To turn off the intercom function completely - just turn off the power to the unit. Your telephones will always work normally when the intercom is not in use (either not activated - or powered off).

© Copyright 2021 - Dextrel / Studio Kingshill