POTS Telephone Intercom

Home Intercom Using Wired Telephones

and Existing House Wiring

Highlighted features

  • The intercom function enables the original phone system to work as before.
  • All phones that are connected to the phone line act also as intercom terminals.
  • The phone line is not occupied during intercom use, incoming calls can are "announced". They can be either be answered or ignored.
  • Compatible with both pulse and tone (DTMF) dialling systems.
  • Falls back to normal phone operation if power is removed or the system is 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.

  IMPORTANT WARNING  

Please be aware that the power supply used in this project contains dangerous mains voltage. It is imperative that you adhere to all proper safety procedures and precautions to prevent any injuries or fatalities. It is crucial that you properly secure the mains wiring to eliminate the risk of mains voltage entering the circuit or phone line. Additionally, the intercom must be constructed within a plastic enclosure. If you choose to use a metallic box, please ensure that the protective earth grounding of the box is implemented.

Background

Back in 1996, I contributed to the Elektor Magazine circuit competition with a project that won the 6th prize. This DIY project may not be very appealing nowadays, especially since most households have moved on from POTS (Plain Old Telephone Service) phones. However, it could still come in handy if you still have the old telephone wiring and some spare old phones lying around.


As an additional challenge, can you simplify the design even further while maintaining its functionality, using less than 30 components? That was the limit for the competition, and I think the current 30 components already do a pretty good job of creating something useful. Just keep in mind that the 8-resistor network (DIP-16) counts as one component.


The problem to be solved (back in 1996!)

Many people living in large homes with multiple rooms, floors, and additional spaces like garages, workshops, or basements often install phones throughout their property. This is because it can be inconvenient or difficult to reach a phone from a distance. Fortunately, modern phones are affordable and don't put much strain on the phone line, allowing for multiple phones to be connected to the same line. Although wireless handsets have helped alleviate this problem, many people still prefer not to carry them around. It's worth noting that mobile phones were not as popular in 1996, making it hard to imagine a world without traditional phones in homes.

If you reside in a big house, it's probable that you share it with more than one family member. While having many phones resolves the issue of communicating with the outside world, it doesn't address communication within the house. Even if you just have two phones far from each other, an intercom system could prove to be beneficial.

In the market, there are numerous intercom systems available. Installation of these systems requires additional wiring and intercom units with the existing phone system. To achieve the same range of reach as your phones, multiple intercom terminals are also needed.


The solution

This solution addresses the issue of having two parallel systems by adding an intercom function to the current phone and house wiring setup. This intercom system operates without disrupting regular phone functions. Imagine the convenience of using the pre-installed phones and wiring also as an intercom, allowing you to communicate with your spouse from your electronics lab when he/she is in another room in the house....


Here is the schematic (sorry for the poor quality - will redraw it someday):

Design background

When I designed this circuit in 1996, the competition rules specified a maximum of 30 components (excluding the PCB) and required that the circuit perform practical real-world functions. Additionally, the components used had to be basic and readily available.

This circuit's main components include a HEX Schmitt-trigger IC, a ULN2003 Darlington array, two DPDT relays, an optocoupler, a 12V 3-pin regulator, and a few passives. Although it appears simple, it's impressive how many complex functions it can perform with so few parts.

Before, I had considered an intercom system and even created some sketches. The initial design included more than 100 components, but the circuit diagram was straightforward to understand.


It took me almost four months to perfect the circuit by removing components one by one. I worked tirelessly, redesigning the circuit over and over again until I had a design that only required 37 components while maintaining the original functionality. However, after reading the competition rules more carefully, I learned that a DIP-resistor network consisting of eight resistors could be counted as a single part. By recalculating the values of these resistors and combining them, I was able to solve my problem. Although this made the schematic more difficult to follow, I will do my best to explain it in detail.

I believe that every component in this circuit is necessary, and building it with fever parts is not possible. If you have the time and interest, I challenge you to try and prove me wrong. However, there is one unused Darlington driver in the circuit, but using it to replace any other part seems impossible.


Functional description of the schematic

The power supply for the intercom consists of a mains transformer having a 2 x 15VAC secondary 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. But there is a good reason for it (other than just saving components). The 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 return path is through the filter capacitor C1. Since this is an AC voltage and the DC offset has no relevance (the phones have always AC-connected ringers) - this is fully OK.  The other 15VAC secondary 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 standard ring voltage frequency is usually lower than 50/60Hz, but modern phones ring fine using the mains frequency.  Old phones with large mechanical ringers may experience ringing issues.

R3, R5, C9, D6, and IC4 form a galvanically isolated phone line pulse detector. C6 and R1C act as an integrator suppressing normal pulse-dialling 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 dialling, and provides a rising edge after each positive voltage transient on the phone line, assuming that these transients are more than about 200mS apart.

When the intercom is not in use, the telephone company's phone line is connected to the phone line in the house through a relay. However, once the intercom is activated, the phones are connected to it and the phone company's line is then fully isolated. Nonetheless, the optocoupler still monitors the incoming line for ring signals, always ensuring complete galvanic isolation between the intercom and the phone company. This is a crucial safety requirement that must not be overlooked. I speak from personal experience, but that's a story for another time.


C7, D7, D8, C5, R1D, and R1A form a pulse frequency discriminator that together with the inverter IC2F triggers a set-reset (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 low and 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 are current-limited by R4). This asymmetric AC level is essential for detecting the off-hook condition, as described later. D1 is also essential, it prevents the ringing circuit in the phones to kick back current into the +15V line when the ringing 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 about 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.

Construction

To download the parts list, click on the link provided. Begin by assembling the resistors and diodes, followed by the capacitors (excluding C1), the IC's, and the two relays. The board does not have any jumpers. Install C1 in a normal position or horizontally, but ensure it is close to R1 and R2 if the board will be built into a low-profile box. Always ensure the polarity of all polarized components (including the semiconductors) is double-checked.


Connection and wiring

To set up the intercom, it should be connected between the incoming phone line from the office and the existing phones. First, locate where the phone line enters the house and the wires go to all house phones. Cut this connection. Then, connect the incoming phone line wires to the board's terminal labelled "OFFICE". It's important to pay attention to the polarity to ensure the intercom functions correctly. To determine the negative wire, use a DC voltmeter with a range of 100VDC. Connect the negative wire to pin 1 (minus) and 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 here.


If you do not use a transformer that is assembled to the PCB then connect the two 15VAC secondaries of a suitable mains 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
No adjustments are necessary, therefore there is no setup procedure. Only a functional test is needed.


Useful and important hints


  • This intercom was tested with many different kinds of phones without any issues. Even old-fashion phones (with mechanical bells and pulse-dialing) work OK. Very old (the Bakelite-era phones) that have very power-hungry ringers will load the system too much. Also, the used ringing frequency  (50/60Hz) is 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 fax machine, answering machine, or any other auto-answer equipment, it should be directly connected to the incoming telephone line, rather than through the intercom system. This ensures proper functioning, as such equipment should be able to operate even during intercom use. By connecting these devices as described, they can be used and work as intended. It is even possible to receive or send a fax while using the intercom simultaneously.

  • IIf the intercom ring signals weaken due to multiple phones being connected in parallel, you can increase the 15VAC between terminals 3 and 4 by using the secondary of a dedicated "ring" transformer to feed it, potentially up to 30VAC.

  • You can use wireless handset phones to answer the intercom, but it might be challenging to start an intercom call with certain phone models. This is because it's not always possible to turn the line on and off quickly enough using the wireless handpiece buttons to initiate an intercom call.


Finally, don't forget to savour your lengthy conversations. The best part is that chatting over the intercom is completely free!

Intercom user instructions


  1. To use the intercom and call someone in the house, simply lift the handset and quickly put it back three times within one second, at a frequency of about 2 to 4 Hz. It's like "knocking" on the phone three times with the handset. After that, just 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). If no one in the house answers the intercom call by lifting a handset, the ringing will automatically stop after approximately 25 seconds. If an incoming call arrives while the intercom is ringing, the intercom ringing will stop right away, and the phones will continue to ring as usual to signal the incoming call. This feature ensures that you won't miss any incoming calls.

  3. If the intercom ringing stops before 25 seconds have passed, it means that someone in the house has answered your intercom call by picking up a handset. To start the intercom conversation, pick up your own handset and start the conversation.

  4. When using the intercom, a clear audible tone will signal a possible incoming call on all active handsets. To receive the incoming call, just return all handsets or choose to ignore it and carry on with your ongoing conversation.

  5. TThe intercom will turn off once all handsets are placed back on the hook, and all phones will resume normal operation.

  6. To turn off the intercom completely - just turn off the power to the unit.

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