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.
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.
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.
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):
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.
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;
The SR flip-flop can be reset because of three reasons;
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.
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.
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.
Finally, don't forget to savour your lengthy conversations. The best part is that chatting over the intercom is completely free!