This project was my contribution to a project competition in Elektor Magazine in 1996. This idea won the 6th price. This DIY project is not anymore that relevant when everyone has gone wireless - even at homes. Nevertheless, this idea might be useful for someone, somewhere. So I decided to publish it. I want also to present a small challenge! Can you further simplify this design and keep the same functionality with less than the here 30 used components? The resistor network (DIP-16) is counted as one component...
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 form 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, and 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.
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):
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 from 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!
The competition for which I originally designed this circuit required that the maximum parts count was 30 components, the PCB excluded. The circuit should also fulfill some real-world useful function. It had also to use 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 regulator and some passives. Very plain...
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 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 in 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 the first look. There is a good reason for it (also other than saving components). This way the secondaries can be connected in series and an AC ring-voltage can be generated with high enough amplitude. The ring voltage (current) return path is actually though the filter capacitor C1. Since this is an AC current and a DC-offset has no relevance (the phones have AC-connected ringers) this is fully OK. The other 15VAC winding is in series with the first one produces 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.
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 it's not activated. When activated, the phones are connected to the intercom and the phone company's line is left idle. However, it 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!
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 flipflop'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.
Click HERE 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.
The intercom must be connected to BETWEEN the incoming phone line (from the office) and the existing phones. Locate the place where the phone line enters the house. Connect the incoming phone line 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 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.
Last but not least, enjoy your even long conversations. Best of all - having a chat over the intercom costs nothing!!
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