Intercoms are devices for voice communication at close distances, for example around home. In classical intercom systems, voice is carried out in its analog form using wires and this is an example of such system. But contrary to most DIY schemes found on the net, this one deals not only with the utmost simplicity but also with reliability and functionality of the system.
The simplest form of intercom
To most readers, principle of intercom operation is well known. An intercom system consists of a single central unit and at least two remote sets placed at distant locaitons. In homebrew systems, a low power audio frequency amplifier able to drive a small loudspeaker usually takes the role of the central unit. Each remote set consists of a loudspeaker and a microphone so that both users are able to access the central unit and talk to each other, one at a time. More often than not, loudspeakers acts as crude microphones which minimises parts count. To be able to talk to the other user, one of them needs to have his or her loudspeaker connected to the power amplifier input while the other user's loudspeaker stays connected to the amplifier output. To swap their roles in terms of being "transmitting" vs "receiving" party, users change amplifier contacts to which their respective loudspeakers are connected.
This diagram illustrates the basic principle. Unfortunately, simplicity usually has its costs. Although devices such as described above are certainly able to transmit voice over a short distance in theory, those users who actually make them and start exploiting them in practice sooner or later notice several issues:
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It is not clear what might be the best way of implementing user switching. If each user wants to have control of system operation, that requires rather complicated switching schemes with multiple mechanical contacts here and there. Things get extremely cumbersome if three or more users are planed to have access to such democratic system. But if a single switching point method is used, then one user gets absolute privilegies over the others - remember the Big Brother?
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It is very hard to ensure reliable commutation as at least one double-contact double-throw mechanical switch has to be used to route low impedance audio signals around the system. Mechanical contacts are by far the most unreliable components one can even think of using in a device or a system - circuits with multiple switching contacts are guarantied to show severe problems in operation.
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Even while contacts work fine, sound quality is often rather poor. The cause can be traced to extremely low level of audio signals that travel from a remote loudspeaker acting as a microphone to the power amplifier input. Since loudspeakers are optimised for reproducing sounds rather than collecting them, they present themselves as very low sensitivity mikes. This gives noise a fair chance to spoil conversations.
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Central amplifier needs to be powered on all the time. If not, nothing would happen if a user talked into the microphone. So batteries are not suitable for powering such simple systems.
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Users have no easy way of checking whether a particular remote unit is in "Tx" state apart from observing the position of its mechanical switch. If switches have no springs to put them back into the default "Rx" position, some of them might be unintentionally left in the "Tx" position which easily leads to jamming of the whole system. B.B. might actually like this, though...
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Calling another party in order to initiate a conversation is a funny process without a kind of a ringer device built into the system.
Dealing with issues
The proposed intercom system deals with each of the potential problems listed above. Logic behind each solution and actual methods of implementation are described on the next two pages. Here are just a few general hints.
Firstly, mechanical switching is avoided where ever it was possible and practical to do it. For example, all remote sets are permanently connected to the central unit while minimal switching takes place inside each remote unit. Each of the remote units contains an active preamplifier so that signal levels that circulate the system are much stronger than raw signals originating in loudspeakers acting as microphones - this effectively prevents noise from entering the system. Since preamplifiers are designed to have high input impedances, clumsy mechanical contacts by which microphones get connected to them have little chance of degrading the sound quality.
Central unit gets its power supply only if there is a remote set acting as a transmitter. This is done by monitoring small DC currents that remote sets draw when transmitting signals in order to control the functioning of the central unit. If no user talks, no power is drained which makes batteries an excellent choice for powering the system. It is worth noting that a single set of four "AA" size 1,5V batteries placed near the central unit is all that is needed to power the whole system for years.
Despite all user equippment is premanently connected into the system, no set is able to transmit sound signals if its "Talk" switch is in "Rx" position. So no secret spying is possible. In addition, there is a clear indication of whether the system is in use and whether a particular remote set is in "Tx" mode via a pair of bright LEDs in each set. And finally, a clean 5V logic level signal for activating the ringer is provided at the central unit location; this signal can be issued by any user remotely by pressing a "Ring" button so that a dedicated amount of DC current is drawn by the calling console - simple and very reliable.
A detailed description of the circuit diagram is given on the next page while several variants of connecting the system together are given on the last page. It is worth noting that "Rx" and "Tx" marks on all units and signal transmission lines age given with respect to remote units: "Rx" line is a line that carries voice signals to remote sets from the central unit, while "Tx" line is a line that carries signals from them to the central unit. Please note that GND line should never be connected to the actual ground such as plumbing as that would severely degrade audio signal quality - the whole system is best to be left "floating" i.e. without any galvanic contacts with the earth ground.
