Classical scheme of intercom system
Together with doorbells almost every building consisting of more than one floor there is built an intercom system. Currently the market offers advanced digital intercom systems in which electronic occupies a significant part. Each company offers its products and respectively provides diagrams and installation schemes.
Кey elements of which are built a classical intercom installation are carbon microphone (M) electro dynamic speaker (T) and power supply. This is one of the first circuits created and serves more to explain the principle of design of the intercom, but is fully operational.
Upon impact of sound pressure (speech) on the carbon microphone diaphragm (1) , it begins to vibrate In synchrony with these vibrations the clamping force of the carbonaceous grains (2) are changing , thereby changing the electrical resistance between the electrodes (3) and (4). When constant voltage U0 ( in the case 8V) is applied, the current through the microphone also is changing. If the microphone is incorporated into the primary coil of the transformer Tr , then in the ends of the secondary coil an AC voltage occurs , the shape of which corresponds to the sound pressure acting on the diaphragm of the carbon microphone.
The structure of a carbon microphone used in telephone from 70-80’s is given in the figure below:
The main advantage of carbon microphone is its high sensitivity, so it can be used without an amplifier.
The disadvantage is unstable and the presence of noise because the useful signal is formed by the interruption and restoration of contact between carbonaceous grains. Non-uniform frequency response is great. Significant non-linear distortions. These disadvantages now practically exclude the use of carbon microphone in cases, which requires high quality of the sound (broadcasting, sound recording and measurement). Used in telephony and the first intercom circuits.
Alternating current created under the influence of carbon microphone is converted into sound by electrodynamic speaker (T) – phone capsule. On the pictures below are shown: on left – telephone loudspeaker (T) and on right – carbon microphone (M):
The effect of electrodynamic loudspeakers is based on the principle of interaction of constant magnetic field with a movable coil through which a current of low frequency sounds flows. The coil is attached to the membrane that vibrates with the frequency and amplitude of audio signals.
In practice, the conductor is wound on a cylindrical bobbin, forming a voice coil (1). It is arranged in the annular gap of the magnet system (2). The voice coil is firmly attached to the membrane (4), which is supported on centering washer (3). Signal to the coil is fed through flexible leads (5). All components are mounted in the housing of the loudspeaker (6).
Intercom system is implemented with three wires for telephone and two wires for electromagnetic lock with section 0.75 mm ² laid in pipes placed in the stairwell . At the front door put a buttonhole with bells embedded in it a common microphone , telephone and electromagnetic locks . Internal intercoms are placed in the entrance lobby of each apartment andalso on them is mounted the button to activate the electromagnetic lock (BB). Power to the intercom is carried out with a constant voltage of 8 V power supply installed in the main switchboard of the building. Installation for electric lock is supplied with AC voltage of 8 -10V bell transformer or power supply on the intercom . Principle of action between two floors conditionally shown in the figure below is as follows:
етаж 1 = floor 1; вх.врата = main entrance; T- phone capsule; M- microphone 
After ringing the bell of the respective apartment by someone, living in it picked up the intercom in which close contacts AK1-1 and AK1-2. They connect the two microphone M (boutonniere located at the main entrance) and M1 (from suite 1 ) in series – the chain is as follows: positive power supply , M , AK1 -1 , M1, AK1 – 2, negative power supply. Similarly, the two telephone capsule are connected in series by means of the closing contact AK1-2. Microphones themselves appear parallel connect two telephone capsule – a change in their resistance will vary and current flowing in telephone capsules. So when one of conversing persons create sound pressure on corresponding microphone, it will change the resistance of the circuit and from there it will be converted into alternating electrical signal having a frequency range accessible to the human ear in the range of 20Hz – 20kHz. This variable electrical signal will be played from both phones capsule
When there is no change in sound pressure, no change in resistance in the branch of the chain where the two microphones included – respectively and current flowing through the telephone capsules is unaltered and telephone capsule doesn’t reproduce speech.
The following figure represents an intercom system connecting the 3 apartments with the possibility of opening the front door of the building.
