Internal Telephone Systems

Internal Telephone Systems Points : internal telephone systems, single-line telephone, inter-com system, manual switchboards with extensions, automatic dial system, float based systems and switches Generally, there are four categories of internal telephone systems used in most of buildings. These categories are:
1. Single-Line Telephone
2. Inter-Com System
3. Manual Switchboards with Extensions
4. Automatic Dial System
5. Float Based Systems and Switches 1. Single-Line Telephone The circuit for simple two-way call and speaking which is suitable for short distances. With the telephone hand set lifted off the cradle and station “A” button pressed, the bell will ring in station “B”. By raising the receiver at “B” both stations can communicate with each other. The battery connections are reversed so that they are connected in series for maximum voltage during the speaking sessions. Lifting the I telephone set and pressing the push of station “B” causes the bell at station “A” to ring. 2. Inter-Com System Inter-corn systems are suitable for 3 to approximately 25 instruments where any instrument or station cart communicate with any other instrument or station. Nearly all instruments are that of push button type, and additional facilities offered are from I to 4 loud- speaking master telephones. The I instruments are fitted with calling buttons in banks of 5, 7, or 1Q. Any extension of ‘the system involves charging the instruments if they have exceeded their capacity.

Intercoms have the facility of immediate connection to any free line. There is no central equipment other than a small power unit or batteries. Multi-core telephone cables link each intercom telephone. The number of cores required being one more than the number of lines required to be operated. Except for the wall mounted type connection is by a rosette or junction box containing the required number of terminals. Figure shows the connection for station inter-corn system. 3. Manual Switchboards with Extensions In manual exchanges, an operator effects the connection between the exchange lines and the extensions. With the switching system in use today, such systems are rare. In such Systems, every call station has excess to each exchange. line on demand. The subscribers switch their extensions on to the required exchange line by a key operation. Internal connections are similarly established by means of assigned call keys. The manual exchanges are suitable for small offices, retail shops, workshops, etc. They can also be connected to existing telephone systems as secondary public exchanges. 4. Automatic Dial System Automatic dial ‘system is implemented in private automatic branch exchange (PABX) systems. The individual call stations are connected each by a twisted pair of wires to the automatic exchange. This is also the termination of exchange lines and if required, the connecting lines to other private branch exchanges. The dialed connections are made automatically. 5. Float Based Systems and Switches Float-based measurement systems are the simplest level transducers. The basic principle states that a floating body experiences two forces: downward force of gravity and an opposing force due to’ its ability to float or buoyancy.

Simple float systems are based on a rigid arm as, and convert the liquid level to an angle which can be measured by an angular position transducer or a float switch. The arrangements of use a vertical moving float and accordingly give a linear output. Uses a counter weight to couple the float to the position transducer. a torque controlled electric motor is used. Other option is a spring-powered take -up drum. In each case, output is a signal proportional to the level. Float-based system that can operate at very high pressures, albeit with a very limited range. A ferromagnetic float is contained in a glass tube similar to a sight glass. The float moves within the coils of linear variable differential transducer (LVDT), the position transducer to give an electrical output dependent on level.

In this application the ‘float” is a fixed sealed tuber connected to the top of the tank via a force transducer. Having no moving parts this arrangement is attractive in dusty and polluted environment. Care must be taken to keep the tuber vertical. The same technique can b used to twist a torque tube whose shear strain can be measured.

Surprisingly float switches can also be used to measured bulk solid level by a method. The technique does not provide a continuous indication of level, but a sampled level of regular intervals. The float is wound to the top of the silo then lowered until the slack wire limit gives a signal to the sequencing logic. The length of the supporting cable paid out then gives the distance between the solid surface and the top of the silo. The depth of the solid can be found by subtraction from the silo height.