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EXTENSION TELEPHONE SWITCHER WITH INDICATOR

INTRODUCTION
Can you imagine how many telephones there are in the world? In the Philippines alone, despite the fact that some of this phones often conk out, there are millions of subscribers. It is indeed very amazing that even with this huge number of telephone subscribers, your own telephone set, representing one telephone station, could be connected to other telephone stations in a matter of minutes. However, this is not a mystery. Having multiple extension telephones is undeniably very convenient, one phone can be in the living room, another in the kitchen, and one could be in the bedroom. One can make or receive calls practically anywhere in the house. However, having multiple phones also increases the risk of other people listening in on an extension.
STATEMENT OF THE PROBLEM
Just imagine, one maybe having a personal conversation without knowing that somebody else is eaves dropping on an extension phone. Of course, this intrusion of privacy happens a lot. It is disgusting, but quite consistent within secure people. This people usually listen to other people's conversation as well as go over their things just to satisfy their need to be godlike, know all and be all.
SIGNIFICANCE OF THE STUDY
This study is significant not only to the families that can afford but to the whole community as well as a to the nation. The study is primarily intended for private conversation. The circuit disable other extension phones connected to the line whenever the master or the extension phone is in use. Although simple and inexpensive, this circuit can guarantee the user that no one will be listening in an extension.
SCOPE AND DELIMITATIONS
The scope of this project study involves only in the conversation privacy of two persons talking in a certain call. This will serves as the security of the confidential conversation of two persons.
REVIEW OF RELATED LITERATURE
Electronics have literally invaded offices and factories across nation. Its main frame is a work reducing the human or physical effort in doing things and the comfort it provides. It helps us in doing jobs that are too tedious with more accuracy and sophistication. The most contribution of electronic in communication is the telephone, which was invented by Alexander Graham Bell in 1874. Telephone can help communicate even at a distance beyond the sensitivity of human ear. A man born from Edinburgh, Scotland in 1847 unlock the history of telecom- munications. His name is Alexander Graham Bell who once dreamed he could send several telegraph messages at once by varying their musical pitch. On June 2, 1875, Bell and Watson were testing the harmonic telegraph when Bell heard a sound come through the receiver. Instead of transmitting a pulse, which it had refused to do in any case, the telegraph passed on the sound of Mr. Watson plucking a tuned spring, one of many set at different pitches. How could that be? Their telegraph, like all others, turned current on and off. But in this instance, a contact screw was set too tightly, allowing current to run continuously, the essential element needed to transmit speech. Bell realized what happened and had Watson build a telephone the next day based on this discovery. The Gallows telephone, so called for its distinctive frame, substituted a diaphragm for the spring. Yet it didn't work. A few odd sounds were transmitted, yet nothing more. No speech. Disheartened, tired, and running out of funds, Bell's experimenting slowed through the remainder of 1871 At this point telephone history become progress came slowly after the original invention. Bell and Watson worked constantly on improving the telephone's range.They made their longest call to date on October 9, 1876. It was a distance of only two miles, but they were so overjoyed that later that night they celebrated, doing so much began dancing that their landlady threatened to throw them out. Watson later recalled "Bell . . . had a habit of celebrating by what he called a war dance and I had got so exposed at it that I could do it quite as well as he could." The rest of 1876, though, was difficult for Bell and his backers. At this point telephone history becomes fragmented and hard to follow. Four different but related stories begin: (1) the further history of the telephone instrument and all its parts, (2) the history of the telephone business, (3) the history of telephone related technology and (4) the history of the telephone system. Due tolimited space I can cover only some major North American events. Of these, the two most important developments were the invention of the vacuum tube and the transistor; today's telephone system could not have been built without them. Progress came slowly after the original invention. Bell and Watson worked constantly on improving the telephone's range. They made their longest call to date on October 9, 1876. It was a distance of only two miles, but they were so overjoyed that later that night they celebrated and to invent the automatic switch are odder still. At this time we should look at Strowger's achievement, before continuing our narrative. The automatic dial system, after all, changed telephony forever. Almon Brown Strowger was born in 1839 in Penfield, New York.. Like Bell, Strowger was not a professional inventor, but a man with a keen interest in things mechanical. This unlikely profession of an inventor so inspired seems odd indeed, but the stories surrounding his motivation In 1893 the first central office exchange with a common battery for talking and signaling began operating in Lexington, Massachusetts. This common battery arrangement provided electricity to all telephones controlled by the central office In 1897 Milo Gifford Kellogg founded the Kellogg Switchboard and Supply Company near Chicago. In 1906 Lee De Forest invented the electron tube. Its amplifying properties led the way to national phone service. Long distance service was still limited. Loading coils helped to a point but no further. Transcontinental phone traffic wasn't possible, consequently, a national network was beyond reach In 1921 the Bell System introduced the first commercial panel switch, an odd best if there ever was one. Developed over eight years, it was AT&T's response to the step by step switch so favored by the independents. It offered many innovations and many problems. On July 1, 1948 the Bell System unveiled the transistor, a joint invention of Bell Laboratories scientists William Shockley, John Bardeen, and Walter Brattain. It would revolutionize every aspect of the telephone industry and all of communication
GENERAL DETAILS OF TELEPHONE LINE
Plain old telephone systems (POTS) telephone line consist of one wire pair which carries full duplex audio and the operating current for the telephone. The telephone connected to line is powered from current limited 48V power source, so phones on-hook, should measure around 48 volts DC. Practically the operating voltages of telephone systems can vary from 24V to 60V depending on the application, although 48V nominal voltage is the most commonly used. Telephone applications often require and use positive grounding in the central office, where the positive conductor of the 48V power supply is connected to earth ground. The telecommunication industry began the positive ground convention in the 1940s and many telecommunication companies still employ the traditionally positive grounded system.This means that generally when telephone is on-hook, one telephone line wire is quite near to the ground potential and other one carries -48V. When telephone is put off-hook the voltage between wires going to telephone drops down to the 3 to 9 volt range and typically a current of 20-60mA will flow through the telephone. The remaining voltage drop occurs over the copper wire path and in the telephone central electronics. Typical telephone line bandwidth is around 200-3200 Hz, which was what phone companies decided years ago be sufficient for speech intelligibility while allowing them to multiplex many calls over coax and twisted pair. The low end is rolled off early to stay away from the mains frequency (50 or 60 Hz) interference. The high end cut off is caused by the telephone transmission system (nowadays the audio is digitized at 8 kHz). The typical signal to noise ratio of a telephone line is approximately 45 dB or somewhat less. The average signal levels on telephone line is -9dBm average speech.The voice on a tip/ring pair is full duplex balanced audio which requires a two wire to four wire hybrid circuit or transformer to convert it into separate transmit and receive audio paths. Bulky and expensive hybrid transformers have been replaced in most telephones by ICs which perform the same function. In a telephone, the biggest contributor to poor audio quality is the handset microphone (it has be cheap and withstand very hard use). Telephone standards world is fragmented. Typically each country has its own standards because of both the historical roots of the phone service and the desire to protect the local phone market from outside competition. Basically the telephone systems work in the same way in different countries, but there are some differences which can mean that a devices designed for one country does not meet the regulations of other country and work poorly or not at all. The differences in local technical standards range from minor to severe and affect many of the signaling conditions on local loops. The most major differences are different wiring practices and connectors, different line impedance, different nominal loop currents, different signaling tones and different electrical safety regulations. Unfortunately nowadays many countries are harmonizing many standards across their boundaries so it is nowadays possible to design devices which work well and meet the regulations in more than one country at the time. For example in Europe the European Commission adopted CTR 21 standard covers non-voice equipment (for example FAX and MODEM) in more than 20 countries.
CIRCUIT DESCRIPTION
The circuit does not require any power supply. It tap its power from the phone line without any noticeable loss in transmit and receive signals. To understand how power is delivered to the phone lines, consider the telephone system model shown in Figure 1. The telephone central office and transmission line loop impedance is represented by the RC block. The no load voltage at the subscriber's terminal when the telephone handset is on-hook is around 48 Volts. When the handset is off-hook, the terminal voltage drops to around 15 Volts to 5 Volts. This caused by the impedance of the transmission line and the telephone set. The voltages at the subscriber terminal is the key factor that controls the operation of the telephone switcher. For the following circuit description, refer to the schematic diagram in Figure 2. Lifting the handset of the telephone will cause the terminal voltage to drop from 48 Volts to 10 Volts (typical value). Note that the drop in voltage does not occur rapidly; therefore, while the terminal voltage is still high (above the threshold voltage), both Zener diodes D13 and D14 are turned on. Current flows through R3 triggering the SCR and providing a link to the telephone set connected to L1a and L2a. When the terminal voltage drops below the threshold voltage of the Zener diode, D13 reverts to its non-conducting state, cutting off the gate drive to SCR1. But note that once the SCR is on, it will remain in this state regardless of the gate drive as long as the current flowing through it does not fall close to zero. So the link is preserved D14 maintains the voltage across R2 and LED1 indicates which telephone is in use.
MATERIALS ( PART LIST )
Bill of materials: Quantity Description Unit Price Total 3 - Resistors 680 Kilo ohms 0.35 1.05 3 - Resistors 180 Ohms 0.35 1.05 3 - Resistors 1 Kilo ohms 0.35 1.05 3 - SCRs 2N5062 12.00 36.00 3 - Zener IN5226 5.50 16.50 Diode 12 - Rectifier IN4750 3.50 42.00 Diode 3 - Capacitors - 0.01 uF ceramic 0.75 2.25 3 - LEDs LED Green Medium 2.50 7.50 1 - Plastic HC811 19.00 19.00 Enclosure 2 - Telephone Tel. 202-4 30.00 60.00 Coupler 1 - PCB 1.5" x 2.5" 5.00 5.00 1 - Lead 1 meter 6.00 6.00 1 - Etching 50 ml 26.00 26.00 Solution 2 - Connecting 2 meters (black/red) 3.00 6.00 Wires (solid)
DEFINITION OF TERMS
1. TELEPHONE - A system for converting sound waves into electric variation can be sent over wires and reproduced at distant point. The Greek word tele means "far" and phone means "sound". 2. CENTRAL OFFICE - A mean telephone office, usually within a few miles of a subscriber, that houses switching gear; most are capable of handling about 10,000 subscriber. 3. TERMINAL VOLTAGE - 4. SCR (Silicon Control Rectifier) - A three-junction, three terminal PNPN thyristor that is normally an open circuit in both direction. 5. ZENER DIODE - A semi-conductor breakdown diode, usually constructed silicon , in which reverse voltage breakdown is based on the Zener effect. 6. RECTIFIER DIODE - A device that converts alternating current into a current which has a large unidirectional components, such as a gas tube, metallic rectifier, semi-conductor diode or vacuum tube. 7. CAPACITOR - A device that consist essentially of two conducting surface separated by a dielectric material like air, paper, mica ceramics or mylar. 8. DIODE - A two electrode semiconductor device that utilize the rectifying properties of a junction between P- and -N type material in a semi-conductor.
CONSTRUCTION PROCEDURE
The foil pattern layout in fig.3 is actual size of the PCB. This layout can be used as a pattern for those who want to make their own PCBs. Review all the components and other materials required. Also, carefully check the PCB foil for any unwanted open or short circuited foil paths. Used your multimeter. Then proceed as follows: 1. Insert and solder all ¼ -Watt resistors into their respective places. 2. Insert and solder diodes D1, D2, D3 to D12, and also the Zener diodes. Make sure that the diodes are place in their proper places with the right orientation or direction. 3. Insert and solder the SCRs; take note of their orientation. 4. Finally, insert and solder the capacitors and LEDs, making that the longer LED's are connected to the positive terminal. 5. Mount the PCB shown in the photograph or you may adapt your own mounting if a different case is to be used.
HOOK UP AND TESTING
Connect the test setup illustrated in fig.4. You should at least two telephones for testing. Terminal L1 and L2 at the junction box and follow the step- by-step procedure. 1. Lift the handset of each telephone to see if the corresponding LED lights up. Return the handset back to its cradle. The LED should turn off. Do this procedure to checked the other phones. 2. Lift the handset of telephone a; each corresponding LED should cut- off(no-dial tone). 3. Lift the handset of phone b. Then return the handset of phone a to its cradle. Telephone b's LED should light up and the dial tone should be heard through the ear piece. 4. Finally, test each phone, making sure that the other phones are cut off while one is in use. The corresponding LED should remain lit as long as the phone's handset is off-hook. This procedure ensures that your circuit is properly working. To add more than 3 telephones, refer to Figure 5.
SUMMARY AND CONCLUSION
The study covered more on telephone features. This is very beneficial in the offices in a certain company and even in our houses. It happened to have an extensions in a telephone lines and if there is a coming call the one or more extension simultaneously rings .Some body want to answer the call and may be having a very personal conversations without knowing that somebody else is eavesdropping on an extension phone. This will happen many times, it is very disgusting for you, but quite consistent with insecure people. These people usually listen to other people's conversation as well as go over their things just to satisfy their needs. This project will avoid the necessary mistakes of every person it is because the circuit disables other extension phones connected to the line whenever the master (main line) or the extension phone is in use. So, we therefore conclude many studies helps a lot to some purposes. Nowadays, confidential conversation should be keep .
LITERATURE CITED
1.Electronic Devices and Circuit Theory ( 5th Edition ) by: Robert Boylestad & Louis Nashelsky 2.Modern Electronic Communication ( 4th Edition ) by: Gary M. Miller
RICHARD C. GUMANIT