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INTRODUCTIONSemiconductors possess conductivity between those of metals and insulators. They find a variety of uses. One such example is an electronic musical doorbell. EXPERIMENT AIM To make an electronic musical doorbell. COMPONENT REQUIRED – 1, One IC chip – UM 66 to 25 2, One power transistor – SL100 (n-p-n) 3, One resistor of 1 k (0.25W) 4, One 3 speaker of impedance 4 5, A full wave rectifier (battery eliminator) with output of 3v 6, A push button. THEORY – An ordinary electric door bell is based on the principle of an electromagnet. When current passes through the coil wound around a soft iron core, it gets magnetised. The magnetised core then attracts a hammer, which strikes against a metallic gong to produce the sound. Such an electric bell suffers from the following drawbacks. 1, when current passes through the coil, eddy currents are produced in the soft iron core, which in turn heats up the coil. As a result, the coil of the bell gets burnt, if the current is passed for a long time. 2) Such a bell gets burnt due to excessive current passing through its coil due to voltage fluctuation. 3) The sound produced by the bell is not soothing to the ears. Since an electric musical bell operates on 3 v d.c. Supply the small voltage fluctuations do not hamper its performance. Moreover as the name suggest, a musical doorbell produces sound, which is quite pleasing to the ears. BRIEF DISCRIPTION OF CIRCUIT DIAGRAMFig. Shows the circuit diagram for a musical doorbell. The positive terminal of the full wave rectifier is connected to the pin 2 of the IC chip UM 66 to 25 through a push button, while the negative terminal is connected to the pin 3 of the IC. The musical notes of the sound produced by the IC are obtained at pin 1 of the IC, which are fed to the base of the power transistors SL 100 through a ressistance of 1 k . The emitter of the transistor is also connected to the negative terminal of the full wave rectifier the speaker of impedance 4 is connected across the collector of the transistor. And the positive terminal of the d.c. Supply from the rectifier. The circuit is based on the simple application of IC chip UM 66 to 25. The whole circuit is wired primarily on the IC chip, which comprises of a ROM memory of 64 notes. It has a built in oscillator and pre-amplifier. Since the signal generated by IC is very weak, the power transistor SL 100 has been use to amplifier the signal. The amplified signal, than drive the speaker, which produces a musical sound. FORMATION OF p-n JUNCTIONA p-n junction is basic semiconductor device. When a p- type crystal is placed in contact with n- type crystal so as to form one piece. This assemble so obtained is called p-n junction or junction diode or crystal diode. The surface of contact of p- and n- type crystals is called junction. In the p-section holes are the majority carriers; while in n-section the majority carriers are electrons. Due to high concentration of different types of charge carriers in the two sections, holes from p- region diffuse into n- region and electron from n- region diffuse to p- region. In both cases when an electron meets a hole, the two cancel the effect on each other and as a result a thin layer at the junctions becomes devoid of charge carriers. This is called Depletion Layer. The thickness of the depletion layer is of the order of 10–6 m. The potential difference developed across the junction due to migration of majority charge carriers is called potential barrier. Thus the formation of p-n junction results in a very strong electric field across the junction. FORWARD BIASINGWhen an external d.c. source is connected to the junction diode with p-section connected to positive pole and n-section to the negative pole, the junction diode is said to be forward biased. Action of p-n junctionWhen the p-n junction is forward biased, the positive holes in the p- section are repelled by positive pole of the battery towards the p-n junction. Simultaneously, the negative electrons in the nsection are repelled by negative pole of the battery towards the junction. However, the movement of electron and holes across the junction is opposed by the factious battery voltage (= 0.3 v to 0.7 v) developed across the junction. Just near the p- n junction electrons and holes combine and cease to exist as mobile charge carriers after the potential barrier is overcome by the applied potential. REVERSE BIASING – When a battery is connected to junction diode with psection connected to negative pole and n- section connected to the positive pole, the junction diode is said to be reverse biased. Action Of p-n Junction – When the p-n junction is reverse biased, the holes in the p- section get attracted towards the negative terminal of battery and therefore, the holes move away from the junction. At the same time the electron in the n- section get attracted toward the positive terminal and move away from the junction. A small electron hole combination current called reverse current is maintained by the minority carriers. The junction diode is represented by the symbol as shown. The arrow head represents the p- section of the junction diode and it points in the direction in which the hole current or conventional current will flow , when junction diode is foreword biased . The electron current or the electronic current will flow in opposite direction. ZENER DIODE – The specially designed junction diodes, which can operate the reverse breakdown voltage region continuously, without being damaged, are called Zener Diodes. A zener diode is represented by the symbol as shown. TRANSISTORSA transistor is formed, when a thin layer of one type of extrinsic semiconductor is sandwiched between two thick layers of the other type of extrinsic semiconductor. Thus, a transistor is a semiconductor device having three sections and two junctions. The three sections are combined, so that the two at extreme ends have the same type of majority carriers; while the section that separates them, has the majority carriers of opposite nature. The semiconductor device so obtained is called n-p-n or p-n-p transistor. Therefore a transistor can be n-p-n or p-n-p type. In other words, in an n-p-n transistor, the p- section is sandwiched between two n- sections. On the other hand, in a p-n-p transistor, the n-section is sandwiched between two p sections. The three section of the transistor are called emitter (E), base (B) and collector (C) symbol for transistor. Symbol For TransistorsIn the symbol for a transistor, the normal points hole currents conventional current. Therefore, the emitter in n-p-n transistor is represented by an arrow pointing away from the base, while the emitter in p-n-p transistor is represented by an arrow pointing towards the base. The symbols for n-p-n and p-n-p transistors are respectively shown. When a transistor is used in a circuit , the base- emitter junction is always forward biased and the base- collector junction is always reverse biased. Action Of TransistorThe action of both the type of transistors i.e. n-p-n and p-n-p is similar, except that the majority and minority carriers in the cases are of apposite nature. A) Action Of n-p-n transistorFig. shows the proper biasing of an n-p-n transistor. Connecting it to negative pole of the battery Vee forward biases the ntype emitter and n- type collector is reverse biased by connecting it to the positive pole of the battery Vcc (collector, base both) B) Action of p-n-p TransistorThe p- type emitter of p-n-p transistor is forward biased by connecting it to positive pole of emitter base battery Vee and the p- type collector is reverse biased by connecting it to the negative pole of the collector base battery Vcc as shown. INTEGRATED CIRCUITS (IC)An integrated circuit consists of a silicon chip incorporating a large no of microelectronic circuits. In other words an IC consists of whole system rather than separate electronic components. Fig. (a) shows a thin slice of silicon crystal about 0.5 cm in thickness. It is called a silicon wafer. It may have diameter ranging between 0.25 cm to 10-cm. Fig (b) shows a small part of such a silicon wafer of dimensions 50 mil 50 mil. This small portion of the wafer is called a silicon chip. The various components such as resistors, inductor, capacitor, diodes, transistors, logic gates etc. can be grown over such one silicon chip. The components are connected internally to produce a desired circuit. Fig(c) shows the mounting of the silicon chip into a casing. The pins are connected internally to the integrated circuit and the other ends of the pins are used to make external connections. Depending on the no of components fabricated on a chip, the integrated circuits are classified as below: i) Medium scale IC (MSI) – Such ICS have about 100 circuits components. ii) Large scale IC (LSI) – They have more than 1000 circuits components. An IS is a multi terminal device. They are of diff. shapes & sizes as shown. A commonly used IC may posses 8,10, or 16 terminals. ICs have following advantages over the conventional electronic circuits: a) They are highly reliable. b) They require very small space as compared to conventional electronic circuits c) They have lower total cost. JUNCTION DIODE AS A RECTIFIERAn electronic device which converts a.c. power into d.c. power is called a rectifier. The junction diode offers a low resistance path, when forward biased, and a high ressistance path, when reverse biased. FULL WAVE RECTIFIERA rectifier, which rectifies both halves of each a.c. input cycle, is called a full wave rectifier. PrincipleIt also works on the principle that a junction diode offers low ressistance during forward bias and high ressistance during reverse biased. Here, two junction diodes are connected in such a manner that if one diode gets forward biased but when the next opposite half cycle comes, the first diode gets reverse biased and the second, forward biased. Thus, output obtained during both the half cycle of the a.c. input. ArrangementThe a.c. supply is fed across the primary coil P of a stepdown transformer. The two ends of the secondary coil S of the transformer are connected to the P- section of the junction diodes D1 &D2. A load ressistance RL is connected across the n-section of the two diodes and the central tapping of the secondary coil. The D.C. Output will be obtained across the load ressistance RL. TheorySuppose that during first half of the input cycle, upper end of the coil S is at positive potential and t6he lower end is at negative potential. The junction diode D1 will get forward biased, while the diode D2 reverse biased. The conventional current due to the diode D1 will flow along the path of full arrows. When the second half of the input cycle comes, the situation will be exactly reversed. Now, the junction diode D2 will conduct and the conventional current will flow along the path of the dotted arrows. Since current during both the half cycle, flows from right to left through the load ressistance RL, the output during both the half cycles will be of the same nature. The right end of the load ressistance w.r.t its left ends. Thus, in a full wave rectifier the output is continues but pulsating in nature. CONCLUSION - Semiconductors devices are being use to most widely in the digital communication, nano technology etc. with their help communication luxury, entertainment, health care, that is have been benefited. They are the most important aspects for the modern life style and have been a real boon for the digital industry. INDEX Introduction Experiment P-n Junction Transistor Rectifier IC’s