Design and Construction of 20watts Wireless Public Address System
Chapter One
AIMS AND OBJECTIVES
Public speaking is the process of speaking to a group of people in a structured, deliberate manner intended to inform, influence, or entertain the audience. In public speaking, as in any form of communication, there are five basic elements, often expressed as “who is saying what to whom using what medium with what effects?”(W.Kleitz;2005 pg134).
The major objective of this project is to design and construct a public address system with relatively unique operational characteristics that is capable of delivering 20 watts of electrical power into an 8 Ohms load (loudspeaker). This will be achieved with adequate tone controls and mixer stages so as to enable communication or reproduction of speech and recorded music in buildings and institutions.
CHAPTER TWO
LITERATURE REVIEW
Communication has been an age long process in which information is passed from generation to generation. The idea of public address system was borne out of the necessity to reach out to a large audience in churches, lecture halls, etc. It would have been impossible to transmit and receive without the use of radio waves and radio receivers (C.G. Montoro and M.C Schneider. 2007). The practical applications of the wireless communication and remote control technology were implemented by Nikola Tesla. The world’s first radio receiver (thunderstorm register) was designed by Alexander Stepanovich Popov, and it was first seen at the All-Russia exhibition in 1896. He was the first to demonstrate the practical application of electromagnetic (radio) waves
ANCIENT MEDIUM OF INFORMATION TRANSMISSION
In the ancient days information is passed across using drums, wooden gongs and gongs to call on people’s attention before passing on the information, a rimmed cattle horn was also used to serve as a microphone (Ezeorah Chidiebere; 2009).
MODERN DAY MEDIUM APPROACH
Recently it has been witnessed that the advent of a durable, wider coverage and more reliable means that could be seen as announcement. It is called a PUBLIC ADDRESS SYSTEM. This system consists of a microphone as the input medium and an amplifier with the speaker as the output medium. These early public address systems made good use of transducers and communication cables. The condenser microphone which is a transducer that converts sound or noise into electrical quantity due to the change in resistance which enables the conversion of sound into electrical signal (Ogundeji;1982, Ogunrem;1997).
This electrical signal is then transmitted to the amplifier section through a transmission cable which is parallel or coaxial. A suitable amplifier circuit is built at the loudspeaker section to amplify the signal from the transmission cable and finally the speaker is a transducer that can equally convert this signal (electrical ) back to sound energy. The above explanation is the operation of early public address system and how it was invented.
Problems Associated with earlier systems
- Risk of open circuit occurrence in the communication cable as a result of the
- There are complex circuitry due to the absence of integrated circuit chips for higher reliability and easier trouble
- Discomfort while handling the microphone due to the cable connecting it to the
- The range of movement is usually restricted by the length of the wire.
Working Principles of the Modern Systems
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Recent public address system were now designed to overcome the above shortcomings. To tackle the problem of complex circuit and additional cooling circuit, integrated circuit have been invented which contains most of these complex circuits with the correct biasing element which have been tested and used in signal amplification, demodulation etc(K.M James; 2001).
Also instead of using the transmission cable a more reliable link, the radio frequency is now employed.
The working principle of this modern public address system is described below; the transducer microphone converts the speech into electrical quantity through change in resistance, this electrical signal is amplified and then used to modulate an electromagnetic carrier wave generated by an oscillator , this modulated signal is then sent into out as radio frequency through the antenna.(W.Kleitz;2005.). The radio frequency is then picked up by the antenna and thus it will require a radio receiver to interpret the signal and obtain the intelligence been conveyed. When the receiver is equipped with a suitable audio amplifier at the output, a speaker can be used at the output to achieve the required performance of a public address system.
CHAPTER THREE
SYSTEM DESCRIPTION AND IMPLEMENTATION
In the design of this project, some existing designs were extensively studied and modified. These different designs were tested and modified using the breadboard.
TRANSISTOR POWER AMPLIFIER
Power amplifiers boost a signal level and provide current to drive a loudspeaker. All output transducers require amplification of the signal by an amplifier, including loudspeakers.
The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification (the “forward gain”) is determined by the external circuit design as well as the active device. Many common active devices in transistor amplifiers are bipolar junction transistors (BJTs) and metal oxide semiconductor field-effect transistor (MOSFETs).
MOSFET POWER AMPLIFIER.
The metal-oxide-semiconductor field-effect transistor (MOSFET) is a device used to amplify or switch electronic signals. The voltage rating of the transistor is a function of the doping and thickness of the N-epitaxial layer, while the current rating is a function of the channel width (the wider the channel, the higher the current). In a planar structure, the current and breakdown voltage ratings are both a function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the “silicon estate”. With the vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness (actually the N-epitaxial layer thickness) is proportional to the breakdown voltage.
It is worth noting that power MOSFETs with lateral structure are mainly used in high-end audio amplifiers. Their advantage is a better behavior in the saturated region (corresponding to the linear region of a bipolar transistor) than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications. A MOSFET operates with the lowest resistance when fully-on and thus has the lowest power dissipation when in that condition, except when fully off.
The MOSFET includes a channel of n-type or p-type semiconductor material. Usually the semiconductor of choice is silicon; unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good semiconductor-to-insulator interfaces and thus are not suitable for MOSFETs.
When a voltage is applied between the gate and source terminals, the electric field generated penetrates through the oxide and creates a so-called “inversion layer” or channel at the semiconductor-insulator interface. The inversion channel is of the same type P-type or N-type as the source and drain, so it provides a channel through which current can pass. Varying the voltage between the gate and body modulates the conductivity of this layer and makes it possible to control the current flow between drain the source.
CHAPTER FOUR
CIRCUIT DESIGN AND ANALYSIS.
Design specifications. The design specifications used in this project work are enumerated below.
- Thepower supply unit
- Thereceiver principle
- Thetransmitter principle
- Inputamplifier
- Natureof the mixer and tone
POWER SUPPLY UNIT DESIGN
The transformer primary voltage is 220V, which is directly from the main supply. The rectifier circuit used in this project makes use of the bridge rectifier shown below which uses four rectifier diodes, the IN4001. The rectified signal is then passed through a smoothening capacitor to remove the ripples which results to a steadier signal output. A voltage regulator is connected at the output to give a constant value of 30V. For the capacitor used for smoothening, a ripple factor of 0.1 was used. The formular is given below;
CHAPTER FIVE
CONSTRUCTION AND TESTING
The components were initially mounted on breadboard as stated earlier and tested; some of the mistakes were corrected before finally transferred onto the vero board for permanent soldering.
THE POWER SUPPLY.
Since the power supply is the hub of the circuit, it made use of high current rating diodes. The diodes were adequately soldered on the vero board to avoid short circuit and excessive heating, which may lead to dry joint problems. The construction was also protected by a fuse.
CHAPTER SIX
CONCLUSION AND RECOMMENDATION
The constructed public address system can stand the test of time in its performance. It is very reliable, and the overall performance is excellent. The design specifications were met.
BILL OF ENGINEERING MEASUREMENT AND EVALUATION
The total expenses made at the cause of the design and implementation of this project is analyzed as follows;
PROBLEMS ENCOUNTERED AND SOLUTION
The major problem was audio feedback
The reception was poor at first and the component was scarce in the market.
AUDIO FEEDBACK
Also known as the Larsen effect after the Danish scientist, Soren Larsen, who first discovered its principles is a special kind of feedback which occurs when a ound loop exists between an audio input (for example, a microphone) and an audio output
(for example, a loudspeaker). In this example, a signal received by the microphone is amplified and passed out of the loudspeaker the sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The frequency of the resulting sound is determined by resonant frequencies in the microphone, amplifier, and loudspeaker. Most audio feedback results in a high-pitched squealing noise familiar to those who have listened to bands at house parties, and other locations where the sound setup is less than ideal-this usually occurs when live microphones are pointed in the general direction of the output speakers.
AUDIO FEEDBACK PREVENTION
To keep the maximal loop gain under 1, the amount of sound energy that is fed back to the microphones has to be as small as possible. As sound pressure falls off with 1/r with respect to the distance in free space or up to a distance known as reverberation distance in closed spaces (and the energy density with 1r2), it is important to keep the microphones at a large enough distance from the speaker
systems. The loudspeakers and microphones should have non-uniform directivity and should stay out of the maximum sensitivity of each other, ideally at a direction of cancellation.
CONCLUSION
Conclusively the public address system successfully achieved the aim of transmitting and receiving voice signals over a distance. I learnt and researched more on the use of modulation and demodulation, the amplifier circuitry etc.
RECOMMENDATION
The only recommendation is that adequate time should be allotted for research; since the project is not only concerned with paperwork but construction and design.
This project is recommended for schools and in a situation where there is need to communicate with a good number of people.
Further improvements in this project is also recommended such as; An infrared switching circuit can be equally be built in the receiver to enable it to be tuned on with a remote control as the case may be and many more with more enhanced modifications.
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