Project.docx

TUMBA COLLEGE OF TECHNOLOGY P.O. Box 6638 Rulindo Tel/fax :( +250) 580381 Northern Province Website: www.tct.ac.rw

FACULTY OF TECHNOLOGY DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION

DESIGN AND IMPLEMENTATION OF DOORBELL WITH SOUND CONTROLLED BY SWITCH

DESIGN AND IMPLEMENTATION OF DOORBELL WITH SOUND CONTROLLED BY SWITCH

In partial fulfillment of the requirements for the award of the diploma A1 in Electronics and Telecommunication Submitted by: Innocent NSENGIYUMVA REG N°: GS09041 JMV NYIRIMANA REG N°:GS08184 Jean Claude SEDEKI NTIRENGANYA REG N°:GS09094 Supervisor: Mr. Kazuhisa WATURU Tumba,……...……..2011

DECLARATION We, JMV NYIRIMANA, Jean Claude SEDEKI NTIRENGANYA and Innocent NSENGIYUMVA declare that this project entitled*doorbell with sound controlled by switch* is my work. It has never been submitted anywhere for the similar award at any university or institution. Submitted in partial fulfillment of the requirements for the award of Diploma A1 in Electronics and Telecommunication (ET) at Tumba College of Technology (TCT).

……………. INNOCENT NSENGIYUMVA REG N°:GS09041

……………. JMV NYIRIMANA REG N°:GS08184

…………….. JEAN CLAUDE SEDEKI NTIRENGANYA REG N°:GS09094

Tumba ,……………… 2011

TUMBA COLLEGE OF TECHNOLOGY

P.O. Box 6638 Rulindo Tel/fax :( +250) 580381 Northern Province Website: www.tct.ac.rw

FACULTY OF TECHNOLOGY DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION CERTIFICATION I, Mr. Kazuhisa WATARU hereby certify that I supervised the study*Design and implementation of Doorbell with sound controlled by Switch* carried out by JMV NYIRIMANA, Innocent NSENGIYUMVA and Jean Claude SEDEKI NTIRENGANYA the study was done as a partial fulfillment of the requirements for the award of a Diploma in Electronics and Telecommunication during the academic year 2011.

Date …/…./2011

Head of department of Electronics and telecommunication

Under the guidance …………………

………………….

Mr. Kazuhisa WATARU

Mr. Jean Pierre MUSABYIMA

DEDICATION We would like to dedicate this final project report to:

  

ACKNOWLEDGEMENT

Our parents Our brethren, sisters and relatives Our friends

We give thanks to the Lord who kept and helped us from the beginning of our life and during this project, and we would also like to express our heartfelt gratitude to our project supervisor Mr. Kazuhisa WATARU, our ET staff for their higher efforts and particularly to our families that may find this modest work, the expression of our recognition.

ABSTRACT

There is no such meaningful word than to ask the Almighty God to bless you. An anti-burglar alarming door bell is disclosed, and in particular, a door bell having dual wires non-polar, with anti-burglar and alarming function. The door bell comprises a detecting mechanism, a door bell/anti-burglar mode selection, a first voicing mechanism, a second voicing mechanism, a micro-computer controlled switch circuit and an alarming activation mechanism having a first indication mechanism and a second mechanism and a press button. The microcomputer receives triggered signals from the detecting mechanism (such as magnetic-spring switch or infrared detecting device) to cause the voicing mechanism to produce a sound. By means of the switching circuit, the first indication mechanism or the second indication mechanism of the alarming activation mechanism is lighted. The microcomputer causes different indication mechanisms to light and causes different voicing mechanisms to produce a sound via the alarming activation mechanism with respect to the detected status by the detecting mechanism and the door bell/antiburglar mode selection.

LIST OF ABREVIATIONS

I: Current V: Voltage Vc: Voltage of capacitor R: Resistance Req: Equivalent Resistance US: United State ET: Electronics and Telecommunication TCT: Tumba College of Technology I T: Information Technology DC: Direct Current C: Capacitance Q: Charge F: Farad C: Coulomb AC: Alternative Current \\: Parallel GND: Ground CTRL: Control DIS: Discharge Vcc: Positive voltage F: Frequency T: Period Ts: Space Time E: Electrolytic Capacitor S: Second Hz: Hertz U: Timers

E mf: Electromagnetic Force S: Switch CRO: Cathode Ray Oscilloscope PCB: Printer circuit Board

CHAPTER 1: INTRODUCTION

1.1. GENERAL INTRODUCTION Doorbell transformers are the components responsible for the electric current that powers the doorbell. They act like an electrical pump, pushing electrons out of one terminal and pulling them. Having a working doorbell is important: You don't want to make your guests wait on your doorstep or miss important package deliveries. The doorbell is one of the first things someone sees when they come to visit you at your home. If your doorbell is cracked and broken, you don't need to continually put off replacing it. A residential doorbell is a simple, low-voltage system made up of a push button at an exterior entrance, a bell or chime, and a transformer. When you push a doorbell, it creates a sound that alerts the people inside a house that you are outside. The ubiquitous doorbell is the message center from the outside world to the home. Through it, your friends, the delivery man or neighborhood prankster announce their presence Doorbells are a useful addition to any home's entryway, offering a distinctive tone and an alternative to harsh knocking sounds. Friedland manufactures a number of different doorbells. Doorbells are a common convenience in homes, giving visitors a way of announcing their presence and preventing residents from missing deliveries or guests. Doorbells are simple pieces of home equipment that let you know a visitor has arrived. They're useful if you are too far from the front door to hear someone knocking. A doorbell is a signaling device typically placed near a door. Most doorbells emit a ringing sound to alert the occupant of the building to a visitor's presence, when the visitor presses a button

1.2.OBJECTIVES The major purpose of a multi-switched musical doorbell with indicators which composes of different switches and a tone attached to it, helps in providing or creating of awareness to the occupant or residents of buildings, that their attention is needed at their door post. It is an electronic informer of building occupants. And it also helps the owner or occupants of a building who have a multi-switch musical doorbell attached to their houses, for the easy recognition of the presence of a visitor in his or her house. The advantages of a musical doorbell are as follows: - It allows an easy positioning of switch at different door steps of buildings i.e. for buildings that have more than one entrance. - It goes beyond the reach of an ordinary knocking with bare-hands and also last as long as possible.

1.3. METHODOLOGY In other to accomplish our project, different ways have been used:    

Searching on internet; Go on ground for extracting information (TCT: ET Lab, IT Lab, and TCT gate); Library; Electronic Laboratory

1.4. SCOPE OF RESEARCH We will take into consideration that Tumba College of Technology (TCT) is too large we could not cover whole TCT,we will do our practice in ET Lab and expand the result to the whole TCTT(TCT Gate).

1.5. PROBLEME STATEMENT

CHAPTER 2: LITTERATURE REVIEW 2.1. ELECTRONICS COMPONENTS There are two important types of components; a. Active components (integrated circuit) b. Passive components (resistor, capacitor) The main difference between active and passive components is that active ones require in some way to them work. Active components can also be used to amplify signals. 2.1.1. ACTIVE COMPONENT 2.1.1.1. Integrated circuit

(fig.2.2.2.1a)

(fig.2.2.2.1b)

Definition Of 555 Pin Functions Pin 1 (Ground): The ground (or common) pin is the most-negative supply potential of the device, which is normally connected to circuit common (ground) when operated from positive supply voltages. Pin 2 (Trigger): This pin is the input to the lower comparator and is used to set the latch, which in turn causes the output to go high. This is the beginning of the timing sequence in monostable operation. Triggering is accomplished by taking the pin from above to below a voltage level of 1/3 V+ (or, in general, one-half the voltage appearing at pin 5). The action of the trigger input is levelsensitive, allowing slow rate-of-change waveforms, as well as pulses, to be used as trigger sources. The trigger pulse must be of shorter duration than the time interval determined by the external R and C. If this pin is held low longer than that, the output will remain high until the trigger input is driven high again. One precaution that should be observed with the trigger input signal is that it must not remain lower than 1/3 V+ for a period of time longer than the timing cycle. If this is allowed to happen, the timer will re-trigger itself upon termination of the first output pulse. Thus, when the timer is driven in the monostable mode with input pulses longer than the desired output pulse width, the input trigger should effectively be shortened by

differentiation. The minimum-allowable pulse width for triggering is somewhat dependent upon pulse level, but in general if it is greater than the 1uS (micro-Second), triggering will be reliable. A second precaution with respect to the trigger input concerns storage time in the lower comparator. This portion of the circuit can exhibit normal turn-off delays of several microseconds after triggering; that is, the latch can still have a trigger input for this period of time after the trigger pulse. In practice, this means the minimum monostable output pulse width should be in the order of 10uS to prevent possible double triggering due to this effect. The voltage range that can safely be applied to the trigger pin is between V+ and ground. A dc current, termed the trigger current, must also flow from this terminal into the external circuit. This current is typically 500nA (nano-amp) and will define the upper limit of resistance allowable from pin 2 to ground. For an astable configuration operating at V+ = 5 volts, this resistance is 3 Mega-ohm; it can be greater for higher V+ levels. Pin 3 (Output): The output of the 555 comes from a high-current totem-pole stage made up of transistors Q20 Q24. Transistors Q21 and Q22 provide drive for source-type loads, and their Darlington connection provides a high-state output voltage about 1.7 volts less than the V+ supply level used. Transistor Q24 provides current-sinking capability for low-state loads referred to V+ (such as typical TTL inputs). Transistor Q24 has a low saturation voltage, which allows it to interface directly, with good noise margin, when driving current-sinking logic. Exact output saturation levels vary markedly with supply voltage, however, for both high and low states. At a V+ of 5 volts, for instance, the low state Vce(sat) is typically 0.25 volts at 5 mA. Operating at 15 volts, however, it can sink 200mA if an output-low voltage level of 2 volts is allowable (power dissipation should be considered in such a case, of course). High-state level is typically 3.3 volts at V+ = 5 volts; 13.3 volts at V+ = 15 volts. Both the rise and fall times of the output waveform are quite fast, typical switching times being 100nS. The state of the output pin will always reflect the inverse of the logic state of the latch, and this fact may be seen by examining Fig. 3. Since the latch itself is not directly accessible, this relationship may be best explained in terms of latchinput trigger conditions. To trigger the output to a high condition, the trigger input is momentarily taken from a higher to a lower level. [see "Pin 2 - Trigger"]. This causes the latch to be set and the output to go high. Actuation of the lower comparator is the only manner in which the output can be placed in the high state. The output can be returned to a low state by causing the threshold to go from a lower to a higher level [see "Pin 6 - Threshold"], which resets the latch. The output can also be made to go low by taking the reset to a low state near ground [see "Pin 4 - Reset"]. The output voltage available at this pin is approximately equal to the Vcc applied to pin 8 minus 1.7V. Pin 4 (Reset): This pin is also used to reset the latch and return the output to a low state. The reset voltage threshold level is 0.7 volt, and a sink current of 0.1mA from this pin is required to reset the device. These levels are relatively independent of operating V+ level; thus the reset input is TTL compatible for any supply voltage. The reset input is an overriding function; that is, it will force the output to a low state regardless of the state of either of the other inputs. It may thus be used to terminate an output pulse prematurely, to gate oscillations from "on" to "off", etc. Delay time from reset to output is typically on the order of 0.5 µS, and the minimum reset pulse width is 0.5

µS. Neither of these figures is guaranteed, however, and may vary from one manufacturer to another. In short, the reset pin is used to reset the flip-flop that controls the state of output pin 3. The pin is activated when a voltage level anywhere between 0 and 0.4 volt is applied to the pin. The reset pin will force the output to go low no matter what state the other inputs to the flip-flop are in. When not used, it is recommended that the reset input be tied to V+ to avoid any possibility of false resetting. Pin 5 (Control Voltage): This pin allows direct access to the 2/3 V+ voltage-divider point, the reference level for the upper comparator. It also allows indirect access to the lower comparator, as there is a 2:1 divider (R8 - R9) from this point to the lower-comparator reference input, Q13. Use of this terminal is the option of the user, but it does allow extreme flexibility by permitting modification of the timing period, resetting of the comparator, etc. When the 555 timer is used in a voltagecontrolled mode, its voltage-controlled operation ranges from about 1 volt less than V+ down to within 2 volts of ground (although this is not guaranteed). Voltages can be safely applied outside these limits, but they should be confined within the limits of V+ and ground for reliability. By applying a voltage to this pin, it is possible to vary the timing of the device independently of the RC network. The control voltage may be varied from 45 to 90% of the Vcc in the monostable mode, making it possible to control the width of the output pulse independently of RC. When it is used in the astable mode, the control voltage can be varied from 1.7V to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output. In the event the control-voltage pin is not used, it is recommended that it be bypassed, to ground, with a capacitor of about 0.01uF (10nF) for immunity to noise, since it is a comparator input. This fact is not obvious in many 555 circuits since I have seen many circuits with 'no-pin-5' connected to anything, but this is the proper procedure. The small ceramic cap may eliminate false triggering. Pin 6 (Threshold): Pin 6 is one input to the upper comparator (the other being pin 5) and is used to reset the latch, which causes the output to go low. Resetting via this terminal is accomplished by taking the terminal from below to above a voltage level of 2/3 V+ (the normal voltage on pin 5). The action of the threshold pin is level sensitive, allowing slow rate-of-change waveforms. The voltage range that can safely be applied to the threshold pin is between V+ and ground. A dc current, termed the threshold current, must also flow into this terminal from the external circuit. This current is typically 0.1µA, and will define the upper limit of total resistance allowable from pin 6 to V+. For either timing configuration operating at V+ = 5 volts, this resistance is 16 Mega-ohm. For 15 volt operation, the maximum value of resistance is 20 Mega Ohms. Pin 7 (Discharge): This pin is connected to the open collector of a npn transistor (Q14), the emitter of which goes to ground, so that when the transistor is turned "on", pin 7 is effectively shorted to ground. Usually the timing capacitor is connected between pin 7 and ground and is discharged when the transistor turns "on". The conduction state of this transistor is identical in timing to that of the output stage. It is "on" (low resistance to ground) when the output is low and "off" (high resistance to ground) when the output is high. In both the monostable and astable time modes, this transistor switch is

used to clamp the appropriate nodes of the timing network to ground. Saturation voltage is typically below 100mV (milli-Volt) for currents of 5 mA or less, and off-state leakage is about 20nA (these parameters are not specified by all manufacturers, however). Maximum collector current is internally limited by design, thereby removing restrictions on capacitor size due to peak pulse-current discharge. In certain applications, this open collector output can be used as an auxiliary output terminal, with current-sinking capability similar to the output (pin 3). Pin 8 (V +): The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the 555 timer IC. Supply-voltage operating range for the 555 is +4.5 volts (minimum) to +16 volts (maximum), and it is specified for operation between +5 volts and +15 volts. The device will operate essentially the same over this range of voltages without change in timing period. Actually, the most significant operational difference is the output drive capability, which increases for both current and voltage range as the supply voltage is increased. Sensitivity of time interval to supply voltage change is low, typically 0.1% per volt. There are special and military devices available that operate at voltages as high as 18 volt

Types of 555timers There are two types of 555timers which are below: a. Astable which produces the square wave b. Monostable which produces a single pulse when is triggered. 2.2.2.1.a Monostable

(fig.2.2.2.1.a) A monostable circuit produces a single output pulse when triggered. It is called a monostable because it is stable in just one state: 'output low'. The 'output high' state is temporary. The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:

time period, T = 1.1 × R1 × C1 T = time period in seconds (s) R1 = resistance in ohms ( ) C1 = capacitance in farads (F) The maximum reliable time period is about 10 minutes. Why 1.1? The capacitor charges to 2/3 = 67% so it is a bit longer than the time constant (R1 × C1) which is the time taken to charge to 63%.    

Choose C1 first (there are relatively few values available). Choose R1 to give the time period you need. R1 should be in the range 1k to 1M , so use a fixed resistor of at least 1k in series if R1 is variable. Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common. Beware that electrolytic capacitors leak charge which substantially increases the time period if you are using a high value resistor - use the formula as only a very rough guide! For example the Timer Project should have a maximum time period of 266s (about 4½ minutes), but many electrolytic capacitors extend this to about 10 minutes!

Monostable operation The timing period is triggered (started) when the trigger input (555 pin 2) is less than 1 /3 Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored. The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches 2/3 Vs the time period is over and the output becomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger. The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled at any time by connecting reset to 0V, this instantly makes the output low and discharges the capacitor. If the reset function is not required the reset pin should be connected to +Vs. Power-on reset or trigger It may be useful to ensure that a monostable circuit is reset or triggered automatically when the power supply is connected or switched on. This is achieved by using a capacitor instead of (or in addition to) a push switch as shown in the diagram. The capacitor takes a short time to charge, briefly holding the input close to 0V when the circuit is switched on. A switch may be connected in parallel with the capacitor if manual operation is also required.

Power-on reset or trigger circuit

Edge-triggering If the trigger input is still less than 1/3 Vs at the end of the time period the output will remain high until the trigger is greater than 1/3 Vs. This situation can occur if the input signal is from an on-off switch or sensor. The monostable can be made edge triggered, responding only to changes of an input signal, by connecting the trigger signal through a capacitor to the trigger input. The capacitor passes sudden changes (AC) but blocks a constant (DC) signal. For further information please see the page on capacitance. The circuit is 'negative edge triggered' because it responds to a sudden fall in the input signal.

edge-triggering circuit

The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high (+Vs). 2.2.2.1.b Astable

(fig.2.2.2.1b) An astable circuit produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'. The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second. T = 0.7 × (R1 + 2R2) × C1 and f =

1.4 (R1 + 2R2) × C1

T = time period in seconds (s) f = frequency in hertz (Hz) R1 = resistance in ohms ( ) R2 = resistance in ohms ( ) C1 = capacitance in farads (F) The time period can be split into two parts: T = Tm + Ts Mark time (output high): Tm = 0.7 × (R1 + R2) × C1 Space time (output low): Ts = 0.7 × R2 × C1 Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is much larger than R1. For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because the output can both sink and source current. For example an LED can be made to flash briefly with long gaps by connecting it (with its resistor) between +Vs and the output. This way the LED is on during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long. If Tm must be less than Ts a diode can be added to the circuit as explained under duty cycle below.

Choosing R1, R2 and C1 R1 and R2 should be in the range 1k to 1M . It is best to choose C1 first because capacitors are available in just a few values.  

 

C1

Choose C1 to suit the frequency range 0.001µF you require (use the table as a guide). 0.01µF Choose R2 to give the frequency (f) 0.1µF you require. Assume that R1 is much smaller than R2 (so that Tm and Ts are 1µF almost equal), then you can use: 10µF R2 =



555 astable frequencies R2 = 10k R1 = 1k

R2 = 100k R1 = 10k

68kHz

6.8kHz

680Hz

6.8kHz

680Hz

68Hz

680Hz

68Hz

6.8Hz

68Hz

6.8Hz

0.68Hz

6.8Hz

R2 = 1M R1 = 100k

0.68Hz

0.068Hz

(41 per min.)

(4 per min.)

0.7 f × C1

Choose R1 to be about a tenth of R2 (1k min.) unless you want the mark time Tm to be significantly longer than the space time Ts. If you wish to use a variable resistor it is best to make it R2. If R1 is variable it must have a fixed resistor of at least 1k in series (this is not required for R2 if it is variable).

Astable operation With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2. The threshold and trigger inputs monitor the capacitor voltage and when it reaches 2/3Vs (threshold voltage) the output becomes low and the discharge pin is connected to 0V. The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to 1/3Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again. This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V. An astable can be used to provide the clock signal for circuits such as counters.

Comparison of multivibrator circuits Monostable multivibrator

Astable multivibrator

1. It has only one state stable. 2. Trigger is required for the operation, to change the state. 3. Two components R and C are necessary with IC555 to obtain the circuit. 4. The pulse width is given by, W=1.1 RC seconds 5. The frequency of operation is controlled by frequency of trigger applied. 6. The applications are time, frequency divider, pulse width modulation, etc .

1. There is no stable state. 2. Trigger is not required to change the state, hence called free running. 3. Three components Ra, Rb and C are necessary with IC 555 to obtain the circuit. 4. The frequency is given by, f=[1.44/(Ra+Rb)]HZ 5. The frequency of operation is controlled by, Ra, Rb and C. 6. The applications are square wave generator, flasher, voltage controlled oscillator, FSK generator ,etc

Loudspeaker A loudspeaker is a device that is used to create the sound in radio, television sets and electric musical instrument amplifier systems. Loudspeaker uses both electric and mechanical principal to convert an electrical signal from a radio televion set or electric musical into sound. For a loudspeaker to produce sound, signal from radio television set or electric musical instrument needs to be connected to an electronic amplifier.

Fig….symbol of loudspeaker A resistor is a two-terminal passive electronic component which implements electrical resistance as a circuit element. When a voltage V is applied across the terminals of a resistor, a current I will flow through the resistor in direct proportion to that voltage. This constant of proportionality is called conductance, G. The reciprocal of the conductance is known as the

resistance R, since, with a given voltage V, a larger value of R further "resists" the flow of current I as given by Ohm's law:

Measurement The value of resistor can be measured with an ohmmeter, which may be one function of a multimeter. Usually, probes on the ends of test leads connect to the resistor. Not that : the resistor does not respect terminals.

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. The reciprocal of resistance R is called conductance G = 1/R and is measured in Siemens (SI unit), sometimes referred to as a mho. Thus a Siemens is the reciprocal of an ohm: S = Ω − 1. Although the concept of conductance is often used in circuit analysis, practical resistors are always specified in terms of their resistance (ohms) rather than conductance. a. b.

fig…..symbols of resistor(a:European, b: American) Series connection In a series configuration, the current through all of the resistors is the same, but the voltage across each resistor will be in proportion to its resistance. The potential difference (voltage) seen across the network is the sum of those voltages, thus the total resistance can be found as the sum of those resistances:

Fig … series connection Req=R1+R2+………+Rn Parallel connection Resistors in a parallel configuration are each subject to the same potential difference (voltage), however the currents through them add. The conductance of the resistors then add to determine the conductance of the network. Thus the equivalent resistance (Req) of the network can be computed: Fig…..parallel connecton.

The parallel equivalent resistance can be represented in equations by two vertical lines "||" (as in geometry) as a simplified notation. For the case of two resistors in parallel, this can be calculated using:

Mixture connection A resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other. For instance,

fig….mixture connection

Potentiometer

Fig…..symbol of potentiometer This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

Fig…..symbol of potentiometer This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.

A potentiometer (a and b), called pot for short, is an electrical component that acts as a variable voltage divider. They usually have three terminals, one of which is connected to a moving center contact. A potentiometer with two terminals is a variable resistor, called a rheostat. Potentiometers come in many forms including rotaries, trimmers, and sliders. Potentiometer is also the term for an old instrument that measured an unknown voltage after being calibrated with a known voltage. These have been replaced by multimeters today.

fig…..potentiometer

…..Capacitor

fig …symbol of capacitor A capacitor is a passive electrical component that stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals

Charging a capacitor

The capacitor (C) in the circuit diagram is being charged from a supply voltage (Vs) with the current passing through a resistor (R). The voltage across the capacitor (Vc) is initially zero but it increases as the capacitor charges. The capacitor is fully charged when Vc = Vs. The charging current (I) is determined by the voltage across the resistor (Vs - Vc):

Charging current, I = (Vs - Vc) / R (note that Vc is increasing) At first Vc = 0V so the initial current, Io = Vs / R Vc increases as soon as charge (Q) starts to build up (Vc = Q/C), this reduces the voltage across the resistor and therefore reduces the charging current. This means that the rate of charging becomes progressively slower. C: Capacitance (in F=Farad) Q: Charge (in C=Coulomb) Vc: Voltage (in VC=Volt) Note that the time constant is a property of the circuit containing the capacitance and resistance, it is not a property of a capacitor alone

Graphs showing the current and voltage for a capacitor charging time constant = RC

. The bottom graph shows how the voltage (V) increases as the capacitor charges. At first the voltage changes rapidly because the current is large; but as the current decreases, the charge builds up more slowly and the voltage increases more slowly. After 5 time constants (5RC) the capacitor is almost fully charged with its voltage almost equal to the supply voltage. We can reasonably say that the capacitor is fully charged after 5RC, although really charging continues for ever (or until the circuit is changed).

Time Voltage Charge 0RC

0.0V

0%

1RC

5.7V

63%

2RC

7.8V

86%

3RC

8.6V

95%

4RC

8.8V

98%

5RC

8.9V

99%

After each time constant the current falls by 1/e (about 1/3). After 5 time constants (5RC) the current has fallen to less than 1% of its initial value and we can reasonably say that the capacitor is fully charged, but in fact the capacitor takes for ever to charge fully!

Discharg ing a capacito r

Graphs showing the current and voltage for a capacitor discharging time constant = RC

The top graph shows how the current (I) decrease s as the capacito r discharges. The initial current (Io) is determined by the initial voltage across the capacitor (Vo) and resistance (R): Initial current, Io = Vo / R.

Note that the current graphs are the same shape for both charging and discharging a capacitor. This type of graph is an example of exponential decay.

The bottom graph shows how the voltage (V) decreases as the capacitor discharges. At first the current is large because the voltage is large, so charge is lost quickly and the voltage decreases rapidly. As charge is lost the voltage is reduced making the current smaller so the rate of discharging becomes progressively slower. After 5 time constants (5RC) the voltage across the capacitor is almost zero and we can reasonably say that the capacitor is fully discharged, although really discharging continues for ever (or until the circuit is changed). Types of capacitors The many types of capacitors are in two groups such as fixed and variable capacitors . Fixed:    

:

Mica capacitor: they consist of alternate layers of metal fail thin sheet of mica. Ceramic capacitors: provide a very high dielectric constant. Plastic film capacitors: they have capacitance range up to 100pf. Electrolytic capacitors: they are polarized.

Variable There are used in a circuit where there is a need to adjust the capacitance value.

Uses of Capacitors Capacitors are used for several purposes:      

Timing - for example with a 555 timer IC controlling the charging and discharging. Smoothing - for example in a power supply. Coupling - for example between stages of an audio system and to connect a loudspeaker. Filtering - for example in the tone control of an audio system. Tuning - for example in a radio system. Storing energy - for example in a camera flash circuit.

CHAPTER 3.DESIGN AND IMPLEMENTATION OFDOORBELL WITH SOUND CONTROLLED BY SWITCH 3.1. Research and Methodology 3.1.1. Material and methodology used during designing process To verify the principal of operation of doorbell with sound controlled by switch used Analog components along with their association with other components. After designing and collecting all designed components, the implementation was carried out using different electronic equipment available in laboratory such as:    

Power Supply Cathode Ray Oscilloscope (CRO) Multimeter Printer Circuit Board (PCB)

3.1.2. Design and implementation process Our design is composed of three main parts such as: input, transfer function and output as it is shown by the following block diagram.

a. Input This part is composed by: 

switch that is used to switch on/off the voltage of 9 v from power supply;

circuit of door bell with sound controlled by switch

This doorbell circuit diagram are consist of two NE555 timer ICs. When some one presses switch S1 momentarily, the loud speaker sounds a bell tone as long as the time period of the monostable multivibrator built around IC1.

When the switch S1 pressed, IC1 is triggered at its pin 2 and output pin 3 goes high for a time period previously set by the values of POT R4 and POT R5.When the output ofIC1 goes high it resets IC2 and it starts to oscillate to make a bell sound through the speaker. The IC2 is configured as an astable multivibrator whose oscillation frequency can be varied with the help of POT R5. By adjusting the values of R4 & R5, modifications on the tone are possible. Power the circuit from a 9V battery or 9V DC power supply. Switch S1 is push button switch. The main part of this doorbell circuit are two NE555 timer ICs. When some one presses switch S1 momentarily ,the loud speaker sounds a bell tone as long as the time period of the monostable multivibrator built around IC1. 

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When the switch S1 pressed, IC1 is triggered at its pin 2 and output pin 3 goes high for a time period previously set by the values of POT R4 and POT R5.When the output ofIC1 goes high it resets IC2 and it starts to oscillate to make a bell sound through the speaker. The IC2 is configured as an astable multivibrator whose oscillation frequency can be varied with the help of POT R5.By adjusting the values of R4 & R5, modifications on the tone are possible. The circuit has to assemble on a good quality PCB or common board. The IC1 & IC2 has to be mounted on IC holders.

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Power the circuit from a 9V battery or 9V DC power supply. Switch S1 is push button switch.

4.2.Discussion C1 is the timing capacitor for the first NE555 timer.Increasing the value of C1; this may cause the frequency to be decreased .thus output value of oscillator is not the same due to the small changes of values of capacitor.As we have seen, if VR turn to its minimum values the pulse width modulation decreased and frequency increased and also if VR turn to its maximum values the pulse width modulation increased and then frequency decreased. Thus the theoritical results are differing to practical results because of some errors measurement. CHAPTER 5: CONCLUSION AND RECOMMENDATION 5.1. CONCLUSION This project has based on designing and implementation of doorbell with sound controlled by switch. Doorbells are a common convenience in homes; giving visitors a way of announcing their presence Doorbells are a common convenience in homes, giving visitors a way of announcing their presence and preventing residents from missing deliveries or guests. Doorbells are simple pieces of home equipment that let you know a visitor has arrived. They're useful if you are too far from the front door to hear someone knockingand preventing residents from missing deliveries or guests. Doorbells are simple pieces of home equipment that let you know a visitor has arrived. They're useful if you are too far from the front door to hear someone knocking. 5.2.RECOMMENDATION It is will be to separate the project period to the normal courses of studies so that students are given a sufficient time to be concentered on their project work. Sometimes projects are not submitted on the specified date because of missing of computer to use during writing project report that is why I recommended TCT and to help final year students by installing laboratory of computers for the final year students. This project was concerned to doorbell with sound controlled by switch, from that we recommended whoever wants to improve this project can go ahead.

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