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OPTIC FIBRE COMMUNICATION SYSTEM
ABSTRACT
The circuit for OPTIC FIBRE COMMUNICATION SYSTEM is designed to demonstrate the transmission and reception of a digital data through an optic fibre cable. The optic signals generated by the transmitter circuit are received by the optical receiver circuit after transmission through an optic fibre cable.This communication is much more effective than ordinary communication. It provides bandwidth in the GHz range.lt provides minimum transmission loss. It finds many applications in communication systems, measuring systems, industrial, medical and military applications.

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DONE BY: GUIDED BY:
ANJUS ANU ANAND ASHA JOHN
Ms.SumoI.N.C Mr. Asini.H

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INTRODUCTION
This project done on communication using optic fibres can be used for data transmission over small distances in computer networks, closed circuit T.V s etc. The information carrying capacity is directly proportional to the frequency or bandwidth of the carrier wave. This system uses light as a carrier wave in the frequency range 10A13 Hz to 10A16 Hz. Hence information transmission capacity increases by several order of magnitude.Thus it overcome almost all the drawbacks of communication systems involving electrical signals.
BLOCK DIAGRAM EXPLANATION
The block diagram given in the figure shows a basic optic fibre communication system.lt mainly consists of three elements
1) Optical transmitter
2) The optic fibre cable
3) The optical receiver
This general description is appropriate for analog as well as digital communication systems.Fibre optic technology and communication technology are involved in this system.
1) The optical transmitter
It consists of electronic components which convert the electrical signals into corresponding optical signals. The data in the form of electrical signal is provided to drive the circuit. This is achieved by using an astable multivibrator which generate a series of digital data in the form of ones and zeroes. This signal is used to turn ON and OFF an LED.This is done by means of a transistor switching circuit.The electrical signals are converted into light signals by an optical source consisting of an LED. These light signals are then transmitted through the optic fibre cable. The LED provides light of constant wavelength and low transmission loss. The light injected into the OFC is a faithful representation of the information.
2) The optic fibre cable
It consists of glass fibres which act as wave guide for optical signal. For long distance transmission^ or more fibres are joined together. The optic fibre is made of three layers namely core, cladding and protective covering.Optic fibre works on the principle of total internal reflection.
3) The optical receiver
It consists of a photo detector, amplifier and a signal indicator. The photodetector converts optical signal into corresponding electrical signal. Here an LDR is used to detect the incoming light signals. The amplifier amplifies the signal. An LED is used to indicate the reception of the data.
TRANSMITTER
CIRCUIT DIAGRAM
RECEIVER
CIRCUIT DIAGRAM EXPLANATION
OPTICAL TRANSMITTER
The circuit for a basic optic fibre communication system for transmitting a series of digital data is shown in the figure. For the purpose of generating the digital signals, an astable multivibrator is designed.
When the circuit is connected as shown in the above figure(pin 2 and 6) connected it triggers itself and free runs as a multivibrator.The external capacitor charges through Rl and R2 and discharges through R2 only. Thus the duty cycle may be precisely set by the ratio of these two resistors. In the astable mode of operational charges and discharges between 1/3 Vcc and 2/3 Vcc. As in the triggered mode ,the charge and discharge times are therefore frequency are independent of the supply voltage.
The charge time(output high) is given by: tl=.693(Rl+R2)Cl And the discharge time (output low) by : t2=0.693(R2)Cl Thus the total period t is given by : T=tl+t2= 0.693(R1+2R2)C1 The frequency of oscillation is then : f=l/T=1.44/(Rl+2R2)Cl The duty cycle is given by =R2/R1+2R2
The output signals thus produced by the astable multivibrator is fed to a
transistor switching circuit.For this a BF 194 transistor is used. Switching circuit
An LED is connected to the collector of the transistor which will be turned ON and OFF according to the input digital data. As the input to the base of the transistor goes high ,the transistor switches to saturation. Current passes through the transistor and therefore LED glows. As the input to the transistor goes low ,the transistor switches to cut off and therefore LED doesn't glow. Superluminiscent LEDs are used here. For proper operation of astable multivibrator ,a +10 V supply and for the switching circuit a +5V supply is used. The LED thus produces the optical signals which are to be transmitted. The LED is coupled to the OFC by means of suitable coupler without any loss of data.Thus the signal is effectively transmitted through the OFC. OPTIC FIBRE CABLE
Here multimode type OFC is used. The OFC is made from silica glass. A plastic coating is also provided. They have larger numerical aperture to facilitate efficient coupling to inherent light sources such as light emitting diodes. They provide bandwidth in the GHz range.
Optic fibre works on the principle of total internal reflection of light. When a ray of light passes from a dielectric medium of refractive
index nl (denser) to other of refractive index n2(rarer) ,and when the angle of incidence is critical angle e, then the refracted ray in the fibre just grazes the surfaces separating the two medium.ie the angle of refraction becomes 90°.When the angle of incidence becomes greater than critical angle, the light ray gets totaly internally reflected into the same medium. This phenomenon is called total internal reflection. Any light ray incident on the fibre edge at an angle greater than 0a meets the core cladding interface at an angle less than critical angle and will not be totally internally reflected and transmitted. Only the light rays that enter the fibre edge within the angle Oa will be accepted by the fibre for total internal reflection. Thus this angle of incidence 0a is called the acceptance angle. The numerical aperture of a fibre deopends on the acceptance angle 0a by the relation Sin Oa=NA.
Optic fibres are very light and easy to handle. Using these the hazards due to short circuit can be avoided. It is also ideal for secret communication because it is very difficult to tap. Optic fibres are unaffected by outdoor atmospheric conditions like lightning. Besides there is no possibility of spark from broken fibre. It will not corrode and is unaffected by most chemicals. They are also immune to electromagnetic interference and avoid crosstalk.Also transmission losses are very low.
OPTICAL RECEIVER
The light transmitted through the OFC has to be properly received. For this optical signal has to be converted into corresponding electrical form . To perform this optical detectors are used. Here an LDR is used for this purpose. The OFC is effectively coupled to the LDR without lossage of incoming data. The LDR is placed in the biasing circuit of the transistor BF547.As the incoming signal goes high, the resistance of the LDR goes low. Current flows and proper biasing is achieved. The transistor then switches to saturation. An LED is connected at the collector of the transistor as an indicator of the incoming signal. As the transistor switches to saturation, current flows and LED glows. When the incoming signal goes low ,the resistance of the LDR becomes high. Current doesn't flow. Transistor switches to cut off and therefore the LED turns OFF. Thus the data has been effectively transmitted from the transmitter circuit to the receiver.
This circuit forms the basis of all optic fibre systems.
POWER SUPPLY
A regulated power supply is an electronic circuit that is designed to provide a constant dc voltage of predetermined value across load
terminals irrespective of ac mains fluctuations or load variations. It mainly consists of an ordinary power supply and a voltage regulating device
The system requires a regulated +5 v supply for the switching circuit and a +10V supply for the astable multivibrator. A +5V supply is also needed for the receiver circuit. These can be delivered from the 230V domestic supply. Before applying this to the system we must step down this high voltage to an appropriate value. After that it should be rectified. This will provide a unidirectional current. To achieve a +5V DC we should regulate this. All these are done in power supply circuitry, which is explained below.
A 12-0-12 V step down transformer is connected to provide the necessary low voltage. The transformer also works as an isolator between the hot and cold end. The hot end refers to the 230 V supply, which is a hazardous one, and the cold one refers to the safe, low voltage. Now the hot portion appears only at the primary of the transformer.The secondary of the transformer deliver 12 V ac pulses along with a ground. This ac supply goes to a center tap rectifier, which converts the ac into a unidirectional voltage.The ripples in the resulting supply is filtered and smoothed by a 2200 microfarad /25V capacitor. The 0.1 microfarad capacitor bypasses any
high frequency noises.The resulting supply has the magnitude above 17 V. This voltage is fed to the regulator IC 7805 and 7810.This IC 7805 provides a regulated 5V positive supply at its 3rdpin.The required input for this is more than 7.5 V. The IC 7810 provides a regulated 10V positive supply at its 3 rd pin
Device Output Maximum Minimum
type voltage in input input
volts voltage in voltage in
volts volts
7805 +5 35 7.3
7810 +10 35 12.5
PCB DESIGNING AND FABRICATION
DESIGN AND PCB FABRICATION
The PCB consists of an insulating base material on which copper conductors are etched by photolithography or screen printing. The insulating materials provides electrical isolation and mechanical rigidity for the printed conductors as such it should possess the essential electrical and mechanical properties and good flexural strength, reasonable high temperature with standing capability, low moisture absorption warpage, good merchantability, good electrical resistance, high dielectric strength, low dielectric constant, low dissipation factor etc.
PHOTOGRAPHIC METHOD OF PCB FABRICATION
Photographic method is another commonly used PCB fabrication method. It is more expensive and widely used for massive production.
SCREEN PRINTING
In this method, a mesh is prepared and is placed over the copper sheets. Screen printing material is pasted over the areas where the circuit is to be land. All other areas are kept open. The different steps used in PCB fabrication are listed below :-
Cutting copper clad lamination
The copper clad laminates are manufactured in 4 inch*3 inch size. From this sheet pieces are cut off to the required size using a shearing machine. For the purpose of handling the PCB during fabrication, borderline of PCB. Hence atleast cutting PCB provides 10 mm of additional space from the actual required PCB size.
Cleaning
The copper oxides may build up on the copper surface. Inorder to remove this following procedure is required :-
a) Wipe with cotton wool socked in trichloro ethylene
b) Dipping in 10% HC1 for 1 minute at room temperature.
c) Scrub with pumice powder.
PCB LAYOUT
TRANSMITTER
PCB SCHEMATIC
CONCLUSION
This circuit can be considered as the basis for all systems utilizing the optic fibre technology. The project explains the transmission of data through an optic fibre cable. Optic fibre sensors like smoke or pollution detector,LDV,crack sensors etc has wide usage today. Besides optic fibres finds many applications in telecommunication, LAN networks, industrial applications like horoscope and remote sensing, medical applications, military applications like antitank missile system, secret communication links etc. It is expected that Photonics ,the light based systems rather than electronics, the electron flow devices will dominate in the coming years.
NE555 SA555 - SE555
GENERAL PURPOSE SINGLE BIPOLAR TIMERS
LOW TURN OFF TIME
MAXIMUM OPERATING FREQUENCY
GREATER THAN 500kHz
TIMING FROM MICROSECONDS TO HOURS
OPERATES IN BOTH ASTABLE AND
MONOSTABLE MODES
HIGH OUTPUT CURRENT CAN SOURCE OR
SINK 200mA
ADJUSTABLE DUTY CYCLE TTL COMPATIBLE
TEMPERATURE STABILITY OF 0.005% PER°C
DESCRIPTION
The NE555 monolithic timing circuit is a highly stable controller capable of producing accurate time delays or oscillation. In the time delay mode of operation, the time is precisely controlled by one external re¬sistor and capacitor. For a stable operation as an os¬cillator, the free running frequency and the duty cy¬cle are both accurately controlled with two external resistors and one capacitor. The circuit may be trig¬gered and reset on falling waveforms, and the out¬put structure can source or sink up to 200mA. The NE555 is available in plastic and ceramic minidip package and in a 8-lead micropackage and in metal can package version.
D S08
(Plastic Micropackage)
PIN CONNECTIONS (top view)
c 1 J
8 J 1 - GND
2 - Trigger
L~ 2 7 J 3 - Output
4 - Reset
5 - Control voltage
L~ 3 6 1 6 - Threshold
7 - Discharge
8-Vcc
C 4 5 J
BLOCK DIAGRAM
R1 4.7k
R2 R3 830 4.7k
R12 6.8k
Q5^|-*-£Q6 Q7^| ¢ JoB Q9^
019*1 f
IQ2C
J Q2
i
1
THRESHOLD o
[,Q1 Q^J
[011 Q12 J 5k
R14
220 >
TRIGGER o
RESET O
DISCHARGE O
roi5
[QIC
¢ _ . Q16J ¢
ro,7
R15
7k
"1 i
014
R5 10k
R6 n 1 r7 n 1
100k 100k I
GND °
TRIGGER COMPARATOR
ABSOLUTE MAXIMUM RATINGS
Symbol Parameter Value Unit
Vcc Supply Voltage 18 V
::=- Operating Free Air Temperature Range for NE555
for SA555 for SE555 0to70 -40 to 105 -55 to 125 °c
Tj Junction Temperature 150 °c
Storage Temperature Range -65 to 150 °c
OPERATING CONDITIONS
Symbol Parameter SE555 NE555 - SA555 Unit
Vcc Supply Voltage 4.5 to 18 4.5 to 18 V
Vthi Vttjg, VC|, Vreset Maximum Input Voltage Vcc Vcc V
ELECTRICAL CHARACTERISTICS
Tamb = +25°C, Vcc = +5V to +15V (unless otherwise specified)
Symbol Parameter SE555 NE555 - SA555 Unit

Min. Typ. Max. Min. Typ. Max.
Ice Supply Current (RL °°) (- note 1) Low State VCc = +5V
Vcc = +15V High State VCc = 5V 3 10 2 5 12 3 10 2 6 15 mA
Timing Error (monostable) (RA= 2k to 100kfl, C = 0.1 uF) Initial Accuracy - (note 2) Drift with Temperature Drift with Supply Voltage 0.5 30 0.05 2
100 0.2 1
50 0.1 3
0.5 %
ppm/°C %N
Timing Error (astable)
(RA, RB = ika to lookn, c = o.iuF,
Vcc = +15V) Initial Accuracy - (note 2) Drift with Temperature Drift with Supply Voltage 1.5 90 0.15 2.25 150 0.3 %
ppm/°C %/V
VCL Control Voltage level
Vcc = +15V Vcc = +5V 9.6
2.9 10
3.33 10.4 3.8 9
2.6 10 3.33 11 4 V
Vth Threshold Voltage
VCC = +15V Vcc = +5V 9.4 2.7 10 3.33 10.6 4 8.8 2.4 10 3.33 11.2 4.2 V
Ith Threshold Current - (note 3) 0.1 0.25 0.1 0.25 uA
vtrig Trigger Voltage
Vcc = +15V Vcc = +5V 4.8 1.45 5 1.67 5.2 1.9 4.5 1.1 5 1.67 5.6 2.2 V
¦trig Trigger Current (Virig = 0V) 0.5 0.9 0.5 2.0 HA
Vreset Reset Voltage - (note 4) 0.4 0.7 1 0.4 0.7 1 V
I reset Reset Current
Vreset = +0.4V Vreset = 0V 0.1 0.4 0.4 1 0.1 0.4 0.4 1.5 mA
VOL Low Level Output Voltage Vcc = +15V, l0(sink)= 10mA lo(sink) = 50mA lo(sink) = 100mA lo(sink) = 200mA Vcc = +5V, lo(sink) = 8mA lO(sink) = 5mA 0.1 0.4 2
2.5 0.1 0.05 0.15 0.5 2.2
0.25 0.2 0.1 0.4 2
2.5 0.3 0.25 0.25 0.75 2.5
0.4
0.35 V
VOH High Level Output Voltage
Vcc = +15V, lo(source) = 200mA lo(source) = 100mA Vcc = +5V, lo(source) = 100mA 13 3 12.5 13.3 3.3 12.75 2.75 12.5 13.3 3.3 V
Notes: 1. Supply current when output is high is typically 1mA less.
2. Tested at Vcc = +5V and Vcc = +15V.
3. This will determine the maximum value of RA + RB for +15V operation the max total is R = 20M£2 and for 5V operation the max total R = 3.5M12.
3/10
ELECTRICAL CHARACTERISTICS (continued)
Figure 4 : Low Output Voltage versus Output Sink Current
0.01
Figure 5 : Low Output Voltage versus Output Sink Current
Figure 6 : Low Output Voltage versus Output Sink Current
vs= 10V
2S*C
ZS'C





55'C -













2 5 10 20 'SINK1"1*1
Figure 7 : High Output Voltage Drop versus Output
Figure 8 : Delay Time versus Supply Voltage
APPLICATION INFORMATION
MONOSTABLE OPERATION In the monostable mode, the timer functions as a one-shot. Referring to figure 10 the external capaci¬tor is initially held discharged by a transistor inside the timer.
Figure 11
The circuit triggers on a negative-going input signal when the level reaches 1/3 Vcc. Once triggered, the circuit remains in this state until the set time has elapsed, even if it is triggered again during this in-terval. The duration of the output HIGH state is given by t = 1.1 R1C1 and is easily determined by figure 12.
Notice that since the charge rate and the threshold level of the comparator are both directly proportional to supply voltage, the timing interval is independent of supply. Applying a negative pulse simultaneously to the reset terminal (pin 4) and the trigger terminal (pin 2) during the timing cycle discharges the exter¬nal capacitor and causes the cycle to start over. The timing cycle now starts on the positive edge of the reset pulse. During the time the reset pulse in ap¬plied, the output is driven to its LOW state. When a negative trigger pulse is applied to pin 2, the flip-flop is set, releasing the short circuit across the external capacitor and driving the output HIGH. The voltage across the capacitor increases exponen¬tially with the time constant x = R1C1. When the volt¬age across the capacitor equals 2/3 Vcc, the compa¬rator resets the flip-flop which then discharge the ca¬pacitor rapidly and drivers the output to its LOW state.
Figure 11 shows the actual waveforms generated in this mode of operation.
When Reset is not used, it should be tied high to avoid any possibly or false triggering.
10 100 1.0 10 100 10 (td) us us ms ms ms s
ASTABLE OPERATION
When the circuit is connected as shown in figure 13 (pin 2 and 6 connected) it triggers itself and free runs as a multivibrator. The external capacitor charges through Ri and R2 and discharges through R2only. Thus the duty cycle may be precisely set by the ratio of these two resistors.
In the astable mode of operation, C1 charges and discharges between 1/3 Vcc and 2/3 Vcc. As in the triggered mode, the charge and discharge times and therefore frequency are independent of the supply voltage.
Figure 13
Figure 15 : Free Running Frequency versus Ri, R2 and Ci
PULSE WIDTH MODULATOR When the timer is connected in the monostable mode and triggered with a continuous pulse train, the output pulse width can be modulated by a signal applied to pin 5. Figure 16 shows the circuit.
Figure 16 : Pulse Width Modulator.
D =
Ri + 2R2
-O Vcc*
Figure 14
Output O
LINEAR RAMP
When the pullup resistor, RA, in the monostable cir-cuit is replaced by a constant current source, a linear ramp is generated. Figure 17 shows a circuit con¬figuration that will perform this function.
¦O Vcc'
Output o
Figure 17.
Figure 18 shows waveforms generator by the linear ramp.
T =
VBE = 0.6V
The time interval is given by :
(2/3 Vcc RE (RI+ R2) C Ri Vcc - VBE (RI+ R2>
Figure 18 : Linear Ramp.
50% DUTY CYCLE OSCILLATOR
For a 50% duty cycle the resistors RA and RE may
be connected as in figure 19. The time preriod forthe
output high is the same as previous,
ti = 0.693 RA C.
For the output low it is t.2 =
[(RARB)/(RA + RB)] CLn1
-i
2RB - RA
Thus the frequency of oscillation is f ,
ti + t2
Note that this circuit will not oscillate if RB is greater Figure 19 : 50% Duty Cycle Oscillator.
than 1/2 RA because the junction of RA and RB can-not bring pin 2 down to 113 Vcc and trigger the lower comparator.
ADDITIONAL INFORMATION Adequate power supply bypassing is necessary to protect associated circuitry. Minimum recom¬mended is 0.1 u.F in parallel with 1uP electrolytic.
Vcc = 5V Top trace : input 3V/DIV
Time = 20us/DIV Middle trace : output 5V/DIV
Ri = 47kfl Bottom trace : output 5V/DIV
R2 = 100kfl Bottom trace : capacitor voltage
Re = 2.7k£2 1V/DIV
C = 0.01N.F
D
n i i n tn
[8 5 I 1 4 u_
Dimensions Millimeters Inches
Min. Typ. Max. Min. Typ. Max.
A 3.32 0.131
a1 0.51 0.020
B 1.15 1.65 0.045 0.065
b 0.356 0.55 0.014 0.022
b1 0.204 0.304 0.008 0.012
D 10.92 0.430
E 7.95 9.75 0.313 0.384
e 2.54 0.100
e3 7.62 0.300
e4 7.62 0.300
F 6.6 0260
i 5.08 0.200
L 3.18 3.81 0.125 0.150
Z 1.52 0.060
e3
ti
u u u u
Dimensions Millimeters Inches
Min. Typ. Max. Min. Typ. Max.
A 1.75 0.069
a1 0.1 0.25 0.004 0.010
a2 1.65 0.065
a3 0.65 0.85 0.026 0.033
b 0.35 0.48 0.014 0.019
b1 0.19 0.25 0.007 0.010
C 0.25 0.5 0.010 0.020
d 45° (typ.)
D 4.8 5.0 0.189 0.197
E 5.8 6.2 0.228 0.244
e 1.27 0.050
e3 3.81 0.150
F 3.8 4.0 0.150 0.157
L 0.4 1.27 0.016 0.050
M 0.6 0.024
S 8° (max.)
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