Friday, 22 February 2013

INVENTION STORY OF TELEPHONE



TELEPHONE:

Like all other inventions, Telephone unfolds a fascinating story that lead to its birth. It all started when an inspired Scotsman named Alexander Graham Bell invented telephone in 1876. Thomas Watson had fashioned the device - a crude thing prepared out of a funnel, wooden stand, acid, and cooper wires. All these simple parts lead to the simple earliest call, belie the complicated past. Bell filed the application only few hours before his competitor Elisha Gray filed the notice to patent a telephone. By using the ideas outlined in the notice of invention by Gray, Bell created an operating telephone 3 weeks later to this.
 
Though Bell had developed novel and new ideas but these were on the basis of older developments and ideas. He succeeded in his approach because he understood the acoustics, electricity as well as study of sound. Other inventor had understood the electricity but could not understand much about acoustic just like Bell. So telephone is joint accomplishment of many pioneers.
 
Telephone history conceivably started with human history only. Man always wished to communicate far and wide. People used jungle drums, smoke signals, mirrors, semaphores and carrier pigeons for sending their message from one place to another. But telephone was certainly something new. Some people believed that Francis Bacon was one that predicted telephone in 1627; though, his book named New Utopia described long speaking tube only. Real telephone was not invented till the start of electric age. And even after it started, telephone did not seem desirable.
 
Electrical principles for building a telephone were recognized in 1831 and in 1854 Bourseul suggested speech transmission electrically. And after long 22 years, this idea turned into reality. While Jules Verne visualized the space travel and Da Vinci envisioned flight, people did not really lie awake with the thought of making a call through centuries. Who among all could have predicted pay phones on street corners or fax machines on their desks? Development of telephone did not gear up in an organized way like powered flight, with series of inventors working one after another to make a common goal a reality. It was rather string of disconnected events, some accidental, mostly electrical that made telephone a reality.
 
In the year 1729, English Chemist named Stephen Gray transmitted electricity through wire. He sent around 300 feet over the moistened thread and brass wire. Electrostatic generator powered his experiments, offering one charge at a time. In 1753, an unknown author suggested that the electricity might help in transmission of messages and he gave a scheme that used separate wires for representing each of the letters. He also posited that electrostatic generator could help in electrifying every line while attracting paper with the static charge on other end. By making a note of which all papers were attracted, one could spell the message out. Need of long wires confined the signals to few miles. Telegraphs were labored in the same fashion for many decades. Experiments in the field were on till 1800.
 
Alessandro Volta had produced earliest battery in 1800. Volta’s battery was considered as a major achievement. It offered low powered electricity current at a high cost. These chemically based batteries were improved within no time and became a source of further experimentation. And in 1820, Christian Oersted, a Danish physicist, demonstrated electromagnetism. It played as a vital idea for developing electrical power as well as to communicate. He pushed compass under the live electric wire at one of his well known experiments. Causing needle to turn northwards, compass acted as a bigger magnet. Oersted had then realized that electric current could create a magnetic field. But the question that loomed in his mind was that if it could create electricity. Electromagnetism principle, fully applied and understood, assured new age of communication.
 
In 1821, Michael Faraday reversed experiment by Oersted. He made weak current to flow in wire that revolved around permanent magnet. By doing this, he created world’s earliest electric generator. He kept working on various electrical problems for 10 years and then published results on the induction in year 1831. Though electrical dynamos were produced but understanding on how to use electromagnetism for communication was still missing.
 
American scientist named Joseph Henry transmitted practical electrical signals for the first time in 1830. In one of his classroom demonstrations, Henry presented forerunner of telegraph. He also helped Samuel Finley Breese Morse in further development. And then in 1837 Morse invented earliest workable telegraph, applied its patent in year 1838 which was granted in year 1848. Long distance operations were made possible with shared efforts of Henry and Morse’s invention of telegraph repeater. Morse was not really a professional inventor but he was encouraged by the electrical experiments. After hearing Faraday’s work on induction, he pondered over electromagnet. Eventually telegraphs became a big business and replaced the messengers, the slow paced channels of communication.
 
Johann Philip Reis completed his earliest non working telephone in year 1861. Enticingly close to speech reproduction, his instrument helped in conveying various sounds. The problem was that this device could not produce intelligent sounds. And even with dawn of 1870s, world did not have any working telephone.
 
But a major breakthrough in this field came when unique combination of voice and electricity led to actual invention of telephone by Graham Bell in 1876. He then got his patent issued for Improvements in Telegraph in the same year. Thomas Edison tried to take advantage of bell’s failure to take patent in Britain for Bell Receiver and got patent for new receiver called electro motograph.
 
In 1877 earliest permanent telephone wire covering distance of around 3 miles was strung. And commercial telephone services started in 1877 in U.S. Soon in 1879, subscribers of telephone were designated numbers instead of names. Dial phones came into being in 1880s. Eventually operators were replaced with Stronger Switch, which received dial pulses. And since then developments in the field of telephone has not come to a halt. From telegraphs to codeless phones, telephones have definitely come a long way. 

Wednesday, 13 February 2013

THEVENIN'S THEOREM


THEVENIN'S THEOREM SOLVED PROBLEMS 


SOLVED PROBLEM




 In order to determine the unknown voltage V0 and the resistance R in the circuit shown in figure 12.1 , power dissipation at the variable resistor RL is measured, for the different values of R. the maximum power dissipation at the variable resistor RL is measured as 147mW when the resistance RL is set to 3kΩ. Determine the values of V& R.




It is reccomended to workout previous problems before this. And you should try to solve this problem without looking at the answers given below. Don’t even look at  single word in the answers if you didn’t try to solve it atleast three times.



Answer:

It says that the maximum power dissipation at RL occurs at 3kΩ. So you may understand that the thevenin resistance RTH is also 3kΩ. This is one of the most important practical usages of thevenin’s theorem. This is the combination of both maximum power transfer theorem and thevenin’s theorem.



If you still can’t understand this method, consider the tehvenin equivalent circuit with the load connected. Now this circuits it exactly equivalent to the original circuit given. So instead of the given circuit we can use the thevenin equivalent circuit with load connected. Now, you may see that there are only two resistances available (RLOAD and RTH). According to maximum power transfer theorem, to get the maximum power output these two resistance values should be equal.

This means,

At maximum power dissipation,

RLOAD = RTH

So as it given in the problem, at maximum power dissipation Rload = 3kΩ

Therefore
RTH = 3KΩ

Now we can find the resistance ‘R’


From figure 12.2

[ (R+2) // 3 ] + 1 = 3 (all values in kΩ)


[ (R+2)*3 / R+5 ] + 1 = 3


3R + 6 = 2R + 10


R = 4Ω


Now we have find R.


We have given that the maximum power dissipation in RLOAD is 147mW at RLOAD = 3.

Apply,

 p = V2/R     to  RLOAD

147mW = V2/ 3kΩ

V = 21v

This is the voltage across load when the maximum power dissipates, See figure 12.2(a).



So

VTH/2 = 21V

VTH = 42V

Now see figure 12.3



Note that VC = VTH = 42V

We have grounded the point B.

Apply nodal equation to point C

(VC – V)/2kΩ  +  (VC – 0)/3kΩ – 12mA – 13mA = 0

(42 – V) /2kΩ + 42/3kΩ -25mA = 0

(42 – V) x 10-3/2 = 11x10-3

42 – V = 22

V = 20v

But this is not what we need. We need to find V0

V = V0 + V1  (see figure 12.3)

V0 + V= 20 ---------------------------(1)

Applying nodal equation to point B

(0 – V1)/ 4kΩ  + (0 - VC)/ 3kΩ  + 13mA = 0

-V1/4 + (-42)/3 +13 = 0
-V1/4 -1 = 0
V = -4v -------------(2)

substitute (2) on (1)

V = 24v

DO YOU ABOUT PIR SENSORS??

                                                            PIR SENSOR


The modern world is filled with gadgets that get excited when they sense human motion. Automatic doors in elevators and shopping malls, burglar alarms at houses and shops, automatic lighting systems, electronic amenities in washrooms are just a few examples where human presence or absence puts the device into active or passive state.  Smart, right? Now, what if we tell you that behind this smart response to motion is a gizmo that does not even reach the 2cm mark in size. Known as Pyroelectric or Passive Infrared Sensor (PIR, in both cases), this small electronic device is the curious case for this Insight.

Every object that has a temperature above perfect zero emits thermal energy (heat) in form of radiation. We, Homo sapiens, radiate at wavelength of 9-10micrometers all time of the day. The PIR sensors are tuned to detect this IR wavelength which only emanates when a human being arrives in their proximity. The term “pyroelectricity” means: heat that generates electricity (here, an electric signal of small amplitude).  Since these sensors do not have an infrared source of their own, they are also termed as passive.
How does PIR sensor selectively responds to human radiated IRs? Upto what range can this sensor work? What lies inside this sensor that makes it work? This and answers to more questions in this Insight on PIR sensors. What adds more charm to this Insight is that the Panasonic 10m sensor taken is also one of the smallest PIR sensors commercially available till date.
Motion Sensor
Image02
Image 02 shows a Panasonic 10m sensor: the PIR sensor unit enclosed in a plastic chamber. The chamber is translucent, donned with a beehive like cap and has an opening at the bottom from which the solder dip legs of the sensor protrude.
Panasonic Sensor
Image 03
On closely observing the top region of the sensor, the beehive structure, curved segments are seen.  These curved segments are Fresnel lenses which constitute an array that increases the detection zone of the sensor. Fresnel lens array is known to capture more infrared radiation and focus it to a relatively smaller point. Detection is more stable and maximum distance for detection is also increased. Fresnel lens has been crafted to be translucent so that it can capture only infrared radiation without getting unwanted radiations from visible spectrum of light.
The number of Fresnel lenses can vary in the array and this sensor has a total of 20 lenses.


Inside Motion Sensor
Image04
PIR Sensor Module
Image05
The Fresnel array loaded cap is tightly placed over the base region of the plastic molding. There is, however, no latch mechanism to hold it, making it curious to lever the Fresnel lens array out from the plastic assembly.   Beneath the Fresnel lens array is the PIR sensor which is firmly placed in the plastic moldings.  Placement of the sensor is crucial as it supposed to receive maximum amount of infrared radiation that comes from the lens array. It is hence placed in the center of the moldings where maximum radiation is converged to drop on.


PIR Filter
Image06
At the top of the sensor is the infrared filter. Looking more like a square shaped glass, this filter selects the desired wavelength at which sensor is desired to respond. Since this sensor is designed to detect human presence, the wavelength chosen is 8micrometer to 14 micrometer which is the range within which human body radiates electromagnetic rays.
TO5 Metal Package
Image07
The body of the sensor is a TO5 metal can structure. TO5 is an industry based standard used for packaging various small modules such as transistors, sensors etc. TO5 metal casing prevents the internal circuitry from external influences such as vibrations or noises which can disturb the normal working of the circuit.
Sensing Element
Image08
Uncapping the sensor shows a small PCB which hosts the sensing module, amplifier and a comparator circuit. Image 09 shows the top part of the PCB where the sensing element is placed.  Sensing elements are typically made from ferroelectric ceramic (contains lead) or lithium tantalite (lead free).  In order to increase the signal receptivity power, multiple sensing elements are taken. The sensor is a quad type and has 4 sensing elements connected in an array.
The IR rays, after passing through the filter, strike this sensing element which generates a charge. The magnitude of the charge produced is directly proportional to the amount of rays that fall on the element.  
JFET Base
Image09
After the charge gets developed, it is passed onto the amplifier circuitry from which it gets transmitted to the comparator. Further embedded in this PCB are the amplifier and comparator. Previously, these two circuits were included in the external circuitry; however, including them in the sensor module makes the circuit more compact. Since these two circuits are already pre-defined for the IR radiation spectrum, the results tend to be more accurate and precise.
Comparators are used in those sensor modules which give a digital output, like the one here.
PIR Sensor
Image10
Ceramic IC
 Image11
The base plate and the remaining structure of the sensor is a Field Effect Transistor, FET. Even though the charge produced gets amplified, it still manages to generate voltages only of the order of 1mV. FETs can work at such low voltage and can easily transmit the voltage to the processor unit through which the sensor is attached.
Image 10 details with the base plate while image 11 shows the processor unit with a few resistors on it.
Connecting Leads
Image12
At the bottom of the sensor lie, as one would expect, the connecting leads. In this sensor, they perform multitasking: allowing the sensor to be soldered, taking care of small power requirements and sending output to the processing unit.
Image12 shows details of the connecting lead placement which are arranged in the same fashion as in a JFET.  The specific function of each lead is shown in the image.
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