ক্যাপাসিটর
Sunday, 15 April 2018
বেসিক ইলেকট্রনিক্স
Photo resistor
What are photoresistors?
Photo resistors, also known as light dependent resistors (LDR),
are light sensitive devices most often used to indicate the presence or absence
of light, or to measure the light intensity. In the dark, their resistance is
very high, sometimes up to 1MΩ, but when the LDR sensor is exposed to light,
the resistance drops dramatically, even down to a few ohms,
depending on the light intensity. LDRs have a sensitivity that varies with the
wavelength of the light applied and are nonlinear devices. They are used in
many applications but are sometimes made obsolete by other devices such as
photodiodes and phototransistors. Some countries have banned LDRs made of lead
or cadmium over environmental safety concerns.
Light dependent resistor
definition
Photo resistors are light sensitive resistors whose resistance
decreases as the intensity of light they are exposed to increases.
Characteristics
Types of photo resistors and
working mechanisms
Based on the materials used, photo resistors can be divided into
two types; intrinsic and extrinsic. Intrinsic photo resistors use undoped
materials such as silicon or germanium. Photons that fall on the device excite
electrons from the valence band to the conduction band, and the result of this
process are more free electrons in the material, which can carry current, and
therefore less resistance. Extrinsic photo resistors are made of materials
doped with impurities, also called dopants. The dopants create a new energy
band above the existing valence band, populated by electrons. These electrons
need less energy to make the transition to the conduction band thanks to the
smaller energy gap. The result is a device sensitive to different wavelengths
of light. Regardless, both types will exhibit a decrease in resistance when
illuminated. The higher the light intensity, the larger the resistance drop is.
Therefore, the resistance of LDRs is an inverse, nonlinear function of light
intensity.
Wavelength dependency
The sensitivity of a photo resistor varies with the light
wavelength. If the wavelength is outside a certain range, it will not affect
the resistance of the device at all. It can be said that the LDR is not
sensitive in that light wavelength range. Different materials have different
unique spectral response curves of wavelength versus sensitivity. Extrinsic
light dependent resistors are generally designed for longer wavelengths of
light, with a tendency towards the infrared (IR). When working in the IR range,
care must be taken to avoid heat buildup, which could affect measurements by
changing the resistance of the device due to thermal effects. The
figure shown here represents the spectral response of photoconductive detectors
made of different materials, with the operating temperature expressed in K and
written in the parentheses.
Sensitivity
Light dependent resistors have a lower sensitivity than photo
diodes and photo transistors. Photo diodes and photo transistors are true
semiconductor devices which use light to control the flow of electrons and
holes across PN-junctions, while light dependent resistors are passive
components, lacking a PN-junction. If the light intensity is kept constant, the
resistance may still vary significantly due to temperature changes, so they are
sensitive to temperature changes as well. This property makes LDRs unsuitable
for precise light intensity measurements.
Latency
Another interesting property of photo resistors is that there is
time latency between changes in illumination and changes in resistance. This
phenomenon is called the resistance recovery rate. It takes usually about 10 ms
for the resistance to drop completely when light is applied after total
darkness, while it can take up to 1 second for the resistance to rise back to
the starting value after the complete removal of light. For this reason the LDR
cannot be used where rapid fluctuations of light are to be recorded or used to
actuate control equipment, but this same property is exploited in some other
devices, such as audio compressors, where the function of the light dependent
resistor is to smooth the response.
Construction
and properties of Photo resistors
Since the discovery of selenium photoconductivity, many materials
have been found with similar characteristics. In the 1930s and 1940s PbS, PbSe
and PbTe were studied following the development of photoconductors made of
silicon and germanium. Modern light dependent resistors are made of lead
sulfide, lead selenide, indium antimonide, and most commonly cadmium sulfide
and cadmium selenide. The popular cadmium sulfide types are often indicated as
CdS photoresistors. To manufacture a cadmium sulfide LDR, highly purified
cadmium sulfide powder and inert binding materials are mixed. This mixture is
then pressed and sintered. Electrodes are vacuum evaporated onto the surface of
one side to form interleaving combs and connection leads are connected. The
disc is then mounted in a glass envelope or encapsulated in transparent plastic
to prevent surface contamination. The spectral response curve of cadmium
sulfide matches that of the human eye. The peak sensitivity wavelength is about
560-600 nm which is in the visible part of the spectrum. It should be noted
that devices containing lead or cadmium are not RoHS compliant and are banned
for use in countries that adhere to RoHS laws.
Typical applications for photoresistors
Photo resistors are most often used as light sensors. They are
often utilized when it is required to detect the presence and absence of light
or measure the light intensity. Examples are night lights and photography light
meters. An interesting hobbyist application for light dependent resistors is
the line following robot, which uses a light source and two or more LDRs to
determine the needed change of course. Sometimes, they are used outside sensing
applications, for example in audio compressors, because their reaction to light
is not instantaneous, and so the function of LDR is to introduce a delayed
response.
Light sensor circuit example
Light sensor
If a basic light sensor is needed, an LDR circuit such as the one
in the figure can be used. The LED lights up when the intensity of the light
reaching the LDR resistor is sufficient. The 10K variable resistor is used to
set the threshold at which the LED will turn on. If the LDR light is below the
threshold intensity, the LED will remain in the off state. In real-world
applications, the LED would be replaced with a relay or the output could be
wired to a microcontroller or some other device. If a darkness sensor was
needed, where the LED would light in the absence of light, the LDR and the two
10K resistors should be swapped.
Audio compressors
Audio compressors are devices which reduce the gain of the audio
amplifier when the amplitude of the signal is above a set value. This is done
to amplify soft sounds while preventing the loud sounds from clipping. Some compressors
use an LDR and a small lamp (LED or electroluminescent panel) connected to the
signal source to create changes in signal gain. This technique is believed by
some to add smoother characteristics to the signal because the response times
of the light and the resistor soften the attack and release. The delay in the
response time in these applications is on the order of 0.1s.
Light dependent resistor symbol
The following symbol is used to depict light dependent or photo
resistors according to the IEC standard. Sometime the resistor symbol is
circled, with the arrows outside the circle.
DC
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জব সীট
তারিখ : 18-04-2018 খ্রিঃ
v বিষয় নামকরণঃ পি.এন.পি
ও এন.পি.এন ট্রানজিস্টর বাছাই করণ।
v উদ্দেশ্যঃ
Ø ক) পি.এন.পি
ও এন.পি.এন ট্রানজিস্টর মাপিয়া বের করা।
Ø খ) এ.ভি.ও
মিটার, ট্রানজিস্টর বাছাই করার কৌশল জ্ঞান অজন করা।
Ø গ) ট্রানজিস্টর
ভাল না মন্দ পরীক্ষাকরণের পদ্ধতি অবহিত হওয়া।
Ø ঘ) ট্রানজিস্টরের
বেজ, কালেক্টর, ইমিটিার বাহির করা।
v প্রয়োজনীয় যন্ত্রপাতিঃ
Ø ১) এ.ভি.ও
মিটার-1টি,
Ø ২) স্ক্রু
ড্রাইভার ফ্লাট-1টি,
Ø ৩) স্ক্রু
ড্রাইভার ফিলিপস-1টি,
Ø ৪) নোজ
প্লায়ার্সৃ-1টি,
Ø ৫) সোল্ডারিং
আয়রন-1টি,
Ø ৬) সোল্ডার
সাকার-১টি,
Ø ৭) চিমটা-1টি,
Ø ৮) এক ব্যান্ড
রেডিও সেট-1টি।
v প্রয়োজনীয় মালামালঃ
Ø ১) পি.এন.পি
(p-n-p) ট্রানজিস্টর
Ø ২) এন.পি.এন
(n-p-n) ট্রানজিস্টর।
v ড্রয়িং/চিত্রঃ
পাতা-02
v ব্যবহারিক পদ্ধতিঃ
Ø (ক) পি.এন.পি ট্রানজিস্টর নিতে হবে।
Ø (খ) এন.পি.এন ট্রানজিস্টর এর বেইজ সনাক্ত করতে হবে।
Ø (গ) এ.ভি.ওস মিটার রেঞ্জ সিলেক্টর নব লো রেঞ্জে স্থাপন করতে হবে।
Ø (ঘ) মিটার এর সাহায্যে ট্রানজিস্টর যে কোন একটি পারে স্থির রাখি এবং মিটারের
প্রোবটি পর পর
দুইটি লেগে ধরতে হবে এবং পাঠ
নিতে হবে।
Ø (ঙ) যদি উভয় ক্ষেত্রে প্রায় সমান
রেজিস্ট্যান্স দেখায় তবে বুঝতে হবে ট্রানজিস্টরটি ভাল আছে।
Ø (চ) যদি উভয় ক্ষেত্রে রেজিস্ট্যান্স সমান না দেখায় তবে বুঝতে হবে ট্রানজিস্টর
খারাপ বা শট বা
ওপেন আছে।
Ø (ছ) মিটারের স্থির রাখা প্রোবটি যদি লাল হয় তবে স্থির রাখা লেগটি (n-type) হবে এবং এটি
হবে (p-n-p) ট্রানজিস্টার এবং প্রোবটি যদি কালো হয় তবে বেসটি (p-type) হবে অর্থাৎ
ট্রানজিস্টরটি (n-p-n) হবে।
Ø (জ) মিটারের স্থির রাখা লেগ বেসের সাথে অপর যে লেগের রেজিস্ট্যান্স সামান্য
বেশি দেখায় সেটি
ইমিটার অপরটি সাথে রেজিস্ট্যান্স কম দেখায় সেটি কালেক্টর হবে।
v সতকতাঃ
Ø (ক) ট্রানজিস্টর সেট হতে খুলার সময় খুব সাবধনতা অবলম্বন করতে হবে যাতে প্যাচ
ভেঙ্গে না যায়।
Ø (খ) ট্রানজিস্টর যে জায়গা হতে খোলা হয় সেখানে সোল্ডার আয়রন বেশীক্ষণ ধরে
রাখা ঠিক নয়।
Ø (গ) সঠিক স্থানে যন্ত্রপাতি ব্যবহার করতে হবে। অন্যথায় কাজের ব্যাঘাত ঘটতে
পারে।
v মন্তব্যঃ
Ø আমরা যদি কাজে মনোনিবেশ করে থাকি তবে ট্রানজিস্টার ভাল-মন্দ পরীক্ষা, পি.এন.পি
ও এন.পি.এন এবং ট্রানজিস্টারের বেজ, কালেক্টর, ইমিটার বাহির করার দক্ষতা অজন করতে পারলাম।
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