Electronics is the field of engineering and applied physics dealing with the design and application of devices, usually electronic circuits, the operation of which depends on the flow of electrons for the generation, transmission, reception, and storage of information. The information can consist of voice or music (audio signals) in a radio receiver, a picture on a television screen, or numbers and other data in a computer.
Electronic circuits provide different functions to process this information, including amplification of weak signals to a usable level; generation of radio waves; extraction of information, such as the recovery of an audio signal from a radio wave (demodulation); control, such as the superimposition of an audio signal onto radio waves (modulation); and logic operations, such as the electronic processes taking place in computers.
Electronic circuits consist of interconnections of electronic components. Components are classified into two categories—active or passive. Passive elements never supply more energy than they absorb; active elements can supply more energy than they absorb. Passive components include resistors, capacitors, and inductors. Components considered active include batteries, generators, vacuum tubes, and transistors.
Monday, October 27, 2008
Resistors
Resistors
The resistor is a one of a most common and reliable electronic component. It is used as a load or a part of a load, in most electronic circuits. Its major purposes are to control current and divide voltage. Some types of resistors can operate at temperature as high as 3000C & they are manufactured in a wide range of resistance value. Resistance of less than 1 ohm to over 100 Meg ohms is available.
Resistors can be classified into two basic categories
· Fixed
· Variable
Fixed Resistors
Fixed resisters are commonly used in electronic circuits. The resistance of them are fixed, but varying in a small range called tolerance mentioned their with its colour code.
Circuit symbol
Resistor Colour Code
As the fixed resisters are very small, we use special standard colour code to represent its resistance value. It is called the resister colour code.
Here is a physical diagram of a 4-colour band fixed resister. The following diagram shows what are the things represented by each colour band.
There is another method of expressing resistor values. This method is normally used when writing the values with digits. When writing values with digits, the decimal points may be erased or accidentally miss. Therefore special characters are used as the decimal point. They are as follows.
Letter Multiplier
R 1
K 1000
M 1000,000
Example:
560R means 560
2K7 means 2.7 k
39K means 39 k
1M0 means 1.0 M
The method has another letters to express the tolerance. They are as follows.
Letter Tolerance
F ±1%
G ±2%
J ±5%
K ±10%
M ±20%
Types of resistors
Resistors are grouped according to the type of material or process used to make the resistive element. The major types and their characteristics are listed bellow.
Carbon composition
The resistance element is made of finally powdered carbon and held together with an inner binding material. The resistance of the element is determined by the ratio of carbon to binding material. The physical size determines the power rating not the resistance. Carbon composition is used for both fixed and variable resistors.
Wire wound Resistor
These are used for all classification of resistors variable, fixed, adjustable and tapped. In fact nearly all taps and adjustable resistors are wire wound and the class as power resistors. The wire for wire-wounded resistor all made of alloy such as nickel-chromium or copper - nickel. The wire is wound on an insulator and the ends are connected to solder coated copper ends.
Fixed wire wound resistors come in many sizes and shapes. Some are looked just like carbon composition resistors. Wire wounded resistors are made with power rating in excess of 1000W.
Wire wound resistors have low temperature coefficient, good stability, and can be made to close tolerance.
There are other kind of fixed resistors. Surface mount, printed resistors and resistor banks are some of them.
Variable Resistor
Variable resisters can be again categorized as potentiometers, rheostat and presets.
Potentiometers
Circuit Symbol
There can be categorized in two ways as,
· Rotary arm
· Slide arm
Or
· Linear (LIN)
· Logarithmic (LOG)
They have 3 terminals. Use to vary voltage. Most potentiometers are linear. That is, one degree of sharp rotation results in the same change of resistance regardless of the shaft location. Logarithmic potentiometers have non-liner tapers. This means that the rate of change of resistance varies as the shaft is rotated. Also called tapped potentiometers. Verities of tapers are available. Tapped potentiometers are some times used as volume and tone control in stereo amplifier.
Rheostats
Circuit Symbol
Rheostats have only two terminals, turning the shaft change the resistance between these two terminals. Rheostats are used to vary current. Just using 2 of the 3 terminals uses very often potentiometers as rheostats
Preset
Circuit Symbol
These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.
Thermistor
This is another kind of resistor. The resistance of these resistance is vary with the heat and use as an input transducer. There are two kinds of thermistors. One has a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. The other one has a positive temperature coefficient (resistance increases as temperature increases).
Power rating
Resistors have a power rating as well as resistance, rating the power of the resistor indicate amount of power the resister can safely dissipate without being destroyed as current pass through the resistor. The resistor is converting electric energy in to heat energy. If the current is allowed to increase, the heat would burn up the resistor. Thus some safe level of heat must be specified and this is what the power rating does.
The manufacturer under specified conditions assigns the power rating of a resistor. These conditions assume free air circulation around the resistor and resistor leads soldered to sizable terminals. Often these conditions are not fully satisfied in the device, which use resistors. Therefore a resistor with a power rating higher than the value calculated for the circuit is used.
The resistor is a one of a most common and reliable electronic component. It is used as a load or a part of a load, in most electronic circuits. Its major purposes are to control current and divide voltage. Some types of resistors can operate at temperature as high as 3000C & they are manufactured in a wide range of resistance value. Resistance of less than 1 ohm to over 100 Meg ohms is available.
Resistors can be classified into two basic categories
· Fixed
· Variable
Fixed Resistors
Fixed resisters are commonly used in electronic circuits. The resistance of them are fixed, but varying in a small range called tolerance mentioned their with its colour code.
Circuit symbol
Resistor Colour Code
As the fixed resisters are very small, we use special standard colour code to represent its resistance value. It is called the resister colour code.
Here is a physical diagram of a 4-colour band fixed resister. The following diagram shows what are the things represented by each colour band.
There is another method of expressing resistor values. This method is normally used when writing the values with digits. When writing values with digits, the decimal points may be erased or accidentally miss. Therefore special characters are used as the decimal point. They are as follows.
Letter Multiplier
R 1
K 1000
M 1000,000
Example:
560R means 560
2K7 means 2.7 k
39K means 39 k
1M0 means 1.0 M
The method has another letters to express the tolerance. They are as follows.
Letter Tolerance
F ±1%
G ±2%
J ±5%
K ±10%
M ±20%
Types of resistors
Resistors are grouped according to the type of material or process used to make the resistive element. The major types and their characteristics are listed bellow.
Carbon composition
The resistance element is made of finally powdered carbon and held together with an inner binding material. The resistance of the element is determined by the ratio of carbon to binding material. The physical size determines the power rating not the resistance. Carbon composition is used for both fixed and variable resistors.
Wire wound Resistor
These are used for all classification of resistors variable, fixed, adjustable and tapped. In fact nearly all taps and adjustable resistors are wire wound and the class as power resistors. The wire for wire-wounded resistor all made of alloy such as nickel-chromium or copper - nickel. The wire is wound on an insulator and the ends are connected to solder coated copper ends.
Fixed wire wound resistors come in many sizes and shapes. Some are looked just like carbon composition resistors. Wire wounded resistors are made with power rating in excess of 1000W.
Wire wound resistors have low temperature coefficient, good stability, and can be made to close tolerance.
There are other kind of fixed resistors. Surface mount, printed resistors and resistor banks are some of them.
Variable Resistor
Variable resisters can be again categorized as potentiometers, rheostat and presets.
Potentiometers
Circuit Symbol
There can be categorized in two ways as,
· Rotary arm
· Slide arm
Or
· Linear (LIN)
· Logarithmic (LOG)
They have 3 terminals. Use to vary voltage. Most potentiometers are linear. That is, one degree of sharp rotation results in the same change of resistance regardless of the shaft location. Logarithmic potentiometers have non-liner tapers. This means that the rate of change of resistance varies as the shaft is rotated. Also called tapped potentiometers. Verities of tapers are available. Tapped potentiometers are some times used as volume and tone control in stereo amplifier.
Rheostats
Circuit Symbol
Rheostats have only two terminals, turning the shaft change the resistance between these two terminals. Rheostats are used to vary current. Just using 2 of the 3 terminals uses very often potentiometers as rheostatsPreset
Circuit Symbol
These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.
Thermistor
This is another kind of resistor. The resistance of these resistance is vary with the heat and use as an input transducer. There are two kinds of thermistors. One has a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. The other one has a positive temperature coefficient (resistance increases as temperature increases).
Power rating
Resistors have a power rating as well as resistance, rating the power of the resistor indicate amount of power the resister can safely dissipate without being destroyed as current pass through the resistor. The resistor is converting electric energy in to heat energy. If the current is allowed to increase, the heat would burn up the resistor. Thus some safe level of heat must be specified and this is what the power rating does.
The manufacturer under specified conditions assigns the power rating of a resistor. These conditions assume free air circulation around the resistor and resistor leads soldered to sizable terminals. Often these conditions are not fully satisfied in the device, which use resistors. Therefore a resistor with a power rating higher than the value calculated for the circuit is used.
Sunday, October 26, 2008
Capacitors
Capacitors
Capacitors are devices, those are store electric charge. They are commonly used to set the time in timing circuits, smooth curves and filter signals. Capacitance is measured with farads. (F)
The capacitance of a capacitor is determined by four factors:
1. Area of the plate
2. Distance between the plates
3. Type of dielectric material
4. Temperature
Types of capacitors
Many different types and styles of capacitors are manufactured to satisfy the needs of the electronic industry. Capacitors may be name to indicate their dielectric material, their enclosure, the process used in their construction, or their intended use.
Polarized Capacitors
Polarized capacitors are not like unpolarized capacitors as they have two poles. There are two polarized capacitors.
Electrolytic capacitors
Electrolytic capacitors provide more capacitance for their size and weight than any other types of capacitors. This is their primary advantage over other capacitors.
A common electrolytic capacitor consists of two aluminium foil plates separated by layer of fine gauze or other absorbent material. The foil plates and separators are long, narrow strips. These strips are rolled up and inserted in an aluminium container. One plate (the negative plate) is usually electrically connected to the aluminium container. Electrical terminals from the foil plates are brought out one end of the container then the container is sealed.
One of the aluminium plates is oxidized. Since aluminium oxide is an insulator, it is used for the dielectric. The separators are saturated with a chemical solution (such as borax), which is called the electrolyte. The electrolytes having relatively good conductivity serves as part of one of the plate of the capacitor. It is in direct contact with both the dielectric and the pure aluminium foil plate. The electrolyte is also necessary in forming and maintaining the oxide on one of the plate.
The method used to produce electrolytic capacitors result in the plates being polarized. When using these capacitors always keep the same voltage polarity as was used in manufacturing them. This polarity is always marked on the body of capacitor. Again the aluminium container is usually connected to the negative plate. It is never connected to positive plate.
Reverse polarity on an electrolytic capacitor cause excessively high current in the capacitor. It causes the capacitor to heat up, and it can cause the capacitor to explode. Thus, the common electrolytic capacitors are limited to use in DC circuits. (Note: - special electrolytic capacitors are manufactured for use in AC circuits, using special techniques)
Tantalum Bead Capacitors
These capacitors also behave like electrolytic, but the only difference is the size. They are smaller. So they are used where a high capacitance and high voltage needed but less space is available.
Some of these capacitors use a colour code to express the values. The upper two colours tell the significant number of the value and the spot tells the multiplier. Other colour is used to express the voltage. Standard colour code is used here, but for the spot, grey is used to mean × 0.01 and white means × 0.1. In the voltage band, the colours are used as follows.
Colour Voltage
Yellow 6.3V
Black 10V
Green 16V
Blue 2 0V
Grey 25V
White 30V
Pink 3 5V
Unpolarized Capacitors
Circuit Symbol
There are several types of unpolarized capacitors. They are small in size as well as small in the voltage handling. The lifetime is short when comparing to the electrolytic capacitors.
Film and paper capacitors
Paper capacitors and film capacitors are constructed using a rolled foil techniques described in previous section. The leads for the capacitor are then connected to the ends of completed role. After the leads are attached, the complete assembly is covered with a protective coating of insulation. Some times the insulation is molded around the assembly to produce molded capacitors. Sometimes it is dipped in plastic insulation material producing dip capacitors. Sometimes it is placed in an insulated tube and the end of the tube is then sealed. This is preferred to as tubular capacitors.
The paper-dielectric capacitor is usually bulkier; les stable, and has less dielectric resistance than the film capacitors. The paper dielectric may absorb moisture from the atmosphere if there is the slightest flow in the end seals. As changing the moisture content and the insulation resistance of the paper, as a dielectric material, it is rapidly being replaced by plastic film material. Such material as polystyrene, polyester and polycarbonate are popular replacements.
Many of the plastic film capacitors use metallised plates with these metallised film capacitors, the metal plates is deposited directly on the film this keeps the distance between plates as small as possible and produces a small compact capacitors.
Film and paper capacitors used in industrial application can be as large as several 100MF. They are often rated in terms of VA as well as DC WV. These large values are usually enclosed in a metal container, which is filled with a special insulation coil. How ever in most electronic circuits, paper and film capacitors are less than 1MF in value.
Ceramic capacitors
For low value capacitors, (less than 0.1MF) ceramic is a popular electric material. The most common style of ceramic capacitor is the disc ceramic.
They are used in a wide variety of application where low values of capacitance are needed.
Mica capacitors
Mica capacitors are limited to even lower values than are ceramic capacitors. This is because mica has a lower dielectric constant than ceramic. However it is easier to control production tolerances with mica dialectics, and mica has good high temperature characteristics.
It takes less space to provide a given value of capacitance with ceramic than with mica. Also it is easier to construct a ceramic capacitor therefore mica capacitors are not as common as ceramic capacitors.
Variable Capacitors
A variable capacitor is constructed in such manner that its value of capacitance can be varied. A typical variable capacitor (adjustable capacitor) is the rotor-stator type. There is another one called Trimmer capacitor.
Rotor-stator Capacitor
Circuit Symbol
It consists of two sets of metal plates arranged so that the rotor plates move between the stator plates. Air is the dielectric. As the position of the rotor is changed, the capacitance value is likewise changed. This type of capacitor is used for tuning most radio receivers.
Trimmer capacitors
Circuit Symbol
This capacitor consists of two plates separated by a sheet of mica. A screw adjustment is used to vary the distance between the plates, thereby changing the capacitance. This is also used as a preset. Set the values when a circuit is made.
Capacitors are devices, those are store electric charge. They are commonly used to set the time in timing circuits, smooth curves and filter signals. Capacitance is measured with farads. (F)
The capacitance of a capacitor is determined by four factors:
1. Area of the plate
2. Distance between the plates
3. Type of dielectric material
4. Temperature
Types of capacitors
Many different types and styles of capacitors are manufactured to satisfy the needs of the electronic industry. Capacitors may be name to indicate their dielectric material, their enclosure, the process used in their construction, or their intended use.
Polarized Capacitors
Polarized capacitors are not like unpolarized capacitors as they have two poles. There are two polarized capacitors.
Electrolytic capacitors
Electrolytic capacitors provide more capacitance for their size and weight than any other types of capacitors. This is their primary advantage over other capacitors.
A common electrolytic capacitor consists of two aluminium foil plates separated by layer of fine gauze or other absorbent material. The foil plates and separators are long, narrow strips. These strips are rolled up and inserted in an aluminium container. One plate (the negative plate) is usually electrically connected to the aluminium container. Electrical terminals from the foil plates are brought out one end of the container then the container is sealed.
One of the aluminium plates is oxidized. Since aluminium oxide is an insulator, it is used for the dielectric. The separators are saturated with a chemical solution (such as borax), which is called the electrolyte. The electrolytes having relatively good conductivity serves as part of one of the plate of the capacitor. It is in direct contact with both the dielectric and the pure aluminium foil plate. The electrolyte is also necessary in forming and maintaining the oxide on one of the plate.
The method used to produce electrolytic capacitors result in the plates being polarized. When using these capacitors always keep the same voltage polarity as was used in manufacturing them. This polarity is always marked on the body of capacitor. Again the aluminium container is usually connected to the negative plate. It is never connected to positive plate.
Reverse polarity on an electrolytic capacitor cause excessively high current in the capacitor. It causes the capacitor to heat up, and it can cause the capacitor to explode. Thus, the common electrolytic capacitors are limited to use in DC circuits. (Note: - special electrolytic capacitors are manufactured for use in AC circuits, using special techniques)
Tantalum Bead Capacitors
These capacitors also behave like electrolytic, but the only difference is the size. They are smaller. So they are used where a high capacitance and high voltage needed but less space is available.
Some of these capacitors use a colour code to express the values. The upper two colours tell the significant number of the value and the spot tells the multiplier. Other colour is used to express the voltage. Standard colour code is used here, but for the spot, grey is used to mean × 0.01 and white means × 0.1. In the voltage band, the colours are used as follows.
Colour Voltage
Yellow 6.3V
Black 10V
Green 16V
Blue 2 0V
Grey 25V
White 30V
Pink 3 5V
Unpolarized Capacitors
Circuit Symbol
There are several types of unpolarized capacitors. They are small in size as well as small in the voltage handling. The lifetime is short when comparing to the electrolytic capacitors.
Film and paper capacitors
Paper capacitors and film capacitors are constructed using a rolled foil techniques described in previous section. The leads for the capacitor are then connected to the ends of completed role. After the leads are attached, the complete assembly is covered with a protective coating of insulation. Some times the insulation is molded around the assembly to produce molded capacitors. Sometimes it is dipped in plastic insulation material producing dip capacitors. Sometimes it is placed in an insulated tube and the end of the tube is then sealed. This is preferred to as tubular capacitors.
The paper-dielectric capacitor is usually bulkier; les stable, and has less dielectric resistance than the film capacitors. The paper dielectric may absorb moisture from the atmosphere if there is the slightest flow in the end seals. As changing the moisture content and the insulation resistance of the paper, as a dielectric material, it is rapidly being replaced by plastic film material. Such material as polystyrene, polyester and polycarbonate are popular replacements.
Many of the plastic film capacitors use metallised plates with these metallised film capacitors, the metal plates is deposited directly on the film this keeps the distance between plates as small as possible and produces a small compact capacitors.
Film and paper capacitors used in industrial application can be as large as several 100MF. They are often rated in terms of VA as well as DC WV. These large values are usually enclosed in a metal container, which is filled with a special insulation coil. How ever in most electronic circuits, paper and film capacitors are less than 1MF in value.
Ceramic capacitors
For low value capacitors, (less than 0.1MF) ceramic is a popular electric material. The most common style of ceramic capacitor is the disc ceramic.
They are used in a wide variety of application where low values of capacitance are needed.
Mica capacitors
Mica capacitors are limited to even lower values than are ceramic capacitors. This is because mica has a lower dielectric constant than ceramic. However it is easier to control production tolerances with mica dialectics, and mica has good high temperature characteristics.
It takes less space to provide a given value of capacitance with ceramic than with mica. Also it is easier to construct a ceramic capacitor therefore mica capacitors are not as common as ceramic capacitors.
Variable Capacitors
A variable capacitor is constructed in such manner that its value of capacitance can be varied. A typical variable capacitor (adjustable capacitor) is the rotor-stator type. There is another one called Trimmer capacitor.
Rotor-stator Capacitor
Circuit Symbol
It consists of two sets of metal plates arranged so that the rotor plates move between the stator plates. Air is the dielectric. As the position of the rotor is changed, the capacitance value is likewise changed. This type of capacitor is used for tuning most radio receivers.
Trimmer capacitors
Circuit Symbol
This capacitor consists of two plates separated by a sheet of mica. A screw adjustment is used to vary the distance between the plates, thereby changing the capacitance. This is also used as a preset. Set the values when a circuit is made.
Diodes
Diodes
The diode is an electronic component, which allows current to flow only in a single direction. The rectifying action of diode is put to good use in radio receivers and power supplies.
Signal Diode and Rectifier Diode
The signal diodes can handle only small currents. But the rectifier diodes can handle much current and they are specially used to rectify alternating currents.
This is a rectify bridge circuit which is used to full wave rectification.
There are pre-made rectifier bridges available. Following are some of them.
Zener Diode
Refer to the characteristic curve of a typical rectifier (diode) in the figure below. The forward characteristic of the curve we have previously described above in the diode section. It is the reverse characteristics we will discuss here.
Notice that as the reverse voltage is increased the leakage current remains essentially constant until the breakdown voltage is reached where the current increases dramatically. This breakdown voltage is the zener voltage for zener diodes. While for the conventional rectifier or diode, it is imperative to operate below this voltage; the zener diode is intended to operate at that voltage, and so finds its greatest application as a voltage regulator.
The basic parameters of a zener diode are:
(a) Obviously, the zener voltage must be specified. The most common range of zener voltage is 3.3 volts to 75 volts, however voltages out of this range are available.
(b) A tolerance of the specified voltage must be stated. While the most popular tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are available. A test current (Iz) must be specified with the voltage and tolerance.
(c) The power handling capability must be specified for the zener diode. Popular power ranges are: 1/4, 1/2, 1, 5, 10, and 50 Watts.
Light emitting diode
The rectifier diode and the zener diode are made by creating a p-n junction between two types of semiconductor (n-type & p-type). This sort of junction acts as a one way valve for current flowing through it.
By using specially selected p-type and n-type semiconductor it is possible to make the p-n junction emit colored light to produce a light emitting diode (LED). The light is emitted when current flows through the LED in the forward-biased direction. The LED is used as a low current indicator lamp in many types of consumer and industrial equipments, such as hi-fi system and machinery control panels. The LED is also used in seven-segment displays to show numbers and letters. By selecting the right type of impurity in the semiconductor compound (usually gallium or arsenide, phospide), the color of the light emitted can be made red, yellow, green or blue. The LED does not emit light when it is reverse biased.
LEDs are used in following ways also in practically.
Bar graph Dot matrix Starburst 7-segment
Photo Diode
In the photodiode a small current flowing in the reverse-bias direction varies with the amount of light, which reaches the junction. The photodiode is used in camera light meters to measure light intensity, and also for rapid counting of the rotation of the wheel in control equipment. The photodiode is able to respond much faster to light change than the light dependent resistor (LDR).
The diode is an electronic component, which allows current to flow only in a single direction. The rectifying action of diode is put to good use in radio receivers and power supplies.
Signal Diode and Rectifier Diode
The signal diodes can handle only small currents. But the rectifier diodes can handle much current and they are specially used to rectify alternating currents.
This is a rectify bridge circuit which is used to full wave rectification.
There are pre-made rectifier bridges available. Following are some of them.
Zener Diode
Refer to the characteristic curve of a typical rectifier (diode) in the figure below. The forward characteristic of the curve we have previously described above in the diode section. It is the reverse characteristics we will discuss here.
Notice that as the reverse voltage is increased the leakage current remains essentially constant until the breakdown voltage is reached where the current increases dramatically. This breakdown voltage is the zener voltage for zener diodes. While for the conventional rectifier or diode, it is imperative to operate below this voltage; the zener diode is intended to operate at that voltage, and so finds its greatest application as a voltage regulator.
The basic parameters of a zener diode are:
(a) Obviously, the zener voltage must be specified. The most common range of zener voltage is 3.3 volts to 75 volts, however voltages out of this range are available.
(b) A tolerance of the specified voltage must be stated. While the most popular tolerances are 5% and 10%, more precision tolerances as low as 0.05 % are available. A test current (Iz) must be specified with the voltage and tolerance.
(c) The power handling capability must be specified for the zener diode. Popular power ranges are: 1/4, 1/2, 1, 5, 10, and 50 Watts.
Light emitting diode
The rectifier diode and the zener diode are made by creating a p-n junction between two types of semiconductor (n-type & p-type). This sort of junction acts as a one way valve for current flowing through it.
By using specially selected p-type and n-type semiconductor it is possible to make the p-n junction emit colored light to produce a light emitting diode (LED). The light is emitted when current flows through the LED in the forward-biased direction. The LED is used as a low current indicator lamp in many types of consumer and industrial equipments, such as hi-fi system and machinery control panels. The LED is also used in seven-segment displays to show numbers and letters. By selecting the right type of impurity in the semiconductor compound (usually gallium or arsenide, phospide), the color of the light emitted can be made red, yellow, green or blue. The LED does not emit light when it is reverse biased.
LEDs are used in following ways also in practically.
Bar graph Dot matrix Starburst 7-segment
Photo Diode
In the photodiode a small current flowing in the reverse-bias direction varies with the amount of light, which reaches the junction. The photodiode is used in camera light meters to measure light intensity, and also for rapid counting of the rotation of the wheel in control equipment. The photodiode is able to respond much faster to light change than the light dependent resistor (LDR).
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