Transistor BC548

Datasheet :

BC548

BC548 is general purpose silicon, NPN, bipolar junction transistor. It is used for amplification and switching purposes. The current gain may vary between 110 and 800. The maximum DC current gain is 800.

 

Its equivalent transistors are 2N3904 and 2SC1815. These equivalent transistors however have different lead assignments. The variants of BC548 are 548A, 548B and 548C which vary in range of current gain and other characteristics.

 

The transistor terminals require a fixed DC voltage to operate in the desired region of its characteristic curves. This is known as the biasing. For amplification applications, the transistor is biased such that it is partly on for all input conditions. The input signal at base is amplified and taken at the emitter. BC548 is used in common emitter configuration for amplifiers. The voltage divider is the commonly used biasing mode. For switching applications, transistor is biased so that it remains fully on if there is a signal at its base. In the absence of base signal, it gets completely off.

Pin Diagram :

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How comparator circuit works? Simple concepts in the study of comparator using opamp

Comparator is a circuit with two input terminals i.e. inverting and non-inverting. A standard voltage known as reference voltage Vref is connected to either inverting or non inverting terminal.

When it is connected to inverting terminal the input voltage Vin to compare is connected to non-inverting terminal or vice versa.

When the Vref = 0 and it is connected to inverting terminal, the circuit is called as INVERTING ZERO REFERENCE COMPARATOR. When it is connected to non-inverting terminal, the circuit is called as NON-INVERTING ZERO REFERENCE COMPARATOR.

When the Vref > 0 and it is connected to inverting terminal, the circuit is called as INVERTING NON-ZERO REFERENCE COMPARATOR. When it is connected to non-inverting terminal, the circuit is called as NON-INVERTING NON-ZERO REFERENCE COMPARATOR.

Whatever may be the input condition, the output of the comparator can have only three possible values i.e. output voltage Vo = +Vsat (Fully saturated positive voltage) or output voltage will be as such that it will beVo = –Vsat (Fully saturated negative voltage).

Now consider inverting zero reference comparator. For this the value ofVref = 0. The input voltage Vi is connected to non-inverting terminal. There are three possible conditions in this circuit, as follows –

When Vi > Vref, Vo = –Vsat

When Vi < Vref, Vo = +Vsat

When Vi = Vref, Vo = 0

Now consider inverting non-zero reference comparator. Let Vref = 3V. The input voltage Vi is connected to inverting terminal. There are three possible conditions in this circuit, as follows –

When Vi > Vref = 3V, Vo = –Vsat

When Vi < Vref = 3V, Vo = +Vsat

When Vi = Vref = 3V, Vo = 0

The above conditions occur because you are connecting higher i.e. positive voltage, which is greater than zero to inverting terminal. And the inverting terminal has the property to revert the polarity or sign of the voltage connected to it. So to understand the working of comparator just remember the property of inverting and non-inverting terminals of the operational amplifier.

Note: All these conditions will be exactly reversed and applicable to other circuit i.e. to the circuit of non-inverting zero reference comparator or the non-inverting non-zero reference comparator.

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Car stereo amplifier circuit using TDA2040

Given below is a car stereo amplifier circuit using  audio amplifier IC – TDA2040

Car Audio Amplifier Circuit:

Description of the circuit:

A car stereo amplifier circuit using TDA2040 is shown here. TDA2040 is a monolithic integrated audio amplifier that operates in Class AB mode. The IC has built in circuitry for short circuit protection and thermal shut down and more over it can be operated from a single supply too. The amplifier can deliver 12 watts into to a 8 ohm speaker.

In the circuit the IC is wired in order to operate from the cars 12V line. Capacitor C7 is the input DC decoupling capacitor and R4 provides feedback. Network consisting of resistor R5 and capacitor C5 provides high frequency stability and prevents any chance of oscillation. Capacitor C6 couples the ICs output to the speaker. C2 and C1 are power supply filters.

Circuit diagram of car stereo circuit using TDA2040

 
 

Notes:

• Quality of the PCB is a very crucial factor in the amplifiers performance.
• The amplifier can be operated from cars 12V line.
• Heat sink is necessary for TDA2040.
• All electrolytic capacitors must be rated 15V.
• Only one channel is shown here. For stereo application you must make one more identical copy.

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The Ways to Hack and Salvage The iPod Video LCD!

Electronics Hacks

You've a broken iPod, which has been leftover in the drawer for years but you just didn't know what you should do about it? Instead of abandoning it, why don't you take up some time and do something with it?

Basically, the 2.5" QVGA TFT LCD in the iPod video is very valuable and it can be salvaged then use in the large-scale project! For this purpose, you should make sure that you're using the iPod, which its hard disk was permanently dead (Well, if you're wealthy enough, then go ahead and use the brand new iPod Nano!).

The first problem that you might be faced in this project will be the connector, where its actually a 0.3 mm pitch FFC, and this will cause a fiddle to connect with. Since the connector is staggered altogether, with those pins on alternate sides, therefore the wires that are soldered on to it are 0.6 mm apart. Your task here is to keep them look tidily and flat, also to stop them fouling the flex cable entry and locking lever as well!

This project will take more time than you thought here, so the patient is the main key, if you want to involve yourself in it! [source]

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FM radio controlled anti-theft alarm

This FM radio-controlled anti- theft alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in the market at reasonable rate, is kept inside. Receiver is tuned to the transmitter’s frequency.

When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transistor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energized.

When an intruder tries to drive the car and takes it a few meters away from the car porch, the radio link between the car (transmitter) and alarm (receiver) is broken. As a result FM radio module gene-rates hissing noise.

Hissing AC signals are coupled to relay switching circuit via audio transformer. These AC signals are rectified and filtered by diode D1 and capacitor C8, and the resulting positive DC voltage provides a forward bias to transistor T2. Thus transistor T2 conducts, and it pulls the base of relay driver transistor T3 to ground level.

The relay thus gets de-activated and the alarm connected via N/C contacts of relay is switched on. If, by chance, the intruder finds out about the wireless alarm and disconnects the transmitter from battery, still remote alarm remains activated because in the absence of signal, the receiver continues to produce hissing noise at its output. So the burglar alarm is fool-proof and highly reliable.

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Fundamentals of resistors

Detailed information about resistors…

Resistor – the resistor is an element which opposes to the flow of electric current. Resistance is the property of the material by which it opposes to the flow of current, the current may be either AC or DC; it offers equal resistance to both. Basically their are two types of resistors : fixed and variable resistors. The fixed resistors have a given fixed value. It does not change by any physical means. A fixed resistor has a specific value printed on its body, either in the form of color codes or in numerical. The general symbols of a fixed resistor is given here. Any fixed resistor is always denoted by “R” and if there are more than one resistors then they are shown as R₁, R₂, R₃ . . . . and so on. Variable resistor has a variable value over a fixed range. Its resistance can be changed by adjusting the knob attached to the shaft of the variable resistor. A variable resistor is also subdivided into four main types : all these types have the same function, only size and shape differs. Symbols of fixed and variable resistor are given below.

The unit of resistance is the ohm, and the chief parameter for any resistor is its resistance. However there are a number of other parameters that are also important. In view of these other resistor parameters there are several different resistor types that are available. In fact choosing the right type of resistor for a given application can be important. Although many resistors will work in a variety of applications the type of resistor can be important in some cases. Accordingly it is necessary to know about the different resistor types, and in which applications each type of resistor can be used.

Basic distinction of resistor types

The first major categories into which the different types of resistor can be fitted is into whether they are fixed or variable. These different resistor types are used for different applications:

• Fixed resistors:   Fixed resistors are by far the most widely used type of resistor. They are used in electronics circuits to set the right conditions in a circuit. Their values are determined during the design phase of the circuit, and they should never need to be changed to "adjust" the circuit. There are many different types of resistor which can be used in different circumstances and these different types of resistor are described in further detail below.

• Variable resistors:   These resistors consist of a fixed resistor element and a slider which taps onto the main resistor element. This gives three connections to the component: two connected to the fixed element, and the third is the slider. In this way the component acts as a variable potential divider if all three connections are used. It is possible to connect to the slider and one end to provide a resistor with variable resistance. Further details of variable resistor can be found on the variable resistors page accessible through the "Related Articles" list which can be found on the left hand side of this page below the main menu.

Fixed resistor types

There are a number of different types of fixed resistor:

• Carbon composition:   These types were once very common, but are now seldom used. They are formed by mixing carbon granules with a binder which was then made into a small rod. This type of resistor was large by today’s standards and suffered from a large negative temperature coefficient. The resistors also suffered from a large and erratic irreversible changes in resistance as a result of heat or age. In addition to this the granular nature of the carbon and binder lead to high levels of noise being generated when current flowed.

• Carbon film:   This resistor type is formed by "cracking" a hydrocarbon onto a ceramic former. The resulting deposited film had its resistance set by cutting a helix into the film. This made these resistors highly inductive and of little use for many RF applications. They exhibited a temperature coefficient of between -100 and -900 parts per million per degree Celcius. The carbon film is protected either by a conformal epoxy coating or a ceramic tube.

• Metal oxide:   This type of resistor is now the most widely used form of resistor. Rather than using a carbon film, this resistor type uses a metal oxide film deposited on a ceramic rod. As with the carbon film, the the resistance can be adjusted by cutting a helical grove in the film. Again the film is protected using a conformal epoxy coating. This type of resistor has a temperature coefficient of around + or – 15 parts per million per degree Celcius, giving it a far superior performance tot hat of any carbon based resistor. Additionally this type of resistor can be supplied to a much closer tolerance, 5% or even 2% being standard, with 1% versions available. They also exhibit a much lower noise level than carbon types of resistor.

• Wire wound:   This resistor type is generally reserved for high power applications. These resistors are made by winding wire with a higher than normal resistance (resistance wire) on a former. The more expensive varieties are wound on a ceramic former and they may be covered by a vitreous or silicone enamel. This resistor type is suited to high powers and exhibits a high level of reliability at high powers along with a comparatively low level of temperature coefficient, although this will depend on a number of factors including the former, wire used, etc.

Classification of variable resistors – they are the variable resistors made up of carbon or wire wound type material. They are especially used for controlling voltage and current in the circuit. According to the quality they are useful for industrial, commercial and military grades circuits. There are also some modern types of variable resistors, which are known as ganged type (often called as tandem type also). They are in pairs which are connected internally to vary the values at a time. It is the most popular type of resistor. It is widely used in the control of volume, bass, treble brightness and contrast controls of tape recorders and TV sets. It has three terminals and its
resistance is fixed between the two outer terminals. The middle terminal is known as the wiper. Between the two adjacent points the resistance can be changed by turning the position of the shaft. This shaft can be attached to a control knob for convenience. It is made-up of carbon material and it has two main types : the linear type and logarithmic type. In linear type [known as LIN] the resistance changes linearly and uniformly. However in logarithmic type [known as LOG] the resistance changes on a log scale. Generally the LIN type resistors are used for controlling of volume and treble. In the control of bass, brightness and contrast the LOG types are used. In general the classification of potentiometers is as follows –

Classification of Potentiometers

Rheostat is an important device, in the high voltage and high current adjustments. They are made up of resistive wires like Nichrome, Tungsten and such high resistive materials. It has three terminals – the two end terminals are the end points of the complete wire and the middle terminals is connected to the wiper which is rested tightly on the naked portion of the wire. Its internal resistance, is decided by the length of the wire used. The basic disadvantage of this device over carbon variable resistor is that it has Ohmic resistance as well as inductive reactance (XL), since the resistive wire is wound in coil fashion. Hence it is not suitable where high frequency current is to be controlled. It is therefore specially used for limiting DC current.

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Voltage and current distribution curves in radiation pattern of a dipole antenna

Basic action of an antenna

As per Maxwell’s theory, we know that the accelerated electrons always radiate energy in the form of EM waves. When a high frequency current is passed through a conductor, then the free electrons inside the conductor get accelerated through it and thus it produces alternating EM wave or a field.

Hence, the EM waves are characterized by the so called electric field (E) and the magnetic field (H) which are always perpendicular to each other.

Concept of distribution of electric current in an antenna

When an antenna is connected in parallel with the output of a transmitter, high frequency alternating current starts flowing through the antenna (along the entire length of the antenna). Now the distribution of current through the antenna can be understood with the help of following important points and the following figure -

1. Suppose at a particular instant of time, point A is negative and point B is positive.
2. Then the flow of charges will be from point A to X and from Y to B, as shown in the above diagram.
3. Now you may wonder that since the ends of the dipole antenna are open, how this flow of charges could be possible?
4. Actually the result is as such that the end point X becomes continuously positively charged due to loss of electrons.
5. At the same time, the end point Y becomes continuously negatively charged as it gains more and more electrons.
6. In this way, the charge is maximum at both the end points X and Y.
7. So the current is zero.
8. However, at the center the charge density is zero and thus current is maximum, as it is shown clearly with a RED curve in the above diagram. It is called as “CURRENT DISTRIBTION CURVE”.
9. Here it is assumed that the feeding of signals to the antenna is a sine wave, so the current wave is also sinusoidal in nature.
10. In this way, when a dipole antenna is fed with a high frequency sinusoidal wave, from the output of a transmitter, then both types of distributions that is current and voltage distribution are produced along the length of the antenna.

Note-1: The conditions explained above are only possible in ideal conditions. When we consider, practical conditions, then some losses take place which can be found out with the help of some formulae.

Note-2: When the length of antenna is equal to half of the wavelength of transmitted signal, then such antenna is called as “Dipole Antenna”.

 Have a look at the radiation pattern animation of a dipole antenna.

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