What is Darlington Pair?

This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. 

 
This gives the Darlington pair a very high current gain such as 10000. 

Darlington pairs are sold as complete packages containing the two transistors. 

They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor.

Static RAM Vs Dynamic RAM

CMOS Vs TTL

The difference between the two Logic Styles are:

• TTL circuits utilize BJTs while CMOS circuits utilize FETs.
• CMOS allows a much higher density of logic functions in a single chip compared to TTL.
• TTL circuits consumes more power compared to CMOS circuits at rest.
•  CMOS chips are a lot more susceptible to static discharge compared to TTL chips
• Propagation delay is more in CMOS compared to TTL
• Switching Speed is More for TTL compared to CMOS.
• CMOS devices are cheaper than TTL Devices. 
• The Power Supply requirement for TTL is  3 to 15V, small fluctuations are tolerated.
• The Power Supply requirement for CMOS is  5V ±0.25V, it must be very smooth, a regulated supply.
• TTL Can handle only less frequency compared to CMOS.

Why CMOS is Slower compared to TTL?

Since TTL has very little parasitic capacitance, the time delay is very small, & TTL is faster.

MOSFETs, based on voltage operations, have much greater capacitances, the charging & discharging of which consumes time (Ref: RC time constant), hence CMOS is slower.

An Introduction to Digital Logic Families

What is a Logic Family?

In Digital Designs, our primary aim is to create an Integrated Circuit (IC).

A Circuit configuration or arrangement of the circuit elements in a special manner will result in a particular Logic Family.

What are the advantages of creating different Logic Families?

Electrical Characteristics of the  IC will be identical. In other words, the different parameters like Noise Margin, Fan In, Fan Out etc will be identical.

Different ICs belonging to the same logic families will be compatible with each other.

Some Characteristics we consider for the selection of a particular Logic Family are:

• Supply voltage range 
• Speed of response 
• Power dissipation 
• Input and output logic levels 
• Current sourcing and sinking capability
• Fan-out 
• Noise margin

The basic Classification of the Logic Families are as follows:

• Bipolar Devices
• MOS Devices
• Hybrid Devices

 Bipolar Families:

1. Diode Logic (DL)
2. Resistor Transistor Logic (RTL)
3. Diode Transistor Logic (DTL)
4. Transistor- Transistor Logic (TTL)
5. Emitter Coupled Logic (ECL) or Current Mode Logic (CML)
6. Integrated Injection Logic (IIL)

MOS Families:

1. P-MOS Family
2. N-MOS Family
3. Complementary-MOS Family 
1.  Standard C-MOS
2. Clocked C-MOS 
3. Bi-CMOS
4. Pseudo N-MOS
5. C-MOS Domino Logic
6. Pass Transistor Logic

 Hybrid Family:

1. Bi-CMOS Family

Diode Logic 

In DL (diode logic), only Diode and Resistors are used for implementing a particular Logic.Remember that the Diode conducts only when it is Forward Biased.

Disadvantages of Diode Logic

• Diode Logic suffers from voltage degradation from one stage to the next.
• Diode Logic only permits OR and AND functions.

Resistor Transistor Logic
In RTL (resistor transistor logic), all the logic are implemented using resistors and transistors. One basic thing about the transistor (NPN), is that HIGH at input causes output to be LOW (i.e. like a inverter). In the case of PNP transistor, the LOW at input causes output to be HIGH.

RTL Circuit

Advantage:

• Less number of Transistors

Disadvantage:

• High Power Dissipation
• Low Fan In

Diode Transistor Logic
In DTL (Diode transistor logic), all the logic is implemented using diodes and transistors.

DTL Logic

Disadvantage:

• Propagation Delay is Larger

Transistor Tansistor Logic
In Transistor Transistor logic or just TTL, logic gates are built only around transistors.
TTL Logic has the following sub-families:

• Standard TTL.
• High Speed TTL
• Low Power TTL.
• Schhottky TTL.
• Low Power Schottky TTL
• Advanced Schottky TTL
• Advanced Low Power Schottky TTL
• Fast Schottky

Emitter Coupled Logic
The main specialty of ECL is that it is operating in Active Region than the Saturation Region. That is the reason for its high speed operation. As you can see in the figure, the Emitters of the Transistors Q1 and Q2 are coupled together.

Emitter Coupled Pair

Disadvantage:

• Large Silicon Area
• Large Power Consumption

Advanced Digital Audio Codecs

DTS standards
DTS (Digital Theater Sound) is a digital sound coding standard created by Universal. Compared with the Dolby Digital standard, DTS uses four times less compression and digitises sound at 20 bits instead of 16. Therefore, DTS's sound quality is theoretically higher, at the cost of a higher bit rate. To be able to play DTS-encoded media, you need a certified DTS decoder.

DTS falls into four different categories:

• DTS 6, the most commonly used 5.1 standard, which can encode six-channel sound with less compression than the Dolby Digital standard. The first five channels are used for the satellite speakers, while the last is reserved for the subwoofer. These devices are normally identified by the presence of this logo:

• DTS ES (Digital Theater Sound Extended Surround), 6.1 standard which uses an additional rear channel (rear central). DTS ES uses less compression than Dolby Digital EX.

The DTS ES standard has two variants:
o DTS ES Matrix, which has a seventh channel interpolated with the primary channels. This is called "virtualisation".
o DTS ES Discrete has an seventh independent channel.

• DTS 24/96 represents an audio format used for storing high-definition music with several channels. This format is primarily used in DVD Audio, or audio tracks which accompany video DVDs. The name comes from the fact that the tracks are recorded in 24 bits at 96 kHz. It may be in either stereo or 5.1.

• DTS Neo:6 is a format for upmixing (virtualising) from a stereo sound source.

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Dolby Digital

Dolby Digital and DTS are six-channel digital surround sound systems and are currently the standard in major motion pictures, music, and digital television.

They both use the 5.1 speaker format The format consists of three speakers across the front and two speakers in the rear. The .1 is a sixth channel called an LFE that is sent to a subwoofer.

Dolby Digital uses the AC-3 file format, which any Dolby Digital Decoder can decoder to produce 5.1 audio. Dolby Digitalis the technical name for Dolby's multi-channel digital sound coding technique, more commonly referred to as Dolby 5.1.

A six-channel sound coding process (one channel each for front, left, center, right surround, left surround and a sub-woofer) originally created by Dolby for theaters, AC-3 was subsequently adapted for home use and is now steadily becoming the most common sound format for DVD.

The difference between Dolby Digital (AC-3) and DTS is:
Both systems are great but statistics for reference only..

• DTS seems to provide a deeper and tighter low frequency presence
• DTS allows the sound to breath - transparency
• AC-3 seems to leave the impression that something is missing from the mix.
• At lower bit-rates AC-3 starts to sound like MP3's encoded at 96kbps (artifacts)

An Introduction to Digital Audio

What is sound? 

Sound is vibrations in the air; that is, a series of rising and falling pressures in the air, deviating from the average, which is represented by atmospheric pressure. The simplest way to create a sound is to make an object vibrate.

In this manner, a violin makes a sound when the bow makes its strikes vibrate, and a piano sounds a note when a key is struck, because a hammer struck a string and made it vibrate.

Speakers are generally used to reproduce these sounds. They are a membrane connected to an electromagnet; as an electrical current travels in front of and behind the magnet very rapidly, it causes vibrations in the air in front of it, and that vibration is sound! This is how sound waves are produced; they can be represented in a diagram as changes in air pressure (or in the electricity level of the magnet) as a function of time.

A sonogram, on the other hand, depicts sound frequencies as a function of time. It should be noted that a sonogram shows fundamental frequency, on top of which higher frequencies, called harmonics, are superimposed. This is what allows us to distinguish between different sources of sound: low notes have low frequencies, while high notes have higher frequencies.
 

Sound as an input and output to the Computer

Sound sampling To play sound on a computer, it must be converted into a digital format, as this is the only kind of information computers can work with. 

 

 

 

A computer program intersperses small samples of the sound (which amount to differences in pressure) at specific intervals of time. This is called sampling or digitising sound. 

 

The period of time between two samples is called the sampling rate. As reproducing audio which sounds continuous to the ear requires samples at least once every few 100,000ths of a second, it is more practical to go by the number of samples per second, expressed in Hertz (Hz). 

 

Here are a few examples of common sampling rates, and what sound quality they correspond to:
Sampling rate -Sound quality 

44,100 Hz -CD quality 

22,000 Hz -Radio quality 

8,000 Hz -Telephone quality 

 

The sampling rate of an audio CD, for example, is not arbitrary. In fact, it follows from Shannon's theorem. 

 

Sampling frequency must be high enough to preserve the form of the signal. 

 

The Nyquist-Shannon theorem stipulates that the sampling rate must be equal to or greater than twice the maximum frequency contained in the signal. 

 

 

Our ears can hear sounds up to about 20,000 Hz. Therefore, for a satisfactory level of sound quality, the sampling rate must be at least on the order of 40,000 Hz. 

 

There are several standardized sampling rates in use:
• 32 kHz: for digital FM radio (band-limited to 15 kHz)
• 44.1 kHz: for professional audio and compact discs • 48 kHz: for professional digital multitrack recording, and consumer recording equipment (like DAT or MiniDisc)

A computer works with bits, so the number of possible values that the sample could have must be determined. This is done by setting the number of bits on which the sample values are encoded.

• With 8-bit coding, there are 28 (= 256) possible values.

• With 16-bit coding, there are 216 (= 65536) possible values. 

 

The second option clearly offers higher sound fidelity, but at the cost of using more computer memory. 

 

Finally, stereo sound requires two channels, with sound recorded individually on each one. One channel is fed into the left speaker, while the other is broadcast from the right speaker. 

 

In computer processing, a sound is therefore represented by several parameters:
• The sampling rate
• The number of bits in a sample
• The number of channels (one for mono, two for stereo, and four for quadrophonic sound
 

 

Memory required for storing a sound file 

 

It is easy to calculate what size an uncompressed audio sequence will be. By knowing how many bits are used to code the sample, you know its size (as the sample size is the number of bits) 

 

To find out the size of a channel, all you need to know is the sample rate, and thus the number of samples per second, and from that the amount of space taken up by one second of music.

 

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Short Notes on Bipolar transistors

NPN Transistor

Placing P-type semiconductor between two N-type semiconductor is NPN transistor

It operates by a small current flow from the emitter to base.current will not  flow from the emitter to the collector until a small voltage (at least 0.7 volts) is applied to the base.

Collector current is much larger than the base current.

SYMBOL

 PNP Transistor

 Placing N-type semiconductor between two P-type semiconductor is PNP transistor.

Its operation is same as NPN but reversed.The voltage relations also reversed.So to turn on the device both the collector and base must be negative.

PNP symbol

Amplification:

 Transistors are used as amplifiers to increase the input signals in TV ,stereo and others applications.It is often called as linear electronics since it contains direct relation with the input and output signal.

If the base is given with the power,it gets biased.It switch on the transistor.An increase or decrease in the input signals makes an increase and decrease in the output signal but with the signal inverse.Frequency remains the same.

Gain:

The measure of amplification is the gain.

For example, if the input signal has an amplitude of 0.2 volts, and the output signal has an amplitude of 10 volts, then:

Power gain =current gain x voltage gain.

Switching

Transistors can also be used for switching in case of digital circuits.Digital circuits needs switch on and off.

When the voltage is below 0.6v no current flows through the circuit.when transistor is not conducting ,it is said to be cut-off.

When the base voltage is about 0.85 volts, sufficient base current flows to turn the transistor fully on. The collector voltage drops to approximately half a volt because of the voltage drop across the collector resistor. A transistor which is conducting the maximum current is said to be in saturation.

Transistor Application

Linear:

Amplifiers

Digital:

Computers

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3D View of PN Junction

P Type and N Type Materials- Before forming the Junction

P and N type Materials Brought together

Recombination Started (Gold Balls- Recombination)

Depletion Region Formed

Courtesy: www.ineer.org
(From the article: 'Education in Three Dimensions: Using Virtual Reality in Education for Illustrating Spatial Relationships' by ALLPORT, Christopher, SINES, Paul, SCHREINER, Brandon & DAS, Biswajit)

What are MEMS?

Think if we can integrate a GPS System on every Parcel we are sending via Courier or Post. It will be very easy for us to track the current location of the Parcel and the Handling of the Parcel. But, the system should be small like a Chip, and it should be of least cost. Don't forget that the GPS System has a Processor and an Antenna to receive the signals from the Satellite. How can we integrate the entire system inside a Small Chip? Here comes the application of MEMS.

MEMS is an emerging technology in which Microscopic Machines are developed by the tools and techniques that were developed for the Integrated Circuit (IC)ndustry.

MEMS structure

Micro-Electro Mechanical systems (MEMS) is a technology that combines computers with tiny mechanical devices such as sensors, valves, gears, mirrors, and actuators embedded in semiconductor chips. 

These machines are built on standard silicon wafers.

MEMS are made up of components between 1 to 100 micrometres in size (i.e. 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometres (20 millionths of a metre) to a millimetre (i.e. 0.02 to 1.0 mm).

They usually consist of a Central Unit that processes data (the microprocessor) and several components that interact with the outside such as Microsensors.

Components of MEMS

What are the Advantages of MEMS?

By utilizing this technology, it is possible to integrate both Microelectronic circuits and Mechanical structures on the same chip, enabling monolithic integration while reducing the microsystem size and cost considerably.

What are the Applications of MEMS? 

SMART BIOMEM (LAB ON A CHIP)

This technology has an enormous number of application areas, including 

• Automotive Eg. Accelerometers for airbag systems, Roll-over detection systems, etc.
• Biomedical Eg.Neural prosthesis devices like hearing and visual aids, Smart drug delivery systems, On Chip body fluid analysis systems, Microsurgery tools, Pacemakers
• Telecommunication Eg. Micromirrors for fiber optic switching for fast internet,Smart Antennas
• Household appliances pressure sensors for water level detection, frost sensors for refrigerators
• Consumer Applications DLP projectors, i-phone,
• Defense applications Eg. Low cost night vision, Smart munitions

Image Courtesy

www.engineersgarage.com 
www.medgadget.com

How Electric fan work

An electric fan is an electric motor with some fan blades attached to its rotating shaft. As the motor spins, the fan blades rotate. Each blade is angled a bit, and as the inclined plane of the blade moves through the air, it forces the air ahead of it forward. Each blade does this on a continuous basis, and the result is a moving air stream. The fan is taking air from the area behind itself and blowing it out the front. The fan generates a movement of air, causing the warm, less dense air to rise, and the cool, dense air to descend, thus creating a feeling of coolness in the air.

HOW METAL DETECTOR WORKS

Transmitter

Inside the metal detector's loop there is a coil of wire called the transmit coil. Electronic current is driven through the coil to create an electromagnetic field. The direction of the current flow is reversed several thousand times every second; the transmit frequency "operating frequency" refers to the number of times per second that the current flow goes from clockwise to counterclockwise and back to clockwise again.
When the current flows in a given direction, a magnetic field is produced whose polarity  points into the ground; when the current flow is reversed, the field's polarity points out of the ground. Any metallic object which happens to be nearby will have a flow of current induced inside of it by the influence of the changing magnetic field, in much the same way that an electric generator produces electricity by moving a coil of wire inside a fixed magnetic field. This current flow inside a metal object in turn produces its own magnetic field, with a polarity that tends to be pointed opposite to the transmit field.

Receiver

A second coil of wire inside the loop, the receive coil, is arranged so that nearly all of the current that would ordinarily flow in it due to the influence of the transmitted field is cancelled out. Therefore, the field produced by the currents flowing in the nearby metal object will cause currents to flow in the receive coil which may be amplified and processed by the metal detector's electronics without being swamped by currents resulting from the much stronger transmitted field.
The resulting received signal will usually appear delayed when compared to the transmitted signal. This delay is due to the tendency of conductors to impede the flow of current (resistance) and to impede changes in the flow of current (inductance). We call this apparent delay "phase shift". The largest phase shift will occur for metal objects which are primarily inductive; large, thick objects made from excellent conductors like gold, silver, and copper. Smaller phase shifts are typical for objects which are primarily resistive; smaller, thinner objects, or those composed of less conductive materials.

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Electrical Jumper

Wire Jumper

A wire jumper is a small piece of uninsulated wire that is place in two adjacent PCB holes and soldered into position based on our requirement.It is called jumper because,The wire jumps the electrical signal or power from one pcb hole to another.

Standard Wire Jumper

Sometimes wires may be used to make a connection between two contacts.

Plastic Jumper

Jumpers are also made up of non-conductive form of plastic for our convenience.When this plastic jumper is placed on the pins it made a connection between two pins.

Courtesy:imagesco.com 

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Band Stop Filter

It is also called band-elimination, band-reject, or notch filters which passes all frequencies above and below a particular range set by the component.

Twin-T Band-stop filter

The low-pass filter section is comprised of R₁, R₂, and C₁ in a “T” configuration. The high-pass filter section is comprised of C₂, C₃, and R₃ in a “T” configuration .Hence it is commonly known as a “Twin-T” filter

A band-stop filter screens out frequencies with in a certain range and thus provides easy way to frequency  outside that range. Also known as band-elimination, band-reject, or notch filters.

It  can be built by placing a low-pass filter in parallel with a high-pass filter. Both the low-pass and high-pass filter sections are of the “T” configuration, giving the name “Twin-T” to the band-stop combination.

The frequency of maximum attenuation is called the notch frequency.

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Band Pass Filter

It passes frequency with in the certain range.Circuits can be designed to accomplish this task by combining the properties of low-pass and high-pass into a single filter. The result is a band-pass filter.

Types:

1. Capacitive band-pass filter.

This construction allows the frequency that are neither too high nor too low.The response of a capacitive bandpass filter peaks within a narrow frequency range.

2.Inductive band-pass filter.

Band-pass filters can also be constructed using inductors. But, the capacitive band pass filter gives more advantage.

Here, the high-pass section comes “first” in this design instead of the low-pass section makes no difference in its overall operation. It will still filter out all frequencies too high or too low.

A band-pass filter works to screen out frequencies that are too low or too high, giving easy passage only to frequencies within a certain range.

Band-pass filters are built by stacking a low-pass filter on the end of a high-pass filter, or vice versa.

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High Pass Filter

A high-pass filter performs  the opposite function  of a low-pass filter.It offer easy passage of a high-frequency signal and difficult passage to a low-frequency signal.

TYPES:

 

1.Capacitive high-pass filter.

The capacitor's impedance increases with decreasing frequency. This high impedance in series leads to block  the low-frequency signals from getting to load.
The response of the capacitive high-pass filter increases with frequency.

2.Inductive high-pass filter.

The inductor's impedance decreases with decreasing frequency. This low impedance in parallel tends to short out low-frequency signals from getting to the load resistor. As a result, most of the voltage gets dropped across series resistor R₁.

The response of the inductive high-pass filter increases with frequency.

By Conclusion,
The capacitive design is the simplest since it requires only one component above and below the load.And, again, the reactive purity of capacitors over inductors tends to favor their use in filter design, especially with high-pass filters where high frequencies commonly cause inductors to behave strangely due to the skin effect and electromagnetic core losses.

Example:

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Low Pass Filter

A low-pass filter is a circuit that allows low-frequency very easily and does not allows high-frequency easily. There are two basic kinds of circuits

1.Capacitive low-pass filter

The capacitor's impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals.It drops most of the voltage across series resistor R1

The response of a capacitive low-pass filter falls off with increasing frequency.

2.Inductive low-pass filter

The inductor's impedance increases with increasing frequency. This high impedance in series tends to block high-frequency signals from getting to the load.

The response of an inductive low-pass filter falls off with increasing frequency.

By Conclusion,

In this case,the  inductive low-pass filter is very simple in structure. The capacitive version of this filter is not that much more complex, with only a resistor and capacitor needed for operation. Due to its complexity, capacitive filter designs are generally preferred over inductive because capacitors tend to be “purer” reactive components than inductors and therefore are more predictable in their behavior. By “pure” I mean that capacitors exhibit little resistive effects than inductors, making them almost 100% reactive. Inductors, on the other hand, typically exhibit significant dissipative (resistor-like) effects, both in the long lengths of wire used to make them, and in the magnetic losses of the core material. Capacitors also tend to participate less in “coupling” effects with other components than inductors, and are less expensive.

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what is filter?

Sometimes, Some kind of operations requires certain range of frequencies to perform its operation more efficiently.Thus a circuit is designed to perform such operation is called filter circuit.It is mostly needed in stereo systems,where certain range of audio frequency is required to be amplified.Also equalizers are used that allow the amplitude of several  frequency ranges to be adjusted to suit the listener's taste and acoustic properties of the listening area.Another one,Cross network which block certain ranges of frequencies from reaching speakers.                     

A tweeter (high-frequency speaker) is inefficient at reproducing low-frequency signals such as drum beats, so a crossover circuit is connected between the tweeter and the stereo's output terminals to block low-frequency signals, only passing high-frequency signals to the speaker's connection terminals. This gives better audio system efficiency and thus better performance. Both equalizers and crossover networks are examples of filters, designed to accomplish filtering of certain frequencies.

Practical Application:

Conditioning of non-sinusoidal voltage waveforms in POWER CIRCUIT.

some devices are sensitive to the harmonics in the power supply and hence requires power conditioning.

If there is a harmonics in the power supply,then the filter can be constructed that allows only the fundamental frequency by blocking the harmonics.

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THE PIEZOELECTRIC ACCELEROMETER (VIBRATION SENSOR) (Compression Type)

Accelerometers for the measurement of acceleration, shock or vibration come in many types using different principles of operation.

 

Inside a piezoelectric version, the sensing element is a crystal which has the property of emitting a charge when subjected to a compressive force.
In the accelerometer, this crystal is bonded to a mass such that when the accelerometer is subjected to a 'g' force, the mass compresses the crystal which emits a signal. This signal value can be related to the imposed 'g' force.

The sensing element is housed in a suitable sensor body to withstand the environmental conditions of the particular application. Body are usually made in stainless steel with welding of the various parts to prevent the ingress of dust, water, etc.
Electrical connection can be via a sealed cable or a plug/socket arrangement.
Many present accelerometers have internal electronic circuitry to give outputs which can be directed used by the associated acquisition or control systems.

Mechanical fixing of the sensor is important in order to achieve true transfer of the vibration or acceleration. Many fixing methods are used including beeswax, hard glues, threaded stud (male or female), magnetic mounts.
Accelerometers are used in many scientific and industrial applications such as predictive maintenance, aerospace, automotive, medical, process control, etc.

DC AND AC CURRENT IN ELECTRIC FIELD

Differences between alternating currents (AC) and direct currents (DC). You can read a brief synopsis of these two electrical systems and then begin your exploration of these inventions by clicking on either AC or DC in the electrical system interactive. By clicking on AC, DC, wire, battery, AC generator, or light bulb you will learn about alternating and direct currents, what exactly an electric current is, and two ways that the currents can be produced.

Alternating Current (AC)

Alternating current (AC) electricity is the type of electricity commonly used in homes and businesses throughout the world. While direct current (DC) electricity flows in one direction through a wire, AC electricity alternates its direction in a back-and-forth motion. The direction alternates between 50 and 60 times per second, depending on the electrical system of the country.

AC electricity is created by an AC electric generator, which determines the frequency. What is special about AC electricity is that the voltage can be readily changed, thus making it more suitable for long-distance transmission than DC electricity. But also, AC can employ capacitors and inductors in electronic circuitry, allowing for a wide range of applications.

Direct Current (DC) Electricity.

Direct current or DC electricity is the continuous movement of electrons from an area of negative (−) charges to an area of positive (+) charges through a conducting material such as a metal wire. Whereas static electricity sparks consist of the sudden movement of electrons from a negative to positive surface, DC electricity is the continuous movement of the electrons through a wire.
A DC circuit is necessary to allow the current or steam of electrons to flow. Such a circuit consists of a source of electrical energy (such as a battery) and a conducting wire running from the positive end of the source to the negative terminal. Electrical devices may be included in the circuit. DC electricity in a circuit consists of voltage, current and resistance. The flow of DC electricity is similar to the flow of water through a hose.

Difference between AC and DC electricity

Electrons have negative (−) electrical charges. Since opposite charges attract, they will move toward an area consisting of positive (+) charges. This movement is made easier in an electrical conductor, such as a metal wire.
Electrons move direct with DC electricity

With DC electricity, connecting a wire from the negative (−) terminal of a battery to the positive (+) terminal will cause the negative charged electrons to rush through the wire toward the positive charged side. The same thing happens with a DC generator, where the motion of coiled wire through a magnetic field pushes electrons out of one terminal and attracts electrons to the other terminal.
Electrons alternate directions in AC electricity

With an AC generator, a slightly different configuration alternates the push and pull of each generator terminal. Thus the electricity in the wire moves in one direction for a short while and then reverses its direction when the generator armature is in a different position.
AC movement of electrons in wire

The charge at the ends of the wire alternates between negative (−) and positive (+). If the charge is negative (−), that pushes the negatively charged electrons away from that terminal. If the charge is positive (+), the electrons are attracted in that direction.

Rate of change
AC electricity alternates back-and-forth in direction 50 or 60 times per second, according to the electrical system in the country. This is called the frequency and is designated as either 50 Hertz (50Hz) or 60 Hertz (60Hz).

Light bulbs in Both AC and DC

Many electrical devices—like light bulbs—only require that the electrons move. They don't care if the electrons flow through the wire or simply move back-and-forth. Thus a light bulb can be used with either AC or DC electricity.