Learn Electronics/Printable version

Learn Electronics

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1. Aim of book: To make it possible for people to read this and start creating their own electronic devices and understanding them. Introducing theories only when they apply to what the reader is doing (follow the Keep It Short and Simple rule).

Although the Electronics wikibook has about the same goal, it introduces too many theories that don't apply to beginners and no hands-on experience, making it impossible to learn out of that book. Links to some theories in that book may be added so they are not replicated on both books, unless it needs to be shortened.

2. Prerequisites: Some knowledge of physics and math is assumed

3. Materials: You may want to consider getting the following materials: Breadboard, and a source of voltage to start with. (under construction)

Sources of Direct current (DC) VoltageEdit

  • For simple hands-on experiences to learn about Electronics it is best to start with BATTERIES as a simple source of electricity. All sorts of types of batteries are available, and it is important to select the correct type for the particular usage.

I have 2 sets of 3-volt (two 1.5 volt each in series) D-size batteries, one new, call it black, and the other used, call it red. The open-circuit actual measured voltages are approximately Red=2.75 Volt, Black=2.97 Volt. Each of these sets of batteries has an internal resistance which can be calculated as follows:

..... Use a load resistance, but which? My ammeter has a range with a full-scale deflection of 0.5 Amperes, which I could get by using a resistor having about 3/0.5 = 6 Ohms (6 Ω). But I do not have such a resistor.

..... Using a 25 Ω resistor as a load, I get a current flow of 95 milliamperes (mA) from the black batteries. Calculating the total approximate resistance in circuit (internal plus external) 2.97/0.097=31.3 Ohms (total). Deduct the external 25 Ω, and get approximately 6 Ω for the internal resistance of the black battery set.

..... Using the same 25 Ω external resistor, I get a current flow of only 60 mA from the red batteries, which indicates an approximate internal resistance of (2.75/0.060) - 25 = 46 - 25 = 21 Ω.

  • Another simple source of DC is a BATTERY CHARGER. This gets its energy from one of the sockets usually available at the bottom in every room. A battery charger starts off with Alternating Current (AC) derived from the mains/line via that socket, and, using one or more rectifiers, converts it into Direct Current (DC) whose voltage keeps changing very quickly all the time from zero to maximum and back again. Often a Capacitor, also sometimes called a Condenser, is used to "smooth" the available DC voltage, and the resulting DC. It is connected across the DC voltage source.
    • An example of a battery charger with 3 different output circuits described on the label:
      • 1) for two or four AA, two or four C or two or four D rechargeable batteries (2.9V, 100 mA for two in series)
      • 2) for only one 9V rechargeable battery (9V, 5 mA), and
      • 3) for only one(?) AAA rechargeable battery (5.8V 18mA).
      • Some manufacturers have on the battery label their instruction showing for how long their rechargeable battery should be charged. Never attempt to "charge" any battery not labelled as being rechargeable - it may be dangerous.
  • Many different types of adapters are available that supply either AC or DC for toys, players/recorders, etc. at the correct voltage as required. The correct adapter must be used for each purpose.

Sources of Alternating current (AC) VoltageEdit

  • CAUTION! Using the wall socket as source can be very dangerous, and should be postponed until much is known about electricity, its uses, and safety measures associated with it.
  • The voltage available at the wall socket is too high for elementary experimentation, but transformers are available that will reduce the voltage to non-dangerous levels. Look for adapters that also are available in many different types and AC voltages.

Next chapter: Basics | Go back to Index


Chapter 1, The BasicsEdit

Electronics use electric power.

  • A battery is a source of electric power.
  • A wall socket is a source of a different kind of electric power.
  • Lightning is also a (very dangerous) source of electric power.
  • Sparks while combing some people's hair is called "Static electricity". Also some rugs can cause static electricity.

Voltage and CurrentEdit

Electronic equipment operates by controlling the flow of electrons. Electrons are part of the atom. The atom also contains neutrons and protons. The number of protons and electrons in an atom are usually equal, but if it is not the atom is called an ion.[1]

Electric current is the flow of charge from one place to another, reminiscent of the way ocean current is the flow of water from one place to another. Charge is like a tiny indivisible packet of electricity, and is carried by electrons. The charge on each electron is fixed, but if more electrons flow in a given time, then more charge is said to be flowing. An electron is a type of charge carrier - it carries charge, but it is not the only type. Later we will introduce another kind of charge carrier used in all electronics, so while we talk about electrons as the entity that carries charge, keep in mind that the term "charge carrier" is sometimes more appropriate.

Charge is important when we examine semiconductors, which is what real electronics is all about. Charges of the same sign (- and -, or + and +) repel each other, just as like poles of a magnet do. Conversely, unlike charges (+ and -) attract each other. This fact is very important. It is this attraction and repulsion of charge that causes charge to move in a conductor, thus creating a current.

Charge flows when there is a difference of potential between two points in the circuit. Potential difference is measured in volts and is sometimes called voltage. The larger the potential difference, the larger the flow of charge, all else being equal. A typical ordinary battery creates a potential difference of about 1.5 volts between its terminals. If the battery is not connected to anything, the potential difference is still there, and can be measured with a voltmeter. Only when a circuit is formed between the terminals of the battery will a path exist for electrons to flow, and electrons will be caused to flow. Current and voltage are linked by Ohm's Law, which is a very simple linear relationship.

Once it was believed that electrons traveled from the positive side of a battery to the negative side. However, when the technology was there to test this it was proven that electrons actually traveled from the negative terminal of the battery to the positive terminal. When talking about circuits and designing them, we use conventional current, which we measure as if charge flows from positive to negative - for practical purposes, the fact that the electrons really go the other way does not make any difference. The rest of this book follows standard practise and always assumes conventional current.

Current is measured in amperes or amps for short, though 1 amp is quite a large current so usually milliamps (1 thousandth of an amp) or microamps (1 millionth of an amp) are often used, especially in electronics.

To summarise: Current is the flow of charge. That charge can be thought of as flowing along the conductor. Voltage or potential difference is the force that gives rise to the current, and appears across a pair of points in a circuit, such as the terminals of a battery.

Conductors: Conductors are materials that are packed full of easily-movable electrons. All metals are conductors. All materials offer some resistance but that shouldn't matter for now as long as you're not using any extremely long lengths of wire.

Open/Closed/Short Circuit: An open circuit is one where there is a physical gap in the wire which does not allow charge to pass. A closed circuit is the path charge travels from one terminal of the battery to the other. A short circuit is where something like a piece of metal would fall onto two or more pieces of metal attached to the circuit and change the flow of electricity, resulting in undesired operation of the electronic device.

Circuit Diagrams: It would be nearly impossible to take a picture of a complicated electronic circuit and expect people to build the same one. To solve this problem, circuit diagrams are used. In circuit diagrams wires are represented by lines. When lines cross over each other they may be connected depending on how the diagram is drawn.

Example 1: Lines that intersect each other connect, and wires that have a loop where they cross are not connected LearnElectronics-Images-Chapter1-diagram1.PNG

Example 2: Lines that intersect each other do not connect. and lines that have a dot where they intersect are connected. LearnElectronics-images-chapter1-Diagram2.PNG

I'll try to make this book use the second example but remember to pay attention to circuit diagrams for things like that.

Electric power, energy, temperature, heat sinks, thermal resistance, and thermal capacitanceEdit

Light bulbs, for example, are rated at 25 Watt (W), 60 W, 100 W, etc. That is the Power they use if and whenever they are switched on, never mind the length of time they are on.

A 100 W light bulb that is switched on uses Energy at the rate of 100 Watthours(Wh) per hour (Wh/h), 100 Wh if one hour, 200 Wh if 2 hours, etc. One Kilowatthour(kWh) is 1000 Wh. Light from lightbulbs is only one of several forms of energy. Another form is heat. Heat is related to temperature.

There is a close relationship between energy and temperature. Electronic devices, such as diodes or transistors, have a "rating" of a number of Watts (W) at a given device temperature that must not be exceeded to prevent damage. For example in data sheets for transistors often there is a graph showing the collector current versus the collector-emitter voltage for different base currents. Always add, if it isn't there already, the power curve. For example, if a power transistor is rated at 50 W, then one point of this curve would be 10 Volt (V) x 5 Amperes (A), another 5V x 10 A, etc. If the outside temperature is higher, or there isn't a free flow of cooling air, then the power rating will be lower - this is called "derating". Often electronic equipment includes an internal fan to keep the temperature down to below rated limits under all reasonable operating conditions.

w:Heat sinks are very important; they are used to reduce the temperature of the electronic device. This is done by mounting the device on a heat sink, usually via a heat-conducting chemical paste, and, if necessary, insulating heat-conducting washers, permitting as much heat as possible to be conducted from the device to the heat sink. The heat sink, which has a comparatively large surface area in touch with not only the device, but also with cooling air, then transfers its heat to that air, thus keeping the device's temperature lower than it would be without the heat sink, and also permitting therefore a larger power rating, and a larger current rating, without causing any damage to the device.

Just as electrical resistance (in DC circuits) is voltage divided by current (Volts per Ampere), similarly "thermal resistance" is defined as degrees Celsius difference per Watt. Also "thermal capacitance" is defined as Wattseconds per degree Celsius.

See also

Hands OnEdit

Basic light bulb circuit: You will need a battery, low power light bulb with socket, and some wire.

Understanding the PartsEdit

Real batteries

Battery: Batteries use internal stored chemical energy to push electrons from one end to the other. The battery will run out eventually when it uses up all its internal stored energy.

Circuit Diagram for battery: Battery.png

solderless breadboard with LEDs and battery

Light Bulb: (I plan on using an LED for the light as it does not need a socket and works with a breadboard) (what's a breadboard?) A breadboard is a plastic board with lots of holes, which electronic components pins are pushed into. The holes are electrically connected in lines. (e.g. all the vertical lines are connected together). (Practical Electronics/Breadboard)


Wires simply provide a pathway for electric current to travel


Now it's time to start building this circuit.

Connect one terminal of the battery to one terminal of the light bulb socket, and the other terminal of the battery to the other terminal of the light bulb socket. That's your first circuit.

Previous chapter Foreword | Next chapter: Resistors | Go back to Index


  1. If the atom is missing electrons it is said to be positive (cation); if it has extra electrons it is said to be negative (anion).


Resistor Circuit DiagramEdit

Resistor symbol.png
Real Resistors


Voltage DropEdit

  • Think of a bucket full of water; nothing happens yet, if there is no hole at the bottom of the bucket. The height of the water in the bucket represents the electrical voltage.
  • If there is a hole at the bottom of the bucket, then water will flow out of it. That water flow represents electric current flow.
  • If the source of the voltage is a battery, then, in time, the battery's voltage will be reduced until there is no voltage left, just as the level of the water in the bucket will get lower and lower until no water is left in the bucket, with zero level.
  • If the source of the voltage is the socket in the wall, then the voltage will remain about the same; normally there will be only slight changes of that kind of voltage.

The voltage is the source of the current, and the current has some purpose, it flows through what is called a "load", which may be one or more resistors. No current flows through a resistor or equivalent, unless there is a voltage across the two terminals of the resistor. If there are at least 2 resistors, or equivalent, in series (like a river flowing via 3 points) then the voltage across each resistor is called voltage drop. Think of a river passing via points A, B, and C, where A is highest and C is lowest. There is a difference in the elevations/levels, with B minus A representing a voltage drop, and C minus B being like another voltage drop. The sum of all of the voltage drops in a circuit always equals the supply voltage, such as the battery voltage.

Ohms LawEdit

Resistors: Resistors "resist" the flow of electricity, reducing the amount of the current (measured in units called amps). Without a resistor in the circuit the current would be too high, you would ruin parts of the circuit, use up the battery too fast, or even blow the battery. Since resistors are too small to print numbers on them they are labeled with color coded bands that have to be interpreted. Resistors have 3 or 4 colorbands on them, but only the first 3 matter for now. The fourth one is either silver or gold. Silver means it is 10% above or below the value shown by the first 3 bands, while gold means 5% above or below. The first 2 bands represent a one decimal number, the third represents the number of zeros that follow.

Resistors follow Ohm's Law: (V=Voltage, I=Current (in amps), R=Resistance (in ohms))

Ohm's law is important to remember since most electronic components have a maximum and minimum voltage and amp rating: if they're too low it wouldn't work, and if they're too high it will destroy the component. As long as you have at least two values you can solve for the last one.

Colour CodeEdit

4 Band Resistor

The Standard EIA Color Code Table per EIA-RS-279 is shown on table as followed : A trick to remember is that the decimal number of a color band is equal to the amount of zeros added to the end if it's the third band.

Colour 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. Coefficient
Black 0 0 ×1
Brown 1 1 ×10
Red 2 2 ×100 (10^2)
Orange 3 3 ×1000 (10^3)
Yellow 4 4 ×10000 (10^4)
Green 5 5 ×100000 (10^5)
Blue 6 6 ×1000000 (10^6) Highest Multiplier band normally seen
Violet 7 7
Gray 8 8
White 9 9
Gold ×0.1 ±5% (J)
Silver ×0.01 ±10% (K)
None ±20% (M)

Series & ParallelEdit

Resistors are very useful in electronic design and there are 2 types of resistor connection mainly series and parallel.


2 resistors arranged in series connection

Definition of series connection: Resistor is connected end-to-end in a line to form a single path through which current can flow.

Resistors in series are summed up as shown to obtain the total resistance of their combination.


2 resistors arranged in parallel connection

Definition of parallel connection: The same wire is connected to one end of all the parts in parallel and another wire is connected to the other end of all the parts in parallel.

Resistors in parallel are the reciprocal (means: one divided by) of the total resistance of the combination of two resistors R1 and R2

Previous chapter Basics | Next chapter: Capacitors | Go back to Index



See also Dielectrics

Charging and DischargingEdit


  • Capacitors can be thought of as temporary sources of voltage. Assume that a capacitor is rated at 10 volts, then, to charge it, any voltage up to 10 V can be supplied to its terminals, possibly directly, or via a series resistance. Many capacitors require proper polarity (direction of applied voltage), otherwise damage could be the result. Capacitors need not be, but can be fully charged.
  • At the moment of first connecting the voltage source to the capacitor a large current flows into the capacitor, and, if desired, that amount of current can be limited by a series resistor. The voltage source usually has an internal resistance in addition, but that resistance may be neglected in some calculations. As the capacitor charges, the voltage at its terminals increases, and because it opposes the applied voltage therefore the charging current decreases until it stops. The capacitor is then fully charged, the voltage at its terminals then being equal to the supplied voltage.
  • Usually, if left alone for some time, a charged capacitor loses its charge slowly. Note that a voltmeter connected across the terminals of a capacitor represents a "load" and in some calculations may not be neglected.
  • If AC is connected to a capacitor, then the capacitor will charge and discharge during each cycle, with an alternating current flowing that can be computed from moment to moment by assuming that the AC voltage is actually a changing DC voltage at each moment, and carefully noting whether the existing voltage across the terminals of the partly charged capacitor adds to, or subtracts from, the applied AC voltage at that moment.


  • There are three voltages in the circuit. 1) Vin the supply voltage, 2) VR the "voltage drop", which is lost across the resistor and is proportional to the current flowing, and 3) VC the voltage across the terminals of the capacitor/condenser.


Capacitors when charged are equivalent to a DC voltage source, and the circuit components connected across the capacitor's terminals determine the magnitude of the discharge current. Any such current reduces the amount of charge, and therefore also reduces the voltage across the terminals of the capacitor.


Capacitor Circuit DiagramEdit

Capacitor symbol.png


The capacitor is another important basic circuit component. They are used as a means of storing energy. They consist of two conducting plates separated by an insulator. Charge will not flow directly through a capacitor. Instead, when a voltage is applied, charge will accumulate on both plates. The accumulation of charges creates an electric field between the two plates, which is the means of energy storage. Eventually, the plates will 'fill up' with charges and no more will accumulate. At this point, the capacitor has an amount of energy stored that is determined by its size and voltage applied across it. The energy can then be discharged into an electrical load via some manner of switch. The size of a capacitor is determined by several factors: physical size, insulator material used, closeness of the plates, etc. Voltage rating is determined similarly. Capacitor size is measured in farads. Typical capacitor sizes used are in the microfarad range, although the lower bound for typical uses can be as low as picofarads, and as high as millifarads.

At low frequencies, capacitors are fairly uninteresting. After they fill up with charge, nothing happens. Thus, at low frequencies (also, after a long time), they act as an open circuit. However, at shorter time spans (higher frequencies), they show more useful behaviors. They can be combined with a resistor in parallel to control the flow of charge into them, which allows them to charge and discharge at a certain rate. These properties are used extensively in timing circuits, such as a 555 timer square wave generator. Capacitors are also used for filtering. By pairing them with resistors in series, capacitors can reduce the amplitude of sin waves. The reduction in amplitude is a function of frequency. By tuning the values of the resistor and capacitor, the frequencies affected and magnitude of reduction can be altered.

See alsoEdit


Construction of capacitors


Resistance-Capacitor circuits

Resistance-Capacitor-Inductance circuits

FHSST: C-L circuits

Wikipedia: Capacitance

Wikipedia: Capacitor

Wikipedia: LC circuit

Wikipedia: RC circuit

Wikipedia: RLC circuit

Previous chapter Resistors | Next chapter: Inductors | Go back to Index




  • Electric circuits include resistors, capacitors, inductors, etc. A piece of wire has a very small amount of inductance, but usually we talk about inductors when we think about coils, which are a lot of wire wound in near-circles around a core which is some kind of magnetic material, such as iron. That core magnifies the inductance of the coil.
  • While the wire has some resistance, the coil has not only that resistance, but also reactance. Reactance is proportional to frequency, and coils are used mainly in AC circuits for that reason. Usually the reactance(L) (in Ohms) is much higher than the resistance(R) of the inductor (also in Ohms), with the L/R ratio being important.


There is a limit to the strength of the magnetic field of an inductor, and therefore also a limit to the inductance of the inductor. Saturation is reached when a small increase in the electric current creating the magnetic field no longer causes an increase in the magnitude of the magnetic field. This is done intentionally in "Saturable reactors".

Previous chapter Capacitors | Next chapter: Semiconductors | Go back to Index


When we take the individuall p-type and n-type semiconductors it conduct with equal facility in both the directions just like a resistor and exbhit linear conduction characteristic.such behavior is known as "OHMIC BEHAVIOUR"


A semiconductor is a material that is neither a conductor nor an insulator - it is somewhere in between. This sounds like a resistor, and indeed it is possible to make resistors from semiconductor material. However, in electronics, semiconductor has a more specific meaning.

If we take a crystal of some pure material, such as silicon, we find that each silicon atom has four bonds (this is called the valency of the atom), and each bond links it to another silicon atom. This bonding forms a crystal lattice and occupies all of the electrons in the silicon atoms. Because there are no free electrons, this material cannot pass a current, and is an insulator.

If we carefully add an additional element to the material, the situation changes. If we add an atom with a valency of five instead of four, we find that wherever an atom of the introduced element is found, four of its bonds will attach to nearby silicon atoms, but the fifth bond will be unattached, leaving a free electron available, which will carry a current. Such a material is said to be doped with the donor material, but because only a small amount of donor material is added, the number of free electrons is quite small and so it will conduct, but rather poorly.

N-type and P-typeEdit

If we dope silicon with material having a valency of five, extra free electrons are created in the crystal. These electrons carry charge in the normal manner, and are called majority carriers. Material doped in this way is referred to as N-type or "negative type" semiconductor (because the charge is carried by electrons, and such charge is actually negative with respect to conventional current).

We can also dope silicon with a donor having a valency of only three instead of four. This also creates a semiconductor material because the shortage of bonds leaves holes in the lattice. These holes are the absence of an electron, but they still carry a charge, equivalent to the charge on an electron, but of the opposite sign. These are called minority carriers. Holes carry charge in the opposite direction to normal current, because their charge has the opposite sign. In fact what happens is that current is carried by electrons as usual, but wherever an electron meets a hole, it falls into the hole, opening up a new hole behind it. So while the current is still really caused by electrons, it appears as if the holes are migrating in the opposite direction. This is exactly equivalent to a positive charge moving the opposite way.

P and N type materials are in themselves not very interesting, but remarkable properties emerge when a sandwich is formed from the two kinds of materials. Such sandwiches are called junctions and are the basis for all kinds of semiconductor devices.


A diode allows current to flow one way but not another. It is important to remember that when looking at a circuit diagram that conventional current flows in the direction of the arrow (positive to negative). The word "di-ode" means two-terminaled.

A diode is formed by bonding a single piece of N-type semiconductor to a single piece of P-type. This two-layer device consists of a single P-N junction, and is the simplest semiconductor device. When current is flowing through the device, it is said to be forward biased and has a fairly low resistance. If the voltage across the diode is reversed, the diode is said to be reverse biased and usually no current will flow; but see also Zener diodes.

How it works is as follows. When reverse biased, free electrons in the N-type material, being negative, are attracted to the positive potential applied to the diode. At the other side, positively charged holes are attracted to the negative terminal. The carriers (holes and electrons) are pulled away from the junction area, leaving it depleted of any charge carriers. This forms a depletion layer and it is an insulator, so no current can flow through the device, except in Zener diodes.

When the potential difference across the diode is reversed, the opposite happens - carriers are pushed away from the terminals towards the junction, until they reach the opposite type of material. Here, electrons are able to "fall into" holes, completing the circuit and allowing current to flow.

Diodes are used also to make DC out of AC. They are then called rectifiers.

Zener diodes are used as voltage sources.

See also more about Diodes and Zener diodes

Light-Emittting DiodeEdit

A light-emitting diode or LED is a 2-layer semiconductor device just as any typical diode is, but it is doped using exotic materials to give it its light-emitting properties. When reverse-biased, an LED blocks the current just as any diode does, but when forward biased, each time an electron falls into a hole, it gives up its energy as a number of photons, which we see as visible light.

The exact amount of energy given up dictates the colour of the LED, and this in turn is controlled by the exact proportions and types of doping material used. Low energy photons are seen as red light, while higher energies gives us orange, green and blue. White LEDs are made by producing a wide range of colours simultaneously.

Most LEDs need to have a carefully limited amount of current, since they are easy to burn out if too much flows. Thus a series resistor is necessary to make sure the current can't be too high. As long as the current is kept within the LED's design limits, they are extremely robust and have very long lifetimes - far greater than ordinary lightbulbs.

A simple experiment:

Take the a LED and the appropriate resistor and battery to make it work and simply switch the two connections of the diode to the resistor and battery. If done correctly it should light in one direction and not the other.

[1]wikipedia definition


A transistor is a semiconductor device with three terminals. There are numerous types of transistors, but in general they all do the same job - they control a current under the influence of another, usually much smaller current or voltage. The most basic type of transistor is the field effect transistor (FET), and as these are what is used in most kinds of integrated circuit, they are also the most common.

An FET consists of a rod of doped semiconductor material (say, N-type) which is surrounded by a metal plate. The metal plate is insulated from the semiconductor and there is no P-N junction here. The metal plate is connected to the control terminal of the device or gate. The two ends of the rod (called the channel) are connected to the source and the drain. A circuit is so arranged so that a current flows through the device from source to drain. Because the material is semiconducting, some current will flow even if the gate terminal is not connected.

If we connect up the gate so that we can change the voltage on it, the metal plate can accumulate charge, just like a capacitor. The charges on this plate can attract or repel the carriers in the channel. If carriers are repelled away from the gate, the channel increases in resistance, because fewer carriers can get through in the region of the repulsion. By changing the design of the gate, it will attract carriers, making available more carriers to carry the current, which will thus increase. So by changing the voltage on the gate, the resistance or conductance of the channel can be varied at will.

When a current is changed by the influence of a voltage, this is called transconductance. The word "transistor" was originally coined to mean a "transconducting resistor".

The type of FET we have just described is sometimes called an "insulated gate" FET or IGFET. It is the simplest kind. There are others, such as the Metal-Oxide Semiconductor FET (MOSFET), or the Junction FET or JFET, but all operate on similar principles.

Historically, the FET was not the first kind of transistor that was invented, though the FET as described above works in a very similar fashion to the much older thermionic valve or tube. When transistors were first invented, the type that was made is known as a bipolar junction transistor. This is the kind usually meant by the unqualified term "transistor". Junction transistors are more complicated than FETs however, which is why we explained the FET first.

Bipolar Junction TransistorsEdit

A Bipolar Junction Transistor or BJT consists of a three-layer sandwich of semiconductor material, arranged N-P-N or P-N-P. The central section is usually much narrower than the other two and is connected to a terminal called the base. The outer areas are connected to terminals called the collector and the emitter.

At first glance a BJT looks a bit like two diodes placed back-to-back, and indeed if one tests a BJT using a multimeter it may be treated as one when testing between the base and either of the other two terminals. However, its physical behaviour is not the same as two diodes back-to-back, because of the very close physical proximity of the two junctions. This is an important point to note to understand the working of a BJT.

In a circuit, a current flows from the collector to the emitter through the device. If the base terminal is not connected, in fact no current will flow because at least one of the internal junctions is reverse biased, so just like the diode, carriers are repelled away from the junction join to form an insulating depletion layer. Unlike the diode, this depletion layer is not altered by reversing the voltage across the device, but by injecting extra carriers via the base terminal.

If a small current is arranged to flow into the base terminal, it adds carriers to the depletion layer, making it narrower. This reduces its resistance and allows more current to flow across the junction from collector to emitter. This current can be very much greater than the current flowing the in the base, yet is proportional to it, thus the base current acts as a control for the collector-emitter current.

The ratio between the base current and the collector current is called the gain of the device (also called its beta), and could be 100 times or even more. Thus in one sense a transistor amplifies the signal - the much larger current changes in sympathy with the smaller base current. However this relationship is not linear - overall it follows a square law - the collector current varies in proportion to the square of the base current. However over limited ranges the output is more or less linear, and usually for amplification purposes a transistor is operated on this part of the curve. The fact that it is a curve is where distortion arises when amplifying music or other small signals.


See also Electronics/RAM and ROM

Previous chapter Foreword | Next chapter: Integrated Circuits | Go back to Index

Integrated Circuits

It is possible to produce microscopic-sized transistors, diodes, resistors, capacitors, and inductors, then connect them together using special custom designs to obtain certain complete circuits, integrated circuits.

See alsoEdit

Also from Wikipedia:

Previous chapter Semiconductors | Next chapter: Analogue and Digital | Go back to Index

Analogue and Digital

Analogue electronics handles analogue signals. Analogue signals are signals having continuously varying amplitudes with respect to the time. For example the loudspeaker of a TV set or of a radio receives a voltage that usually is continually changing, which in turn causes a current to flow that changes in the same way and that activates the speaker cone to move, causing sound. This is an analogue device.

Digital electronics handles digital signals. Digital signals are discrete signals. They have constant amplitude for a specific time period. Walking could be regarded as similar to a digital device. The number of steps per minute can vary, but each step is usually nearly the same. A digital signal can be regarded as a number "packages" that are almost the same, but the number of these packages that are delivered per minute varies.

In another context, there are analogue watches and digital watches. The analogue watches have a minute hand and an hour hand, they keep moving all the time, while a digital watch has numbers showing, they change only once in a while.

See also, from WikipediaEdit

Previous chapter Integrated Circuits | Next chapter: Digital Circuits | Go back to Index

Digital Circuits

Digital CircuitsEdit

Digital electronics handles digital signals. Digital signals are discrete signals. They have constant amplitude for a specific time period. Digital Circuits are a very important part of Electronics, and learning at least their basic concepts is highly recommended.

See alsoEdit

From Wikipedia:

From the Web:

Previous chapter Analogue and Digital | Next chapter: Microprocessors | Go back to Index


Microprocessors do a lot of things, but before buying one it is very important to ensure it will be the right one for the specific application(s) in which it will be used. Compare with Central Processing Unit (CPU). There is an urgent need to ensure that sufficient information about usage and instructions are available.

An exampleEdit

Reference: "Commodore 64 Personal Computer Programmer's Reference Guide" published in 1982 by Commodore Business Machines, Inc. There is a detailed description of that computer's "brain", the 6510 microprocessor chip's specifications, starting at page 402. This microprocessor looks like an insect with a rectangular body, and 20 legs on each of two sides of that body.

  • The power supply should normally be close to 5 volts DC, although the absolute maximum voltage range is minus 0.3 to plus 7.0 Volts, with about 125 milliamps (mA) normally used.
  • A 2-phase non-overlapping clock signal is required as well, also at about 5 Volt.
  • There are 16 connections to the address bus, and an address can therefore be any number between 0 and 65535 (=216minus 1)
  • The data bus has 8 connections, so that any address can have any number between 0 and 255 (=28minus 1) in it.
  • A part of the external memory can be used as a set of instructions, the rest being available for data; there is a lot that this microprocessor can do.
  • For example, feeding in the instruction 169 followed by 123 will put the number 123 into a small memory (address 780) called the Accumulator; this is called "Accumulator addressing". There are also "Immediate addressing", "Absolute addressing", "Zero page addressing", "Indexed zero page addressing", "Indexed absolute addressing", "Implied addressing","Relative addressing", "Indexed indirect addressing", "Indirect indexed addressing", and "Absolute indirect".
  • Various kinds of calculations are possible, some of them quite involved, using proper software. This microprocessor can be used also in very many other applications.

See alsoEdit


Previous chapter Digital Circuits | Next chapter: Alternating Current | Go back to Index

Alternating Current

A (coil of) wire and a magnet, generating ACEdit

Magnets attract iron and a few other metals, but here we are concerned with generating electricity. A simple experiment with iron filings and a magnet shows that there is such a thing as a magnetic field; that field is strongest (the force of attraction is highest) close to the poles of the magnet, and gets weaker further away from the poles.

A wire is usually made of copper, and copper is neither attracted nor repelled by a magnet, however if a wire, usually of copper, is inside a CHANGING magnetic field, or if the wire itself is moved to a place where the magnetic field is of DIFFERENT strength, then a voltage is generated between the two ends of that wire. If there are two wires that are connected in series, then that voltage will be twice as much, and if it is a coil, in that CHANGING magnetic field, with n number of turns, then the voltage will be n times as large. If a voltmeter is connected to the terminals of that coil, and the coil is moved, or the magnet is moved, then the voltage will be shown on the voltmeter. That voltage's direction will keep changing, it will ALTERNATE, as the changes are made, therefore that voltage is an Alternating Voltage, and if there is a circuit completed between the two terminals of the coil, then an Alternating Current (AC) will flow in that circuit.

  • Usually one or more coils of wire are fixed on the outside of a circle, and an electromagnet is rotated inside, causing an alternating voltage and/or alternating current (AC) to be generated.

1 Think of a simple magnet that rotates; what happens? Starting with the magnet in a position so that the coil is half-way between the poles of the magnet, the magnetic field strength is zero at the coil, and therefore the voltage across the terminals of the coil is zero.

2. As the magnet rotates a little bit further, one of the poles gets closer to the coil, the fieldstrength changes, and therefore a voltage appears across the coil's terminals; the polarity of that voltage depends on how the voltmeter is connected; reversing the leads will reverse the indicated polarity of the voltage. Also the polarity depends on whether the North-pole or the South-pole is closest to the coil. Because the magnet rotates, its poles ALTERNATE getting closer to the coil, and therefore the indicated voltage ALTERNATES as well.

  • Normally the resulting AC voltage is connected to a transformer in order to increase the voltage to a suitable level for transmission over great distances, but in addition there are sensors and switches designed to switch off if the current is too high, and also to ensure the rotation of the magnet or equivalent is then stopped - or else the speed of rotation would increase, causing considerable destruction, even, maybe, loss of life.


This is a ruf draft. Some chapters are likely to be removed others may be added. The idea is to keep a structure as Theory->Some Exercises->Experiment. This may not be possible for all chapters. Please add comments, more topics or new chapters. --Patrik 12:16, 20 July 2005 (UTC)

Part 1 DC: Direct CurrentEdit

What is electricity?Edit

What is electricity physicaly, What is voltage, current and resistance?

Voltage, current and Resistance. Ohm’s Law.Edit

Math. relation between U, I and R. Where it applies, where it doesn’t. Examples of magnitudes of values for U, I and R.
Experiment: measuring current and voltage in a simple circuit. (battery + small light bulb). Introduces how to measure these quantities.

The Resistor.Edit

What is a resistor, how does it looks like, how does it work. simple circuits with resistors (serie, parallel, and combinations) and how to solve them.
Expermiment: measuring resistance. Check if your calculation is correct with an ohm meter.

Resistance of wires.Edit

How to calculate the resistance of a certain length of wire. Loses in wires.
Experiment: Calculate the resistance of a piece of wire and the check your result.

The Voltage divider.Edit

What it is, how it works, how to calculate it. How a load (resistor from divider to ground) modifies its output. The pot-meter.
Experiment: building a voltage divider and check out the math and the load. Try out a voltage divider build with a pot-meter.

Special resistors.Edit

Light Dependent Resistors, Temperature dependent resistors.
Experiment: Measuring resistance of these resistors in different conditions.

The Capacitor.Edit

What is a capacitor. construction, uses. Capacity (what it is, where it depends on) Charging and discharging. The RC circuit (charge and discharge graph. The meaning of the value of R*C (or tau), meaning of 5 tau.
Experiment: measuring and drawing the charge and discharge graph of an RC circuit. Check if tau (calcutated) is correct. Same for 5 tau.


What are semiconductors. the PN junction. Diode, led and zener diode.
experiment: simple circuit (battery, diode and resistor), measure in which direction the diode blocks and in which it conducts. Same experiment with led, Getting a fixed voltage with a zener.

Next chapters of this part are optional. They may be kept for a differend part or something. These chapters may benefit from having AC to explain them completely.

Transistors in DC.Edit

What’s a transistor, how does it work, transistor models, basic circuits. Why its beta (HFE and Hfe. how much the transistor amplifies the basis current) is pretty useless for designing circuit (it varies too much in real life transistors. this is commonly noted as a value “between 100 and 300” or simular.).
Experiment: test a few transistor circuits. Measure the beta of a few transistors of the same type.

Light-sensitive semiconductors.Edit

Phototransistor, opto-couplers, photo-diode.
Experiment: Measuring some values in simple circuits with phototransistor and photodiode. building a light meter. Simple circuit with an optocoupler.

The Field Effect Transistor or FET.Edit

What’s a FET, how does it work. different types, uses, basic circuits. FET- transistor (dis)simularities. (dis)advantages.
Experiment: Similar experiments as the transistor.

Part 2 Digital ElectronicsEdit

What is digital electronicsEdit

what does digital means, benefits of digital technology (in contrast to analog electronics), limits and problems of digital electronics.

Digital Numbers and MathEdit

What are binary numbers, how to add, substract, multiply and divide them. what are shifts. Boolean operators.

Boolean algebraEdit

boolean algebra, Demorgan

Karnaugh diagramsEdit

diagrams for simplifying digital equations

Real Life Digital Cicuits: TTL IC'sEdit

What are IC's, what is TTL, voltage levels, how to interface them, push-pull and open collector outputs, fan-in and fan-out.
Experiment: build a digital circuit, build a larger digital circuit, apply boolean algebra to symplify and build simpler circuit, verify they are equal. Measure on which voltage levels a gate switches.


What is CMOS. How CMOS differs from TTL, how to interface them, voltage levels, how to mix TTL and CMOS.

Latches and FlipflopsEdit

basic Latch with gates, master-slave flipflop, J-K-Flipflop, D-flipflop.
experiment: Build a basic batch.

Shift RegistersEdit

What are shift registers, how are they constructed

Digital OscillatorsEdit

What, how, examples
Experiment: Build an oscillator with a 555 timer IC.