Chapter 3
Introduction to Microcontrollers
3.1 Just a Start Up
A microcontroller (also MCU or ¥ìC) is a computer-on-a-chip. It is a type of micro-computer element that emphasizing high integration, low power consumption, self-sufficiency and cost-effectiveness, in contrast to a general-purpose microprocessor. In addition to the usual arithmetic and logic elements of a general purpose microprocessor, the microcontroller typically integrates additional elements such as read-write memory for data storage, read-only memory, such as flash memory for code storage and permanent data storage memory, peripheral devices, and input/output interfaces.
Microcontrollers often operate at very low speed compared to modern day microprocessors, but this is adequate for many typical applications. They consume relatively little power (milliwatts), and will generally have the ability to sleep while waiting for an interesting peripheral event such as a button press to wake them up again to do something. Power consumption while sleeping may be just nanowatts, making them ideal for low power and long lasting battery applications.
Microcontrollers are frequently used in automatically controlled products and devices, such as automobile engine control systems, remote controls, office machines, appliances, power tools, and toys. By reducing the size, cost, and power consumption compared to a design using a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to electronically control many more processes.
This chapter will present a foundation for new engineers interested in Microcontroller application. The chapter starts with the basic electrical concepts that may be many of reader will be familiar with. The idea behind that is to give a comprehensive background that is sufficient for all knowledge levels. The chapter also provide many step by step tutorials that enable the reader to execute interesting project and give a physical feeling about the microcontroller capabilites.
3.2 The Breadboard (Socket Board)
The breadboad comes very handy in any electronic project (Figure 3-1). Many electronics components could be assembled in a very short time using the breadboard. Circuit modification could be easly be done for tuning and optimization purpose. The final circuit layout could be assembled together in a printed circuit board (PCB).
The bread board has many strips of metal (copper usually) which run underneath the board. The metal strips are laid out as shown in Figure 3-2. These strips connect the holes on the top of the board. This makes it easy to connect components together to build circuits. To use the bread board, the legs of components are placed in the holes (the sockets). The holes are made so that they will hold the component in place.
Each hole is connected to one of the metal strips running underneath the board. Each wire forms a node. A node is a point in a circuit where two components are connected. Connections between different components
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Figure 3-1: The Breadboard Upper Surface
Figure 3-2: Metal Strips Underneath the Breadboard are formed by putting their legs in a common node. On the breadboard, a node is the row of holes that are connected by the strip of metal underneath. The long top and bottom row of holes are usually used for power supply connections.
The rest of the circuit is built by placing components and connecting them together with jumper wires.
Then when a path is formed by wires and components from the positive supply node to the negative supply node, we can turn on the power and current flows through the path and the circuit comes alive. For chips with many legs (ICs), place them in the middle of the board so that half of the legs are on one side of the middle line and half are on the other side. A completed circuit might look like the following circuit shown in Figure 3-3.
3.3 Semiconductor Materials
A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator; it can vary over that wide range either permanently or dynamically. The ability to control resistance/conductivity in regions of semiconductor material dynamically through the application of electric fields is the feature that makes semiconductors useful. It has led to the development of a broad range of semiconductor devices, like transistors and diodes. Semiconductor devices that have dynamically controllable conductivity, such as transistors, are the building blocks of integrated circuits devices like the microprocessor and microcontrollers.
Semiconductors are tremendously important in technology. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern electrical devices. Examples range from computers to cellular phones to digital audio players. Silicon is used to create most semiconductors com95
Figure 3-3: Example of a Complete Circuit on a Breadboard
Figure 3-4: Hydrogen and Helium Atoms mercially, but dozens of other materials are used as well.
An atom is the smallest particle of an element that retains the characteristics of the element. Each known element has atoms that are different from the atoms of all other elements. This gives each element a unique atomic structure. Atom structure consists of a muscles surrounded by orbiting electrons. The nucleus consists of positively charged particles called protons and uncharged particles called neutrons. Electrons are the basic particles of negative charge.
Each type of atom has a certain number of electrons and protons that distinguishes it from the atoms of all other elements. For example the simplest atom is that of hydrogen which is shown in Figure 3-4. It has one proton and one electron. The helium atom shown has tow protons and two neutrons in the nucleus orbited by tow electrons.
In there normal, or neutral, state, all atoms of a given element have the same number of electrons as protons; the positive charges cancel the negative charges, and the atom has charge of 0. Electrons orbit nucleus at certain distances from the nucleus. Electrons near the nucleus have less energy than those in more distant orbits. It is known that only discrete values of electron energies exist within atomic structures.
Therefore, electrons must orbital at discrete distances from the nucleus.
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Figure 3-5: Energy Levels Increase as Distance from Nucleus Increases
Each discrete distance (orbit) from the nucleus corresponds to a certain energy level. In an atom, orbits are grouped into energy bands known as shells as shown in Figure 3-5. A given atom has a fixed number of shells. Each shell has a fixed maximum number of electrons at permissible energy levels (orbits). The differences in energy levels within a shall are much smaller than the difference in energy between shells .The shells are designated K, L, M, N, and so on , with K being closet to the nucleus.
Valence Electrons
Electrons in orbits farther from the nucleus are less tightly bound to the atom than those closer to the nucleus. This is because the force of the attraction between the positively charged nucleus and the negatively charged electron increases with decreasing distance. Electrons with the highest energy levels exist in the outermost shell of an atom and are relatively loosely bound to the atom. These valence electrons are responsible for the bonding within the structure of the material. The valence of an atom is the number of electrons in its outermost shell.
When an atom absorbs energy a from heat source or from light, for example, the energy levels of the electrons are raised. When electron gains energy, it moves to an orbit farther from the nucleus. Since the valence electrons possess more energy and more loosely bound to the atom than inner electrons, they can jump to higher orbits more easily when external energy is absorbed.
If a valence electron acquires a sufficient amount of energy it can be completely removed from the outer shell and the atoms influence. The departure of a valence electron leaves a previously neutral atom with an excess of positive charge (more protons than electrons). The process of losing a valence electron is known as ionization and the resulting positively charged atom is called a positive ion. The escaped valence electron is called a free electron When a free electron falls into the outer shell of an atom, then atom becomes negatively charged (more electrons that protons) and us called a negative ion.
Silicon and Germanium Atoms
Two widely used types of semiconductor materials are silicon and germanium. Both the silicon and the germanium atoms have four valence electrons. They differ in that silicon has fourteen protons its nucleus and germanium gas 32 Figure 3-6 shows the atomic structure for both materials.
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Figure 3-6: Silicon and Germanium Atoms
Figure 3-7: Covalent Bonds in Silicon
When silicon atoms combine into molecules to a form a solid material, they arrange them- selves in a fixed pattern called a crystal. The atoms within the crystal structure are held together by covalent bonds, which are created by interaction of the valence electrons of each atom.
Figure 3-7 (a) shows how each silicon atom positions itself with four adjacent atoms. Since an atom can have up to eight electrons in its outer shell, a silicon atom with its four valence electrons shares an electron with each its four neighbors. This sharing of valence electrons produces the covalent bonds that hold the atoms together, because each shared electrons is attracted equally by the tow adjacent atoms which share it.
Covalent bonding of a pure (intrinsic) silicon crystal is shown in 3-7 (b). Bonding for germanium is similar because it also has four valence electrons.
Conduction in Semiconductor Materials
As you have seen, the electrons of an atom can exist only within prescribed energy bands. Each shell around the nucleus corresponds to a certain energy band and is separated from adjacent shells by energy gaps, in which no electrons can exist. This is shown in Figure 3-8 for an unexcited silicon atom (no external energy).
This condition occurs only at absolute zero temperatures.
Absolute zero is the lowest possible temperature where nothing could be colder, and no heat energy remains in a substance. Absolute zero is the point at which molecules do not move (relative to the rest of the body) more than they are required to by a quantum mechanical effect called zero-point energy. Absolute zero is defined as precisely 0 K on the Kelvin scale, which is a thermodynamic (absolute) temperature scale, and
-273.15 on the Celsius (centigrade) scale.
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Figure 3-8: Energy Band Diagram
Conduction Electrons and Holes
A pure silicon crystal at room temperature derives heat (thermal) energy from the surrounding air, causing some valence electrons to gain sufficient energy to jump the gap from the valence band into the conduction band, becoming free electrons. When an electron jumps to the conduction band, a vacancy is left in the valence band. This vacancy is called a hole. For every electron raised to the conduction band by thermal or light energy, there is one hole left in the valence band, creating what is called an electron-hole pair.
Recombination occurs when a conduction band electron loses energy and falls back into a hole in the valence band. To summarize, a piece of pure silicon at room temperature has, at any instant, a number of conduction band (free) electrons that are unattached to any atom and are essentially drifting randomly throughout the material. There are also an equal number of holes in the valence band created when these electrons jump into the conduction band.
Electron and Hole Current
When a voltage is applied across a piece of silicon, as shown in Figure 3-9, the free electrons in the conduction band are easily attracted toward the positive end. This movement of free electrons is one type of current in a semiconductor material, called electron current. Another current mechanism occurs at the valence level, where the holes created by the free electrons exist. Electrons remaining in the valence band are still attached to their atoms and are not free to move randomly in the crystal structure. However a valence electron can fall into a nearby hole, with little change in its energy level, thus leaving another hole where it came from.
Effectively the hole has moved from one place to another in the crystal structure, as illustrated in Figure
3-10. This is called hole current.
Semiconductors, Conductors, and Insulators
In a pure (intrinsic) semiconductor, there are relatively few free electrons; so neither silicon nor germanium is very useful in its intrinsic state. They are neither insulators nor good conductors because current in a material depends directly on the number of free electrons.
A comparison of the energy bands in Figure 3-11 for the three types of materials shows the essential differences among them regarding conduction. The energy gap for an insulator is so wide that hardly any electrons acquire enough energy jumps into the conduction band. The valence band and the conduction band in a conductor (like copper) overlap so that there are always many conduction electrons, even without the application of external energy. The semiconductor, as the figure shows, has an energy gap that is much narrower than that in an insulator.
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Figure 3-9: Free Electron Current
Figure 3-10: Hole Current
Figure 3-11: Energy Diagrams for three Categories of Materials
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Figure 3-12: N-Type Semiconductor
N-Type and P-Type Semiconductors
Intrinsic semiconductor materials do not conduct very well because of the limited number of free electrons in the conduction band. This means that the resistively of a semiconductor is much greater than that of a conductor. For example, a one-cubic-centimeter sample of silver has a resistively of 10.6¥Ø.cm, whereas the resistively is about 45¥Ø.cm for pure germanium and several thousand ohms. cm for pure silicon.
Doping
The resistivity of silicon and germanium can be drastically reduced and controlled by the addition of impurities to the pure semiconductor material. This process, called doping increases the number of current carriers
(electrons or holes) thus increasing the conductivity and decreasing the resistivity. The tow categories of impurities are N-type and P-type.
N-Type Semiconductor
As illustrated in Figure 3-12 each atom (antimony, in this case) forms covalent bonds with four adjacent silicon atoms. Four of the antimony atoms valence electrons are used to form the covalent bonds, leaving one extra electron. This extra electron becomes a conduction electron because it is not attached to any atom.
The number of conduction electrons can be controlled by the amount of impurity added to the silicon.
Since most of the current carriers are electrons, silicon (or germanium) doped in this way is an N-type semiconductor material where the N stands for the negative charge on an electron. The electrons are called the majority carriers in N-type material. Although the great majority of current carriers in N-type material are electrons, there some holes. Holes in an N-type material are called minority carriers.
P-Type Semiconductor
To increase the number of holes in pure silicon, impurity atoms are added. These are atoms with three valence electrons such as aluminum, boron, and gallium. As illustrated in Figure 3-13, each atom (boron, in this case) forms covalent bonds with four adjacent silicon atoms. All three of the boron atom¡¯s valence electrons are used in the covalent bonds. Since four electrons are required, a hole is formed with each
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Figure 3-13: P-Type Semiconductor trivalent. Since most of the current carriers are holes, silicon (or germanium) doped in this way is called a P-type semiconductor material because holes can be thought of as positive charges. The holes are the majority carriers in P-type material. Although the great majority of current carriers in P-type material are holes, there are some electrons in P-type material are called minority carriers.
PN Junction
When a piece of silicon is doped so that half is N-type and the other half is P-type, a PN junction is formed between the tow regions as shown in Figure 3-14. This device is known as a semiconductor diode. The N region has many conduction electrons and the P region has holes, as shown in Figure 3-14. The PN jection is fundamental to the operation not only of diodes but also of transistors and other solid state devices.
The Depletion Layer
With no external voltage, the conduction electrons in the n region are aimlessly drifting in all directions. At the instant of junction formation, some of the electrons near the junction diffuse across into the P region and recombine with holes near the junction. For each electron that crosses the junction and recombines with a hole an atom is left with a net positive charge in the N region near the junction, making it a positive ion.
Also when the electro recombines with a hole in the P region, The atom acquires net negative charge making it a negative ion.
As a result of this recombination process a large number of positive and negative ions build up near the
PN junction. As this build-up occurs the electrons in the N region must overcome both the attraction of the positive ions and the repulsion of the negative ions in order to migrate into the P region. Thus, as the ion layers build up, the area on both sides of the junction becomes essentially depleted of any conduction electrons or holes and is known as the deletion layer. This condition is illustrated in Figure 3-15. When an
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Figure 3-14: Basic PN Structure at the Instant of Junction Formation
Figure 3-15: PN Junction Equilibrium Condition equilibrium conditions is reached, the depletion layer has widened to a point where no further electrons can cross the PN junction.
The existence of the positive and negative ions on opposite sides of the junction creates a barrier potential across the depletion layer. At 25 C, the barrier potential is approximately 0.7 V for silicon and 0.3 V for germanium. As the junction temperature increases, the barrier potential decreases, and vice versa.
More About The PN Junction
As you have seen there is on current across a PN junction at equilibrium. The primary usefulness of the
PN junction diode is its ability to allow current in only one direction and to prevent current in the other direction as determined by the bias. There two bias conditions for a PN junction, forward and reverse.
Either of these conditions is created by application of an external voltage.
Forward Bias
The term bias in electronics normally refers to a fixed voltage that sets the operating conditions for a semiconductor device. Forward bias is the condition that permits current across a PN junction. Figure 3-16,(A) shows a DC voltage connected in a direction to forward-bias the diode. Notice that the negative terminal of the battery is connected to the N region (called the cathode), and the positive terminal is connected to the
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(A) (B)
Figure 3-16: PN Junction Forward Bias
P region (called the anode).
This is the basic operation of forward bias: the negative terminal of the battery pushes the conductionband electrons in the N region toward the junction, while the positive terminal pushes the holes in the
P region also toward the junction. When it overcomes the barrier potential, the external voltage source provides the N region electrons with enough energy to penetrate depletion layer and cross the junction, where they combine with the P region holes. As electrons leave the N region more flow in from the negative terminal of the battery. So, current through the N region is the movement of conduction electrons (majority carriers) toward the junction.
Once the conduction electrons enter the P region and combine with holes they become valence electrons.
They then move as valence electrons from hole to hole toward the positive connection of the battery. The movement of these valence electrons is the same as the movement of holes in the opposite direction. So, current in the P region is the movement of holes (majority carriers) toward the junction. Figure 3-16,(B) illustrates current in forward biased diodes.
Reverse Bias
Reverse bias is the condition that prevents current across the PN junction Figure 3-17,(A) shows a DC voltage source connected to reverse bias diode. Notice that the negative terminal of the battery is connected to the P region, and the positive terminal to the N region. The negative terminal of the battery attracts holes in the P region away from the PN junction while the positive terminal also attracts away from the junction As electrons and holes move away from the junction, the depletion layer widens, more positive ions are created in the N region and more negative ions are created the P region as shown in Figure 3-17,(B).
The depletion layer widens until the potential difference across it the external bias voltage. At this point, the holes and electrons stop moving away from the junction and majority current cease. The initial movement of majority carriers from the junction is called transient current and lasts only for a very short time upon application of reverse.
Reverse leakage Current. As you have learned, majority current very quickly becomes 0 when reverse bias is applied. There is however, a very small leakage current produced by minority during reverse bias.
Germanium, as a rule, has greater leakage current than silicon. This current is typically in the ¥ìA or n A range. A relatively small number of thermally produced electron-hole pairs exist in the depletion layer.
Under the influence of the external voltage, some electrons manage to diffuse across the PN junction before recombination. This process establishes a small minority carrier currier throughout the material . The reverse leakage current is dependent primarily on the junction temperature and not on the amount of reverse-biased voltage. A temperature increase causes an increase in leakage current.
Reverse Breakdown. If the external reverse-biased voltage is increased to large enough value, breakdown occurs. Here is what happens: Assume that one minority conduction-band electron acquires enough energy from the external source to accelerate it toward the positive end of the diode. During its travel, it collides
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(A) (B)
Figure 3-17: PN Junction Reverse Bias with an atom and imparts energy to knock a valence electron into the conduction band. There are now two conduction-band electrons. Each will collide with an atom, knocking two more valence electrons into the conduction band. There are now four conduction band electrons which, in turn, knock four more into the conduction band. This rapid multiplication of conduction-band electrons, known as an avalanche effect, result in a rapid build-up of reverse current. Most diodes are normally not operated in a reverse break-down and can be damaged by the resulting excessive power. However, a particular type of diodes, known as the zener diode, is optimized for reverse-breakdown operation.
3.4 Introduction to Common Electronic Components
In this section a fundamental idea about common electronic component that generally used in many electronic circuit will be given.
3.4.1 Resistors
Resistors are components that have a predetermined resistance. Resistance determines how much current will flow through a component. Resistors are used to control voltages and currents. A very high resistance allows very little current to flow. Air has very high resistance. Current almost never flows through air.
(Sparks and lightning are brief displays of current flowing through air. The light is created as the current burns parts of the air.) A low resistance allows a large amount of current to flow. Metals have very low resistance. That is why wires are made of metal. They allow current to flow from one point to another point without any resistance. Wires are usually covered with rubber or plastic. This keeps the wires from coming in contact with other wires and creating short circuits. High voltage power lines are covered with thick layers of plastic to make them safe, but they become very dangerous when the line breaks and the wire is exposed and is no longer separated from other things by insulation.
Resistance is given in units of ohms. Common resistor values are from 100 ohms to 100,000 ohms. The letter k is often used with resistors to mean 1000. For example, a 10,000 ohm resistor is written as 10k ohms.
Each resistor is marked with colored stripes to indicate its resistance. To learn how to calculate the value of a resistor by looking at the stripes on the resistor, go to the resistor values tutorial, Section 1.3, which includes more information about resistors.
Finding the Value of a Resistor by Color Codes
The simplest way ti find out the value of a resistor is to use a Multimeter. You can also calculate the value of a resistor using the color coded stripes on the resistor by using the following procedure:
. Step One: Turn the resistor so that the gold or silver stripe is at the right end of the resistor.
. Step Two: Look at the color of the first two stripes on the left end. These correspond to the first two digits of the resistor value. Use Table 3.1 below to determine the first two digits.
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Color First Stripe Second Stripe Third Stripe Fourth Stripe
Black 0 0 x1
Brown 1 1 x10
Red 2 2 x100
Orange 3 3 x1,000
Yellow 4 4 x10,000
Green 5 5 x100,000
Blue 6 6 x1,000,000
Purple 7 7
Gray 8 8
White 9 9
Gold 5%
Silver 10%
Table 3.1: Resistors Strips Color Coding
Figure 3-18: A Typical Diodes
. Step Three: Look at the third stripe from the left. This corresponds to a multiplication value. Find the value using the table below.
. Step Four: Multiply the two digit number from step two by the number from step three. This is the value of the resistor in ohms. The fourth stripe indicates the accuracy of the resistor. A gold stripe means the value of the resistor may vary by 5% from the value given by the stripes.
3.4.2 Diodes
Diodes are components that allow current to flow in only one direction. They have a positive side (leg) and a negative side. When the voltage on the positive leg is higher than on the negative leg then current flows through the diode (the resistance is very low). When the voltage is lower on the positive leg than on the negative leg then the current does not flow (the resistance is very high). The positive leg of a diode is the one with the line closest to it (Figure 3-18). Usually when current is flowing through a diode, the voltage on the positive leg is 0.65 volts higher than on the negative leg.
3.4.3 The Light Emitting Diodes (LED)
An LED is the device shown physically in Figure 3-19, a. Besides red, they can also be yellow, green and blue. The letters LED stand for Light Emitting Diode. The important thing to remember about diodes
(including LEDs) is that current can only flow in one direction. To make an LED work, you need a voltage supply and a resistor. If you try to use an LED without a resistor, you will probably burn out the LED. The
LED has very little resistance so large amounts of current will try to flow through it unless you limit the
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(A)
(B)
Figure 3-19: Light Emitting Diode (LED)
A) Typical LED Picture, B) LED Circuit Schematic current with a resistor. If you try to use an LED without a power supply, you will be highly disappointed.
So first of all we will make our LED light up by setting up the circuit as shown in Figure 3-19, b.
Here is the steps in details that you will need to follow to build this LED tuotorial using a breadboard:
1. First you have to find the positive leg of the LED. The easiest way to do this is to look for the leg that is longer.
2. Once you know which side is positive, put the LED on your breadboard so the positive leg is in one row and the negative leg is in another row.
3. Place one leg of a 2.2k ohm resistor (does not matter which leg) in the same row as the negative leg of the LED. Then place the other leg of the resistor in an empty row.
4. Unplug the power supply adapter from the power supply. Next, put the ground (black wire) end of the power supply adapter in the sideways row with the blue stripe beside it. Then put the positive
(red wire) end of the power supply adapter in the sideways row with the red stripe beside it.
5. Use a short jumper wire (use red since it will be connected to the positive voltage) to go from the positive power row (the one with the red stripe beside it) to the positive leg of the LED (not in the same hole, but in the same row). Use another short jumper wire (use black) to go from the ground row to the resistor (the leg that is not connected to the LED). Refer to the picture below in Figure
3-20 if necessary.
Now plug the power supply into the wall and then plug the other end into the power supply adapter and the LED should light up. Current is flowing from the positive leg of the LED through the LED to the negative leg. Try turning the LED around. It should not light up. No current can flow from the negative leg of the LED to the positive leg.
People often think that the resistor must come first in the path from positive to negative, to limit the amount of current flowing through the LED. But, the current is limited by the resistor no matter where the resistor is. Even when you first turn on the power, the current will be limited to a certain amount, and can be found using ohms law.
Revisiting Ohm¡¯s Law
Ohm¡¯s Law can be used with resistors to find the current flowing through a circuit. The law is I =
V/R (whereI = current, V = voltageacrossresistor, andR = resistance). For the circuit above we can only use Ohm¡¯s law for the resistor so we must use the fact that when the LED is on, there is a 1.4 voltage drop across it. This means that if the positive leg is connected to 12 volts, the negative leg will be at 10.6 volts. Now we know the voltage on both sides of the resistor and can use Ohm¡¯s law to calculate the current.
The current is (10.6 . 0)/2200 = 0.0048 Amperes = 4.8 mA. This is the current flowing through the path from 12V to GND. This means that 4.8 mA is flowing through the LED and the resistor. If we want to change the current flowing through the LED (changing the brightness) we can change the resistor. A smaller resistor will let more current flow and a larger resistor will let less current flow. Be careful when using smaller resistors because they will get hot. The maximum current that can flow through the LED
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Figure 3-20: Simple LED Circuit on Breadboard
(A)
(B)
Figure 3-21: A Typical Capacitor
A) Capacitor Picture, B)Capacitor Symbol Schematic without burning it could be found from the LED data sheet.
Next, we want to be able to turn the LED on and off without changing the circuit. To do this we will learn to use another electronic component, the transistor.
3.4.4 Capacitor
If you already understand capacitors you can skip this part. The picture shown in Figure 3-21 on the left shows two typical capacitors. Capacitors usually have two legs. One leg is the positive leg and the other is the negative leg. The positive leg is the one that is longer. Right on Figure 3-21 is the symbol used for capacitors in circuit drawings (schematics). When you put one in a circuit, you must make sure the positive leg and the negative leg go in the right place. Capacitors do not always have a positive leg and a negative leg. The smallest capacitors usually do not. It does not matter which way you put them in a circuit.
A capacitor is similar to a rechargeable battery in the way it works. The difference is that a capacitor can only hold a small fraction of the energy that a battery can. (Except for really big capacitors like the ones found in old TVs, These can hold a lot of charge. Even if a TV has been disconnected from the wall for a long time, these capacitors can still make lots of sparks and hurt people.) As with a rechargeable battery, it takes a while for the capacitor to charge. So if we have a 12 volt supply and start charging the capacitor, it will start with 0 volts and go from 0 volts to 12 volts.
The same idea is true when the capacitor is discharging. If the capacitor has been charged to 12 volts and then we connect both legs to ground, the capacitor will start discharging but it will take some time for the voltage to go to 0 volts. Below is a graph of what the voltage is in the capacitor while it is discharging.
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(A) (B)
Figure 3-22: A Typical Capacitor voltage graph
A) Charging, B)Discharging
(A) (B)
Figure 3-23: Transistor
A) Transistor Picture, B) Transistor Schematic Symbol
Figure 3-22 shows a graph of what the voltage is in the capacitor while it is charging and discharging. We can control the speed of the capacitor¡¯s charging and discharging using resistors.
Capacitors are given values based on how much electricity they can store. Larger capacitors can store more energy and take more time to charge and discharge. The values are given in Farads but a Farad is a really large unit of measure for common capacitors. Common capacitors use measurements of pf and ¥ìF. pF means picofarad and ¥ìF means microfarad. A picofarad is 0.000000000001 Farads. So a 33pF capacitor has a value of 33 picofarads or 0.000000000033 Farads. A microfarad is 0.000001 Farads. So a 10uf capacitor is 0.00001 Farads and a 220¥ìF capacitor is 0.000220 Farads. If you do any calculations with formulas using the value of the capacitor you have to use the Farad value rather than the picofarad or microfarad value.
Capacitors are also rated by the maximum voltage they can take. This value is always written on the larger can shaped capacitors. For example, the 220¥ìF capacitor some times comes with a maximum voltage rating of 25 volts. If you apply more than 25 volts to them they will die.
3.4.5 The Transistor
Transistors are basic components in all of today¡¯s electronics. They are just simple switches that we can use to turn things on and off. Even though they are simple, they are the most important electrical component.
For example, transistors are almost the only components used to build a Pentium processor. A single Pentium chip has about 3.5 million transistors. The ones in the Pentium are smaller than the ones we will use but they work the same way. A typical Transistor picture and schematic symbol that we will use through this text is shown in Figure 3-23.
3.4.6 Basic Transistor Circuit
The Base (B) is the On/Off switch for the transistor. If a current is flowing to the Base, there will be a path from the Collector (C) to the Emitter (E) where current can flow (The Switch is On.) If there is no current flowing to the Base, then no current can flow from the Collector to the Emitter (The Switch is Off). Figure
3-24 shows the basic circuit that could be implemented to use transistor as a switch.
To build this circuit we only need to add the transistor and another resistor to the circuit we built above for the LED. Unplug the power supply from the power supply adapter before making any changes on the
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10.6V
0.2V
C
E
B
0.6V
( 12V, or 0V )
Figure 3-24: Transistor Circuit Schematic
Figure 3-25: Transistor Circuit on Breadboard breadboard. To put the transistor in the breadboard, seperate the legs slightly and place it on the breadboard so each leg is in a different row. The collector leg should be in the same row as the leg of the resistor that is connected to ground (with the black jumper wire). Next move the jumper wire going from ground to the 2.2k ohm resistor to the Emitter of the transistor.
Next place one leg of the 100k ohm resistor in the row with the Base of the transistor and the other leg in an empty row and your breadboard should look like the picture shown in Figure 3-25.
Now put one end of a yellow jumper wire in the positive row (beside the red line) and the other end in the row with the leg of the 100k ohm resistor (the end not connected to the Base). Reconnect the power supply and the transistor will come on and the LED will light up. Now move the one end of the yellow jumper wire from the positive row to the ground row (beside the blue line). As soon as you remove the yellow jumper wire from the positive power supply, there is no current flowing to the base. This makes the transistor turn off and current can not flow through the LED. As we will see later, there is very little current flowing through the 100k resistor. This is very important because it means we can control a large current in one part of the circuit (the current flowing through the LED) with only a small current from the input.
Back to Ohm¡¯s Law
We want to use Ohm¡¯s law to find the current in the path from the Input to the Base of the transistor and the current flowing through the LED. To do this we need to use two basic facts about the transistor.
1. If the transistor is on, then the Base voltage is 0.6 volts higher than the Emitter voltage.
2. If the transistor is on, the Collector voltage is 0.2 volts higher than the Emitter voltage.
110
Unregulated 7805
Voltage Input
7.5 V
I c o 270
2N2222
Bipolar
Transistor
Regulator
DC
1000
LED Relay
Lamp
Figure 3-26: Transistor-Relay Driver
So the current flowing through the 100k resistor is (12. 0.6)/100000 = 0.000114 A = 0.114 mA.
The current flowing through the 2.2k ohm resistor is (10.6 . 0.2)/2200 = 0.0047 A = 4.7 mA.
If we want more current flowing through the LED, we can use a smaller resistor (instead of 2200) and we will get more current through the LED without changing the amount of current that comes from the Input line. This means we can control things that use a lot of power (like electric motors) with cheap, low power circuits. Soon you will learn how to use a microcontroller. Even though the microcontroller can not supply enough current to turn lights and motors on and off, the microcontroller can turn transistors on and off and the transistors can control lots of current for lights and motors.
3.4.7 Transistor-Relay Driver
Figure 3-26 shows how to use a transistor to activate a relay. The relay could be used to operate a lamp or any other power system such as DC motor.
3.4.8 Optical Encoder
Figure 3-27 shows how to use a slotted optical switch to indicate the exitance of non-transparent element in the optical switch slot. This could be used as optical position encoder if slotted disk is used. A 2N222 transistor is used to magnify the output signal to a level that is acceptable by many microcontrollers (0-5
Volt).
3.5 Introduction to Digital Devices - The Inverter
In digital devices there are only two values, usually referred to as 0 and 1. 1 means there is a voltage (usually
5 volts, or there is a high level of volts) and 0 means the voltage is 0 volts or there is a low level of volts. An inverter (also called a NOT gate) is a basic digital device found in all modern electronics. So for an inverter, as the name suggests, its output is the opposite of the input (Output is NOT the Input). If the input is
0 then the output is 1 and if the input is 1 then the output is 0. We can summarize the operation of this device as shonwn in Table 3.2 (usually called the truth table).
To help us practice with transistors we will build an inverter. Actually we have already built an inverter.
The transistor circuit we just built is an inverter circuit. To help see the inverter working, we will build
111
Unregulated 7805
Voltage Input
7.5 V
I c o +
-
560
E D +
H21A1
Optical Switch
2N2222
Bipolar
Transistor
Regulator
Figure 3-27: Optical Encoder
Input Output
1 0
0 1
Table 3.2: The Inverter Truth Table
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First Inverter Second Inverter
Figure 3-28: Two Inverter Circuit Schematic
Figure 3-29: Inverter Circuit Schematic another inverters circuit. The circuits we will use are shown in Figure 3-28.
Now we will integrate the two inverters circuit into circuit. To build this circuit, use the transistor circuit we just built as the first inverter. The first inverter input is the end of the 100k ohm resistor. Connect the output of the first inverter to the input of the second inverter by putting one end of a jumper wire in the same row of holes as the 2.2k ohm resistor and the Collector of the transistor (the output of the first inverter) and putting the other end in the same row of holes as the leg of the 100k ohm resistor of the second inverter (the input to the second inverter).
Here is how to check if you built it correctly. Connect the first inverter input to 12V. The LED in the first inverter should come on and the LED in the second inverter should stay off. Then connect the first inverter input to 0V. (You are turning off the switch of the first inverter.) The first LED should go off and the second LED should come on. If this does not happen, check to make sure no metal parts are touching.
Check to make sure all the parts are connected correctly.
The input can either be connected to 12V or 0V. When the Inverter Input is 12V, the transistor in the first inverter will turn on and the LED will come on and the Inverter Output voltage will be 0.2V. The first
Inverter Output is connected to the input of the second inverter. The 0.2V at the input of the second inverter is small enough that the second transistor is turned off. The circuit voltages are shown in the inverter circuit schematic shown in Figure 3-29.
When the Inverter Input is connected to 0V, the transistor in the first inverter is turned off and the LED will get very dim. There is a small amount of current still flowing through the LED to the second inverter.
The voltage at the first Inverter Output will go up, forcing the second inverter transistor to come on. When the second inverter transistor comes on, the second inverter LED will come on. To find the voltage at the output of the first inverter (10.4V), use Ohm¡¯s law. There is no current flowing through the transistor in
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Figure 3-30: Inverter Circuit Schematic
Figure 3-31: Different Shapes of Integrated Circuits the first inverter so the path of the current is through the first LED, through the 2.2k resistor, through the
100k resistor, through the second transistor to ground. The voltage at the negative side of the first LED is fixed at 10.6V by the LED. The voltage at the second transistor base is fixed at 0.6V by the transistor.
Then given those two voltages, you should be able to find the voltage at the point in the middle (10.4V) using Ohms law. This second voltage arrangment is shown in Figure 3-30.
3.6 Introduction to Integrated Circuits ICs
As electronic designs get bigger, it becomes difficult to build the complete circuit. So we will use pre-built circuits that come in packages like the one shown in Figure 3-31. This pre-built circuit is called an IC. IC stands for Integrated Circuit. An IC has many transistors inside it that are connected together to form a circuit. Metal pins are connected to the circuit and the circuit is stuck into a piece of plastic or ceramic so that the metal pins are sticking out of the side. These pins allow you to connect other devices to the circuit inside. We can buy simple ICs that have several inverter circuits like the one we built before or we can buy complex ICs like a Pentium Processor.
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Figure 3-32: A Graph of Pulse Voltage
Figure 3-33: A Clock Signal Voltage Diagram
3.6.1 The 555 Timer
Before we talke about the 555 timer, we must fisrt review and study two important fundamental concepts.
The Pulse - More Than Just an On/Off Switch
So far the circuits we have built have been stable, meaning that the output voltage stays the same. If you change the input voltage, the output voltage changes and once it changes it will stay at the same voltage level. The 555 integrated circuit (IC) is designed so that when the input changes, the output goes from 0 volts to Vcc (where Vcc is the voltage of the power supply). Then the output stays at Vcc for a certain length of time and then it goes back to 0 volts. This is a pulse. A graph of the output voltage is shown in Figure
3-32.
The Oscillator (A Clock) - More Than Just a Pulse
The pulse is nice but it only happens one time. If you want something that does something interesting forever rather than just once, you need an oscillator. An oscillator puts out an endless series of pulses. The output constantly goes from 0 volts to Vcc and back to 0 volts again. Almost all digital circuits have some type of oscillator. This stream of output pulses is often called a clock. You can count the number of pulses to tell how much time has gone by. We will see how the 555 timer can be used to generate this clock. A graph of a clock signal is shown in Figure 3-33.
Creating a Pulse
The 555 is made out of simple transistors that are about the same as on / off switches. They do not have any sense of time. When you apply a voltage they turn on and when you take away the voltage they turn off. So by itself, the 555 can not create a pulse. The way the pulse is created is by using some components in a circuit attached to the 555 (Figure 3-34). This circuit is made of a capacitor and a resistor. We can flip a switch and start charging the capacitor. The resistor is used to control how fast the capacitor charges.
The bigger the resistance, the longer it takes to charge the capacitor. The voltage in the capacitor can then be used as an input to another switch. Since the voltage starts at 0, nothing happens to the second switch.
But eventually the capacitor will charge up to some point where the second switch comes on.
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Figure 3-34: A 555 Timer Integrated Circuit
The way the 555 timer works is that when you flip the first switch, the Output pin goes to Vcc (the positive power supply voltage) and starts charging the capacitor. When the capacitor voltage gets to 2/3Vcc the second switch turns on which makes the output go to 0 volts. The pinout for the 555 timer is shown in
Figure 3-34
Deep Details about the 555 Timer
Pin 2 (Trigger) is the ¡¯on¡¯ switch for the pulse. The line over the word Trigger tells us that the voltage levels are the opposite of what you would normally expect. To turn the switch on you apply 0 volts to pin 2. The technical term for this opposite behavior is ¡¯Active Low¡¯. It is common to see this ¡¯Active Low¡¯ behavior for
IC inputs because of the inverting nature of transistor circuits like we saw in the LED and Transistor Tutorial.
Pin 6 is the off switch for the pulse. We connect the positive side of the capacitor to this pin and the negative side of the capacitor to ground. When Pin 2 (Trigger) is at Vcc, the 555 holds Pin 7 at 0 volts.
When Pin 2 goes to 0 volts, the 555 stops holding Pin 7 at 0 volts. Then the capacitor starts charging. The capacitor is charged through a resistor connected to Vcc. The current starts flowing into the capacitor, and the voltage in the capacitor starts to increase.
Pin 3 is the output (where the actual pulse comes out). The voltage on this pin starts at 0 volts. When
0 volts is applied to the trigger (Pin 2), the 555 puts out Vcc on Pin 3 and holds it at Vcc until Pin 6 reaches
2/3 of Vcc. Then the 555 pulls the voltage at Pin 3 to ground and you have created a pulse. The voltage on
Pin 7 is also pulled to ground, connecting the capacitor to ground and discharging it. To see the pulse we will use an LED connected to the 555 output, Pin 3. When the output is 0 volts the LED will be off. When the output is Vcc the LED will be on.
Building the 555 Timer Circuit
Place the 555 across the middle line of the breadboard so that 4 pins are on one side and 4 pins are on the other side. (You may need to bend the pins in a little so they will go in the holes.) Leave the power disconnected until you finish building the circuit. The diagram in Figure ?? shows how the pins on the 555 are numbered. You can find pin 1 by looking for the half circle in the end of the chip. Sometimes instead of a half circle, there will be a dot or shallow hole by pin 1.
Before you start building the circuit, use jumper wires to connect the red and blue power rows to the red and blue power rows on the other side of the board. Then you will be able to easily reach Vcc and Ground lines from both sides of the board.
. Connect Pin 1 to ground.
. Connect Pin 8 to Vcc.
. Connect Pin 4 to Vcc.
. Connect the positive leg of the LED to a 330 ohm resistor and connect the negative end of the LED to ground. Connect the other leg of the 330 ohm resistor to the output, Pin 3.
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Figure 3-35: Schematic of Using 555 Timer as Oscillator
. Connect Pin 7 to Vcc with a 10k resistor (RA = 10K).
. Connect Pin 7 to Pin 6 with a jumper wire.
. Connect Pin 6 to the positive leg of the 220¥ìF Capacitor (C = 220¥ìF).
. Connect the negative leg of the capacitor to ground.
. Connect a wire to Pin 2 to use as the trigger. Start with Pin 2 connected to Vcc.
Now connect the power. The LED will come on and stay on for about 2 seconds. Remove the wire connected to Pin 2 from Vcc. You should be able to trigger the 555 again by touching the wire connected to pin 2 with your finger or by connecting it to ground and removing it.
Making the 555 Timer Oscillate
Next we will make the LED flash continually without having to trigger it. We will hook up the 555 so that it triggers itself. The way this works is that we add in a resistor between the capacitor and the discharge pin, Pin 7 as shown in Figure 3-35. Now, the capacitor will charge up (through RA and RB) and when it reaches 2/3Vcc, Pin 3 and Pin 7 will go to ground. But the capacitor can not discharge immediately because of RB. It takes some time for the charge to drain through RB. The more resistance RB has, the longer it takes to discharge. The time it takes to discharge the capacitor will be the time the LED is off.
To trigger the 555 again, we connect Pin 6 to the trigger (Pin 2). As the capacitor is discharging, the voltage in the capacitor gets lower and lower. When it gets down to 1/3Vcc this triggers Pin 2 causing Pin
3 to go to Vcc and the LED to come on. The 555 disconnects Pin 7 from ground, and the capacitor starts to charge up again through RA and RB.
Refere to Figure 3-35 To build this circuit from the previous circuit, do the following.
. Disconnect the power.
. Take out the jumper wire between Pin 6 and Pin 7 and replace it with a 2.2k resistor (RB = 2.2K).
. Use the jumper wire at pin 2 to connect Pin 2 to Pin 6.
. Now reconnect the power and the LED should flash forever (as long as you pay your electricity bill).
Experiment with different resistor values of RA and RB to see how it changes the length of time that the
LED flashes. (You are changing the amount of time that it takes for the Capacitor to charge and discharge.)
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Figure 3-36: Microprocessor Buses Configuration
Timer Basic Formulas
These are the formulas we use for the 555 to control the length of the pulses. t1 = charge time (how long the LED is on) = 0.693 . (RA + RB) . C t2 = discharge time (how long LED is off) = 0.693 . RB . C
T = period = t1 + t2 = 0.693 . (RA +2 . RB) . C
Frequency = 1/T = 1.44/((RA+ 2 . RB) . C) where t1 and t2 are the time in seconds. C is the capacitor value in Farads. 220¥ìF = 0.000220F. So for our circuit we have: t1 = 0.693 . (10000 + 2200) . 0.000220 = 1.86 seconds t2 = 0.693 . 2200 . 0.000220 = 0.335 seconds
T = 1.86 + 0.335 = 2.195 seconds
Frequency = 0.456 (cycles per second)
3.7 Introduction to Microcontroller
Here is some physical definition of microcontroller to make the reader catch in hand what is a microcontroller really means:
This is like a scaled-down computer designed for a very specific task, unlike a desktop computer, which has many uses. An example of an application for a microcontroller would be a traffic light, or the chip that controls the suspension system of your new car.
A highly integrated microprocessor designed specifically for use in embedded systems. Microcontrollers typically include an integrated CPU, memory (a small amount of RAM, ROM, or both), and other peripherals on the same chip.
A highly integrated chip that contains all the components comprising a controller. Typically, this includes a CPU, RAM, some form of ROM, I/O ports, and timers. Unlike a general-purpose computer, which also
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Figure 3-37: Microcontroller RAM Interaction includes all of these components, a microcontroller is designed for a very specific task to control a particular system. As a result, the parts can be simplified and reduced, which cuts down on production costs single purpose processing units designed to execute small control programs, sometimes in real time. The program is frequently stored on the microcontroller in an area of nonvolatile memory.
A single-chip microcomputer with on-board program ROM and I/O that can be programmed for various control functions.
Also called an embedded, or dedicated, computer; the smallest category of computer.
A microcontroller is a computer-on-a-chip optimised to control electronic devices. It is a type of microprocessor emphasizing self-sufficiency and cost-effectiveness, in contrast to a general-purpose microprocessor, the kind used in a PC. A typical microcontroller contains all the memory and I/O interfaces needed, whereas a general purpose microprocessor requires additional chips to provide these necessary functions.
3.8 The ATMEL 2051 Microcontroller
The 2051 microcontroller is a complex integrated circuit that is programmable. You can give it a set of commands to follow and it will run through those commands and do exactly what you want it to do. This section will give a quick overview of the pins of the 2051 and then the next section will show how to program the 2051. The ATMEL2051 Microcontroller pin layout is shown in Figure 3-38.
Pin 1 is reset pin. This pin can be used to force the 2051 to start over at the beginning of the program.
Pin 2 and Pin 3 can be used to communicate with the computer or other devices (RXD is receive and TXD is transmit). Pin 2 and Pin 3 are also part of Port 3. Port 3 includes P3.0, P3.1, P3.2, P3.3, P3.4, P3.5
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Figure 3-38: Pin Layout of the ATMEL 2051 Microcontroller and P3.7 (there is no P3.6). These pins are usually used as general digital input/output pins. They can be connected to LEDs to turn them on and off (this would be using them as outputs). Or they can be connected to switches so that the 2051 can look and see if a user has turned a switch on or off (this would be using them as inputs).
Pins 4 and 5 are connected to the 11.0592 MHz crystal. The 2051 uses this crystal to create a clock. The speed of the crystal determines the speed that the 2051 runs at. You can make programs run faster by using a faster crystal such as 24 MHz. Pin 10 is the ground connection for the 2051. Pins 12 to 19 make up Port
1. This is another set of pins that can be used as general digital inputs and outputs