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5332 Class Notes
Week 1
Rudiments of electricity
Electrons and electric charges
Electrons and protons are the building blocks of nature.
Electrons and protons have equal but opposite electric charges. Electrons carry the (-) charge, while protons
carry the (+) charge.
While protons are relatively static, electrons are adventurous little fellows who don’t like to sit around.
Electrons travels from one place to another through conductors.
Conventional current vs real current
See: http://www.rare-earth-magnets.com/t-conventional-vs-electron-flow.aspx
Conductivity
The electrons of different types of atoms have different degrees of freedom to move around. With some types
of materials, such as metals, the outermost electrons in the atoms are so loosely bound that they chaotically
move in the space between the atoms of that material by nothing more than the influence of roomtemperature heat energy.
In other types of materials such as glass, the atoms' electrons have very little freedom to move around. While
external forces such as physical rubbing can force some of these electrons to leave their respective atoms and
transfer to the atoms of another material, they do not move between atoms within that material very easily.
This relative mobility of electrons within a material is known as electric conductivity. Materials with high
electron mobility (many free electrons) are good conductors.
Physical dimension also impacts conductivity. For instance, if we take two strips of the same conductive
material - one thin and the other thick - the thick strip will prove to be a better conductor than the thin for
the same length (think water pipe).
Voltage (V)
We need more than just a continuous path (circuit) before a continuous flow of electrons will occur: we also
need a force to push these electrons around the circuit.
The excess of electrons (imbalance) creates an electric charge. An attractive force will provoke electrons to
flow in a uniform direction until the charges neutralize. This attractive force between positive and negative
charges is an electromotive force called voltage.
The unit for voltage is the volt, or V.
At power stations the voltage is much higher and it is measured in kilovolts, symbol kV. One kilovolt equals
to one thousand volts. Small voltages are measured in millivolt, symbol mV. A millivolt is one-thousandth of
a volt.
(Think water pressure): increasing water pressure causes more water to flow through the pipe. This is
analogous to increasing voltage, which causes more electrons to flow, producing a greater electric current.
Current (A)
Following the metaphor of water moving through a pipe, a continuous, uniform flow of electrons through the
circuit is called a current. So long as the voltage source keeps "pushing" in the same direction, the electron
flow will continue to move in the same direction in the circuit. This single-direction flow of electrons is called
a Direct Current, or DC. This is the type of current that battery cells produce.
Because of the way electricity is generated in a power plant, the direction in which the electrons flow changes
120 times a second. This change in electron flow is called Alternating Current, or AC.
The unit of electric current is the ampere. Its symbol is A. Smaller amounts of current is measured in
milliamps (mA). One mA is one thousandth of an amp. A microamp (µA) is one millionth of an amp.
Electric clocks and watches take only a few µA. A single AAA cell can power a digital clock for many months.
Resistance (ohm)
Resistance is the measure of opposition to electric current.
The unit for resistance is the Ohm, symbol Ω.
Voltage Drop / Uniform Current
Although the amount of current is uniform in a simple circuit, the amount of voltage between different sets of
points in a single circuit may vary considerably.
Ohm's Law
Voltage = Current x Resistance
V=IXR
Power (W)
Electrical Power in a circuit is the amount of energy that is absorbed or produced within the circuit. For
example a room heater converts electrical energy into heat energy. The rate at which it does this is its power,
expressed in watts. The symbol for watts is W.
An average lamp runs at 75 W. A typical two-bar heater is rated at 2000 W. The power of a device is
proportional to the amount of current and voltage flowing through it.b Power (W) = current (A) x voltage (V).
Introduction to Common Electronic Components (1)
Resistors
Limit current to another component
Reduce voltage
NON-polarized
Resistor values
Capacitors
Used to store charges
Smooth out voltage
Block DC current
Electrolytic = POLARIZED
Tantalum = NON-polarized
In additional to the capacitance there is also a voltage rating. The voltage rating can be quite low (6V for
example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list
does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply
voltage.
Capacitor values
Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the
smaller values. Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico).
Electrolytic caps: the values are printed on the body. Tantalum caps: a number code is often used on small
tantalum capacitors where printing is difficult. Nobody remember them all, and labelling is not standardized
– google the value if unsure.
Diodes
A Semi-conductor: allows electricity to flow in only one direction
Connection is from + to Convert AC current to DC current
POLARIZED
Light-emitting diodes (LED)
Long leg = +, short leg = Never connect an LED directly to a battery or a power supply! LEDs must have a resistor in series to limit the
current to a safe value, for quick testing purpose a 1K ohm resistor is suitable for most LEDs if your supply
voltage is 12V or less. For a more “exact” approximation, follow these guidelines: 3.3 - 5 V = 330 ohms / 6 - 9
V = 560 ohms / 12 - 15 V = 1000 ohms.
Transistors
2 types: NPN and PNP. Most transistors today are NPN
Transistors amplify current / voltage
Can also be used as a switch
The leads are labelled base (B), collector (C), and emitter (E)
Using transistor as a switch
When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make
LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from
its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.
When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs
are off.
Integrated Circuits (IC)
These are complex circuits that have been etched onto a semiconductor.
The pins are numbered anti-clockwise around the IC starting near the notch or dot.
ICs tend to be static sensitive - earth your hands before handling an IC by touching a window frame or a
water pipe.
Potentiometers
Potentiometers are basically resistors that have variable values, depending on where the knob or slider is.
The more common potentiometers are of the knob variety.
The resistance between the outer 2 terminals (A and C) of the pot is fixed at the designated value, which is its
absolute maximum resistance.
As you rotate the shaft clockwise the resistance between the center terminal B and the outer terminal C goes
up from 0 ohms to the maximum value; while the resistance between B and A goes down from the maximum
to 0.
Photocells
A photocell is also a type of resistor. When light strikes the cell, it allows current to flow more freely. When
dark, its resistance increases dramatically. NON-polarized.
They are very low cost, easy to get in many sizes and specifications, but are very inaccurate.
For this reason, they shouldn't be used to try to determine precise light levels. Instead, you can expect to only
be able to determine basic light changes.
For most light-sensitive applications like "is it light or dark out", "is there something in front of the sensor
(that would block light)", "is there something interrupting a laser beam" (break-beam sensors), or "which of
multiple sensors has the most light hitting it", photocells can be a good choice.
Voltage Regulators
Voltage regulators serve a very important purpose: they take in a higher voltage, and output a lower voltage
that is regulated at a constant value.
The most common voltage regulators you will come across in your projects will be the simple 7805. It's a
basic regulator that outputs +5V, provided you power it with at least 7V of power or more from a DC power
supply. POLARIZED.
Speakers (Transducers)
Loudspeakers are output transducers which convert an electrical signal to sound.
They require a driver circuit, such as a 555 astable or an audio amplifier, to provide a signal.
There is a wide range available, but for many electronics projects a 300mW miniature loudspeaker is ideal.
This type is about 70mm diameter and it is usually available with resistances of 8ohm and 64ohm.
Miniature loudspeakers can also be used as a microphone and they work surprisingly well, certainly good
enough for speech in an intercom system for example.
If a project specifies a 64ohm speaker you must use this or higher resistance to prevent damage to the driving
circuit.
Piezo
Piezo also converts electrical signal to sound.
They also require a driver circuit (such as a 555 IC) to provide a signal, and if this is near their natural
(resonant) frequency of about 3kHz they will produce a particularly loud sound. Piezo transducers require a
small current, usually less than 10mA, so they can be connected directly to the outputs of most ICs. They are
ideal for buzzes and beeps, but are not suitable for speech or music because they distort the sound.
Piezo can also be used as an input transducers for detecting sudden loud noises or impacts, effectively
behaving as a crude microphone.