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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.