Before building an 8-bit computer, it is extremely useful to have a grasp on the elemental properties of electricity and analog circuitry. There are parts on the computer you will build will need analog components. There are many electronics self teaching guides available for a minimal cost that provide a crash-course in electrical engineering. I personally found Electronics Self Teaching Guide by Harry Kybet and Earl Boysen to be a wonderful book for tackling the world of analog electronics.



Electronics Self Teaching Guide: http://www.amazon.com/Electronics-Self-Teaching-Guide-Teaching-Guides/dp/0470289619/



Common Components:



Resistor - Limits current, measured in ohms.



Capacitor - Stores charge, can either be polar or non-polar (polar meaning that it must be placed in the correct direction to work). Measured in farads.



Diode - Only allows current to flow in one direction, breaks down at a certain voltage when placed in the wrong direction.



Transistor - A current gate that is controlled by a third pin that acts as a mediator. There are many types of transistors, but here we will be talking about the BJT (bipolar junction transistor) which comes in two types: NPN and PNP.



Current, voltage and resistance go hand-in-hand in a circuit. The relation between the three can be expressed with Ohm's law: V = IR. In other words, Voltage equals the current in amperes multiplied by the resistance in ohms. Ohm's law is one of the most important formulas in electronics and it is well worth knowing off of the top of your head.



To apply Ohm's law you need to know the resistance of a circuit. To find the value of a resistor you have to use its color code. The resistor color code is based upon the visible spectrum and can be memorized in many different fashions. For those who don't care to memorize it, there is a plethora of tools that exist to help you find the correct value for your resistor. To calculate total resistance in a circuit you need two formulas for two different configurations of resistors: series and parallel. In series one resistor follows the other one, whereas in parallel they work alongside each other. In series the formula is very simple:



Resistors in Series: R(total) = R(1) + R(2) + . . . + R(N)



Meaning that you just have to add up the values of the resistors.



Resistors in Parallel: R(total) = 1/{ 1/R(1) + 1/R(2) + . . . + 1/R(N) }



A good tool to find resistance from color code: http://www.csgnetwork.com/resistcolcalc.html



It is easier to understand the formula for resistors in parallel if you think of the resistors as working together like two people working together on a project. The same formula is used for word problems where you are given the rate at which two person operate and you must find out how fast their project will be completed if the work together.



To find how much current is supplied to a given component with a given resistance value you would simply plug in the resistance and voltage values into Ohm's law and solve for I. For instance:



A light is in a circuit and and two 1K (one thousand ohm) resistors are placed in front of it in parallel. With a power supply of 9 volts, how much current is supplied to the light?

1.) Calculate R(total):

R(total) = 1/( 1/1000 + 1/1000 ) = 1/( 2/1000) = 1000/2 = 500 ohms

2.) Calculate current using Ohm's law:

9 = I * 500

I = 9/500 = .018 A = 18 mA (milliamps)



You can also arrange resistors in a circuit to regulate voltage. This is called a voltage divider and involves two resistors in series. The voltage output of the two resistors is at their junction. For a better idea, look at the picture that I have attached. In this arrangement the formula for voltage output is:



V(out) = V(source) * R(2)/{ R(1) + R(2) }



Capacitors will be useful in your computer with the construction of the clock. The clock is simply a circuit that turns on and off at a constant rate. Just like resistors, capacitors have two formulas for finding the total value for both series and parallel configurations.



Series: C(total) = 1/{ 1/C(1) + 1/C(2) + . . . + 1/C(N) }



Parallel: C(total) = C(1) + C(2) + . . . + C(N)



The rate at which a capacitor charges depends upon the resistance of the circuit before (or after if you are discharging) the capacitor as well as its capacitance. The charging of a capacitor is measured in time constants. It takes 5 time constants to fully charge or discharge a capacitor. The formula for finding the time constant of a capacitor in seconds is:



T(constant) = Resistance * Capacitance



Diodes are simple in operation and come in handy when building a TTL computer. They only allow current to flow in one direction. When they are placed in the correct direction they are what is called forward-biased. When they are reversed they break down at a certain voltage. When a diode is working against the current it is reverse-biased.



A Transistor operates like a valve that is operated by current. A BJT has three pins: the collector, the emitter and the base. For sake of simplicity in this step I will describe a NPN transistor in which current flows from the collector to the emitter. The current applied at the base controls how much of the current flows from the collector to the emitter. Transistors are ideal for many applications due to their ability to amplify a signal. This is because the current applied at the base of the transistor can be considerably less than the current controlled. This gain in current is called the current gain of the transistor, or beta. The formula for beta is:



Beta = Current(Collector)/Current(Base)



When a transistor is completely on it is said to be saturated. A boolean transistor is one that is either in its saturated or off state and never in between. This is the type of transistor that you will be dealing with mostly in digital electronics. Transistors form the logic gates needed for a computer to function. These will be described later.



Useful Links:



http://en.wikipedia.org/wiki/Resistor

http://en.wikipedia.org/wiki/Capacitor

http://en.wikipedia.org/wiki/Diode

http://en.wikipedia.org/wiki/Transistor





