To built a PC, no-one in their right mind would start using basic digital building blocks, you'd prabably hobby around with mother boards, CPU's, various disk drives, of which there is quite a variety. but it will never teach you how those things realy work, and know quite a lot of electrical engineers (not even to mention informatics students)would have few clues as to how the basic circuitry in a PC can be put together without simply plugging and playing++, let alone how one could engineer a real computer from REAL basic building blocks.
I decided to do this page in a semi-course way, so I'll even start at the transistor level, and give some examples on how standard, cheap TTL based IC's can be put to real work. Simply put, I want to explain the mechanics, understandable for a reasonably intelligent and interested layman, and end up making serious circuitry understandable to the point of building them yourself.
If I feel up to it (time and effort-wise), I might do this more full-blown,
complete fashion, hell I might even make a book out of it. Some kiddos
are bount to find it enjoyable, at least.
a light, a switch and power supplyWhen lit, the lamp consumes power, that is it extracts power from the supply through the current that runs through it. In electrical language, the voltage over the lamp (in Volts) times the current though it (In Amperes).
A valid comparison is that the voltage is the height difference between
two water levels, a supply creates the height difference by 'pumping up'
the water on one end, and the amount of water that is transfered between
two water levels, by 'falling though' the lamp and conversely being pumped
up by the supply is a measure for the current.
In mathematicians formulas:
V = I x R
==> R = V / I
==> I = V / R
Where V is the voltage measured in Volts (V), R the resistance in Ohms (*), and I the current in Amperes (A). This law is called Ohms' law, after its inventor, and it gouverns most fundamental electrical circuit behaviour.
In daily life, it is not common use to apply it, except when replacing
fuses, the lamp acts as a resistor, the mains supply is the supply, and
the current rating is on the fuse. The total resistance of the network
'behind' the fuse in Europe can have a resistance of R= V/I= 220 V/16 Amps
= 13.75 Ohm.
To let the same current run though a resistor (the equivalent of the
circuit of lights and appliances) fed by 110 Volts, a resistor of R = V/I
= 110/16 = 6.875 Ohms. In other words, when the voltage is reduced to a
half, the same current will be drawn when the resistance is halved as well.
How about the power that is consumed? We all know that more power requires
a fuse with higher rating, and in fact a proportional relation holds: the
more current, the more power, and the more voltage, the more power, in
formula:
P = V x I
==> V = P / I
==> I = P / V
Where P is power in Watts (W). When A light bulb is seen as a resistor,
that is that the current depends on the voltage applied in a straightforward
way, it is noteworthy that when the voltage is doubled, the current automatically
doubles as well, so that the amount of power quadruples!
Supppose we draw a graph of the voltage over a lamp against the current through it, a simplified graph is a straight line: more voltage --> more current. When the power rating of the lamp is low, the current goes up only slow with the voltage, when the power rating is higher, the sampe voltage will draw more current, so the graph is steeper.
Actually, and this is where a graph has a clear advantage over a formula,
the current through a lamp is not realy proportional to the voltage applied
to it, but a low voltage is relatively high, because the resistance of
the lamp is higher at low temperatures. The resistance can be seen as the
slope
of the graph for a certain voltage.
Voltage versus current over an idealized and actual lamp
Lamp
On
_____ _____
______
____|
|____| |____|
Off
|<-------->|
1 Second
---> Time
The lamp is then pulsed with a frequency of 1 Herz (Hz). Suppose it
would flash 5 times on and off per second, that would be a flashing frequency
of 5 Herz. The human eye can see about 15 Herz 'flashing' at least, higher
than 25 to 50 Herz, it just sees the average, 50% of the total lamp power.
Lamps themselves are slow, too. When they carry high currents, for instance
halogen lamps that run at twelve volts and for instance I= P/U = 60/12
= 5 Amps, they contain thick wires are heated up, so they might glow for
another second or so when turned off. Ordinary european light bulbs of
60 watts run at 220 Volts, and therefore draw a current of about I= 60W/220V
= ca. 0.28 Amps, or 280 milli Amperes. Such a light bulb has a much thinner
wire in it, that lights up and cools down a lot quicker, and is therefore
more suitable for light-organs, for instance.
In frequencies: neon lights could show frequencies up to the sensitivity
of the eye: up to about 25 Herz (Old 'flicker' movies), ordinary lights
a few herz, and low voltage, high power lamps about 0.5 Herz.
Apart from the logic, the simplified models of parts can cause inacuracies in the behaviour of a circuit composed of it. For instance assuming that two lamps of 60 Watts will allways produce the same amount of light is not correct. A halogene lamp or TL tube has higher efficiency than an ordinary light bulb, meaning that they produce less heat and more light for the same voltage and current fed to it. But still, assuming that measuring the amount of light coming out of them is enough to accurately model them, would put a photographer of, because he (or she) would know that the colors of these types of lamps are quite different.
The same holds for the way a lamp is switched on and off.
A more accurate graph would include the heating up and cooling down
curve of the lamp, and it would look a lot smoother. When switched on,
the lamp first heats up quite quickly, but at first without producing light
(until a few hundred degrees Celcius or so), and eventually it slowly reaches
its maximum temperature and light yield (for some spotlights that may take
up to minutes).
The electrical picture makes this even more complicated.
A combination of the way the lights' wire heats up against the temperature
of the environment, and the amount of power per lets say milli second the
lamp consumes as a result of its resistance and the supply voltage, which
also depends on the current the lamp draws, determines the accurate curve
of how much light is produced at each time instance.
Forgetting all this is enough for more than 99% of the times one thinks of a light, switch on: light, switch off: dark! Simple.
In other instances, it is sufficient to simply know that a lamp more or less smoothly heats up in a fraction of a second or so after the switch is operated, and for real advanced thoughts, one may want to compute exactly what happens.
The second example is the way an electronician may look at it, the third a scientist, and the first way is what a light show or digital designer likes to limit himself to, because that way a complex circuit still can be easily modeled: light on or light off !
When she (he?) wants to make a photograph of what is going on onstage or in a computer, the second and even the third way of looking at what's going on may be needed to get to the core of things. This should be remembered when following this section we'll look into the wonderfull world of digital circuits, because they too exhibit quite complicated behaviour when things get realy though.
Imagine a lamp is used as a morse signal transmitter: light on '1' or
'dash', light off '0' or blank. A fast morse operator could not use a stage
light at full speed: it wouldn't turn on and off fast enough! Then again,
a good morse reader might see it dim and light up as morse is fed through
it, and still try to decipher the message.
A computer has tens of millions of equivalent light switches and lights
in it, turned of up to billions of times a second (Giga Herz, 1,000,000,000
Herz), and each one of them can mess up the same way as I just described.
Then is pays to realize that the 'switches' and 'lights' are not ideal,
but models.
In case of doubt, or unavialability of a 5V stablelized wall-wart,which normally should be readily and cheaply available, electronics stores will sell dedicated 5V supplies for reasonable prices as well, and they are of course preferable.
In case of a plug-supply, be aware of the + and - of the connector, usually it is indicated on the box or on some label. Logic parts usually DO NOT SURVIVE a reverse voltage!!! Plus is + and - is minus or 'ground' or 'earth'.
They virtually live forever when treated well, they are complety shock-proof (unless you hammer it to pieces), and are very suitable as digital indicators.