Download Computers – “From The Bottom Up”

Computers –
“From The Ground Up”
Electricity,
Binary Numbers,
Logic Gates,
Binary Adders,
The Computer Power Supply
Electricity
• For our purposes “electricity” is the flowing
of an electric charge.
• We can view that “flowing charge” as a
rope passing through a pulley or as a
stream of water passing through a pipe.
• When talking about electricity we use the
terms “voltage”, “current”, “resistance” and
“power”.
• Voltage (V = volts): The difference in electric
potential between two points.
• Current (I = amps): The amount of electrical flow
across a conductor.
• Power (P = watts): The amount of “work” done
by an electric charge. P = I x V.
• Resistance (R = ohms): The amount of
resistance to electrical flow from a given
substance. Ohm’s Law: V = I x R
– Resistors, Conductors, Semi-conductors
• Analogy: Consider the water pressure
(voltage) required to pass a litre of water
(current) through a copper pipe of a
certain small diameter (resistance).
• Example: A 60 Watt light bulb hooked to a
standard 120 Volt household circuit draws
0.5 amps of current (P = I * V) and thus
the filament in the light bulb must be
offering 240 ohms of resistance (V = I * R).
• Electricity can be used to provide “work” in a
variety of ways by:
– Heating a light bulb filament.
– Forcing a magnet to move inside an electric motor.
– “Flipping the switch” in a transistor.
Binary Numbers
• The binary number system works under
the exact same principles as the decimal
system, only it operates in base 2 rather
than base 10. In other words, instead of
columns being:
– | 10^2 | 10^1 | 10^0 |
they are
– | 2^2 | 2^1 | 2^0 |
Boolean Logic Gates
• Binary numbers (0 and 1) can be used to
represent whether or not an electrical
circuit is off (0) or on (1).
• Boolean logic gates (built using
transistors) can be used to conduct simple
logical operations (such as addition).
And Gate
X
0
0
1
1
Y
0
1
0
1
Output
0
0
0
1
Other Logic Gates
•
•
•
•
•
•
•
NOT
OR
NAND
NOR
XAND
XOR
These gates can be put together to make
logic circuits, like adders.
Binary Adders
• If you can add (and store numbers), you
can do anything:
• http://woodgears.ca/marbleadd/
The Power Supply
• The power supply converts the alternating current (AC)
power from the grid to the direct current (DC) needed by
a personal computer.
• In a personal computer (PC), the power supply is the
metal box usually found in a corner of the case. It
contains the power-cord receptacle and the cooling fan.
• Power supplies convert AC input to lower DC voltages.
The typical voltages supplied are:
–
–
–
* 3.3 volts
* 5 volts
* 12 volts
• The power supply is the one really “dangerous” part
of a computer. NEVER EVER open it.
Elector Static Discharge (ESD)
• The human body can build up static
charges of up to 25,000 volts. ESD
is not usually dangerous to humans
because it lasts only microseconds.
• ESD poses a risk to some
electronic circuitry, where even a
micro-amp current discharge can
damage miniature transistors.
• Make sure you “ground” yourself
before touching electronic
equipment and/or wear an antistatic wrist strap.
For more information:
• Electricity:
– http://www.swansontec.com/set.html
• Binary Numbers:
– http://www.math.grin.edu/~rebelsky/Courses/152/97F/
Readings/student-binary
• Boolean Logic Gates & Binary Adders:
– http://computer.howstuffworks.com/boolean.htm
• The Binary Marble Adder (Info):
• http://www.exploringbinary.com/the-binarymarble-adding-machine/
• The Computer Power Supply