One bit is a short way of saying one `binary digit'. It is a single 0 or 1.
Interesting fact
If you have eight bits, you can store a binary number from 00000000 to 11111111 (0 to 255 in denary). This gives you enough permutations of 0s and 1s to have one for each letter of the alphabet (in upper and lower case), each digit from 0 to 9, each punctuation mark and each control code used by a computer in storing a document. When you type text into a word processor, each character is stored as a set of eight bits. Each set of eight bits is called a
byte . Computer memories are graded according to how many bytes they store. There are 1024 bytes in a kilobyte (kB),
bytes in a megabyte (MB), and
bytes in a gigabyte (GB).
To store a bit we need a circuit which can `remember' a 0 or a 1. This is called a
bistable circuit because it has two stable states. It can stay indefinitely either as a 0 or a 1. An example of a bistable circuit is shown in
[link] . It is made from two NOR gates.
A bistable circuit made from two NOR gates. This circuit is able to store one bit of digital information. With the two inputs set to 0, you can see that the output could be (and will remain) either 0 or 1. The circuit on the top shows an output of 0, the circuit underneath shows an output of 1. Wires carrying high logic levels (1) are drawn thicker. The output of the bistable is labelled Q.
To store the 0 or the 1 in the bistable circuit, you set one of the inputs to 1, then put it back to 0 again. If the input labelled `S' (set) is raised, the output will immediately become 1. This is shown in
[link] .
The output of a bistable circuit is
set (made 1) by raising the `S' input to 1. Wires carrying high logic levels (1) are shown with thicker lines.
To store a 0, you raise the `R' (reset) input to 1. This is shown in
[link] .
The output of a bistable circuit is
reset (made 0) by raising the `R' input to 1. Wires carrying high logic levels (1) are shown with thicker lines.
Once you have used the S or R inputs to set or reset the bistable circuit, you then bring both inputs back to 0. The bistable `remembers' the state. Because of the ease with which the circuit can be Reset and Set it is also called an
RS flip flop circuit.
Computer memory can store millions or billions of bits. If it used our circuit above, it would need millions or billions of NOR gates, each of which is made from several transistors. Computer memory is made of many millions of transistors.
Interesting fact
The bistable circuits drawn here don't remember 0s or 1s forever — they lose the information if the power is turned off. The same is true for the RAM (Random Access Memory) used to store working and temporary data in a computer. Some modern circuits contain special memory which can remember its state even if the power is turned off. This is used in FLASH drives, commonly found in USB data sticks and on the memory cards used with digital cameras. These bistable circuits are much more complex.
You can also make T flip flops out of logic gates, however these are more complicated to design.
Counting circuits
What is the term
bit short for?
What is 43 in binary?
What is 1100101 in denary?
What is the highest number a modulo 64 counter can count to? How many T flip flops does it contain?
What is the difference between an RS flip flop and a T flip flop?
Draw a circuit diagram for a bistable circuit (RS flip flop). Make three extra copies of your diagram. On the first diagram, colour in the wires which will carry high voltage levels (digital 1) if the R input is low, and the S input is high. On the second diagram, colour in the wires which carry high voltage levels if the S input of the first circuit is now made low. On the third diagram, colour in the wires which carry high voltage levels if the R input is now made high. On the final diagram, colour in the wires carrying high voltage levels if the R input is now made low again.
Justify the statement: a modern computer contains millions of transistors.
End of chapter exercises
Calculate the reactance of a 3 mH inductor at a frequency of 50 Hz.
Calculate the reactance of a 30
F capacitor at a frequency of 1 kHz.
Calculate the impedance of a series circuit containing a 5 mH inductor, a 400
F capacitor and a 2 k
resistor at a frequency of 50 kHz.
Calculate the frequency at which the impedance of the circuit in the previous question will be the smallest.
Which component can be used to block low frequencies?
Draw a circuit diagram with a battery, diode and resistor in series. Make sure that the diode is forward biased so that a current will flow through it.
When building a complex electronic circuit which is going to be powered by a battery, it is always a good idea to put a diode in series with the battery. Explain how this will protect the circuit if the user puts the battery in the wrong way round.
Summarize the differences betwen a bipolar and field effect transistor.
What does an operational amplifier (op-amp) do?
What is the difference between a digital signal and an analogue signal?
What are the advantages of digital signals over analogue signals?
Draw the symbols for the five logic gates, and write down their truth tables.
Draw a circuit diagram with an AND gate. Each input should be connected to the output of a separate NOT gate. By writing truth tables show that this whole circuit behaves as a NOR gate.
Convert the denary number 99 into binary.
Convert the binary number 11100111 into denary.
Explain how three T flip flops can be connected together to make a modulo 8 counter. What is the highest number it can count up to?
Draw the circuit diagram for an RS flip flop (bistable) using two NOR gates.
Show how the circuit you have just drawn can have a stable output of 0 or 1 when both inputs are 0.
Operational (and other) amplifiers, logic gates, and flip flops all contain transistors, and would not work without them. Write a short newspaper article for an intelligent reader who knows nothing about electronics. Explain how important transistors are in modern society.
