New Students Day Activity

Transcription

1 Course: S ELECTRONICS New Students Day ctivity Introduction: In S Level Electronics you need to gain an understanding of the electronic circuits so that you can then start to design your own circuits like the one shown below. You will set up and test circuits in the electronics laboratory using electronics kits as well as computer software programs. t the beginning of the level course we will be concentrating on the fundamentals of digital and analogue circuits. We should start by ensuring that you understand the difference between a digital signal and an analogue signal. 1

2 n analogue signal. This is a signal that can have any value between the minimum and maximum of the power supply. Voltage (V) Max signal. Min time (s) digital This is a signal that can only have two finite values, usually at the minimum and maximum of the power supply. Voltage (V) Max For Min time being we will concentrate on digital systems. time (s) the When an input or output signal is at the minimum power supply voltage (usually 0V) this is referred to as a LOW signal or LOGIC 0 signal. When an input or output signal is at the maximum power supply voltage this is referred to as a HIGH signal or LOGIC 1 signal. Logic Gates The term logic gate actually gives a clue as to the function of these devices in an electronic circuit. Logic implies some sort of rational thought process taking place and a gate in everyday language allows something through when it is opened. 2

3 logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. They are the decision making units in electronic systems and there are many different types for different applications. 1.The NOT gate (or inverter) This is the simplest form of logic gate and has only 1 input and 1 output. So how can it make a decision if it only has 1 input? Simply the purpose of this gate is to invert the input signal so if a Logic 0 is at the input, the output will be at Logic 1 and vice versa. The symbol for a NOT gate is as follows. You will notice that the input has been given the letter and the output the letter. Traditionally inputs are given letters from the start of the alphabet,, C etc. but this is more a rule of thumb and is not written in stone. The output of a logic gate can also be summarised in the form of a table, called a Truth Table. The truth table for a NOT gate is the simplest of all Truth Tables and is shown below. Input Output There is also a shorthand way of writing down the function of this logic gate, using a special type of algebra called oolean lgebra. The oolean expression for a NOT gate is The bar over the indicates that the output is the opposite of. We will now consider four of the most common logic gates in use in electronic circuits. These are the : ND gate OR gate NND gate NOR gate EXOR gate 2. The ND gate. 3

4 We will start with a 2 input ND gate. The symbol for a 2 input ND gate is as follows. The truth table for the 2 input ND gate is shown below We can see that the output is only at a Logic 1 when Input ND Input are at a Logic 1. The oolean expression for a 2 input ND gate is. The. between the and means ND in oolean lgebra. Real World Example: The ND function can be demonstrated by thinking about a door with two locks. The only way to get through this door is to unlock lock 1 ND to unlock lock 2. Otherwise you will be stuck outside. 3.The OR gate. We will start with a 2 input OR gate. The symbol for a 2 input OR gate is as follows. The truth table for the 2 input OR gate is shown below

5 1 1 1 We can see that the output is at a Logic 1 when Input OR Input OR both are at a Logic 1. The oolean expression for a 2 input OR gate is The + between the and means OR in oolean lgebra. Real World Example: Picture a house that has a doorbell at the front door and a doorbell at the back door. oth doorbells are connected to the same buzzer inside the house. This buzzer will ring when EITHER OR OTH of the doorbells are pressed. 5

6 4.The NND gate. We will start with a 2 input NND gate. The symbol for a 2 input NND gate is as follows. The truth table for the 2 input NND gate is shown below If you compare this truth table with that for the ND gate, you will find that the output is the exact opposite to the ND. The oolean expression for a 2 input NND gate is s before the. between the and invert the output in oolean lgebra.. means ND, and the bar means 6

7 5.The NOR gate. We will start with a 2 input NOR gate. The symbol for a 2 input NOR gate is as follows. The truth table for the 2 input NOR gate is shown below If you compare this truth table with that for the OR gate, you will find that the output is the exact opposite of the OR. The oolean expression for a 2 input NOR gate is s before the + between the and invert the result in oolean lgebra. means OR and the bar means Now we will consider a 3 input NOR gate. 7

8 6.The EXOR gate. The EXOR gate has 2 inputs and is a specialised version of the OR gate. The symbol for a 2 input EXOR gate is as follows. The truth table for the 2 input EXOR gate is shown below Comparison with the 2 input OR gate will reveal that is a Logic 1 when either or is a Logic 1, but not when and are Logic 1. The oolean expression for a 2 input EXOR gate is alternatively The between the and means Exclusive OR, however the alternative form will prove to be more useful later on in the course when simplifying oolean expressions... Real World Example: n example of an XOR gate would be a game show buzzer. If two contestants buzz in, only one of them, the first to buzz, will activate the circuit. The other contestant will be locked out from buzzing. Now try this: 1. Look at the following logic symbols labelled G. 8

9 i. Which is the correct symbol for an ND gate. ii. Which is the correct symbol for a NOT gate. iii. Which is the correct symbol for a NOR gate. iv. Which is the correct symbol for an EXOR gate. v. Which is the correct symbol for a NND gate. vi. Which is the correct symbol for an OR gate. 1. Complete the following truth tables. i. ND gate

10 ii. NOR gate iii. NND gate iv. OR gate The oolean equations labelled I, below are to be used to answer the following questions. 10 ). ) C) D).. E) F).. G)

11 H). i. Which expression is correct for an ND gate. ii. Which expression is correct for a NOT gate. iii. Which expression is correct for a NOR gate. iv. Which two expressions are correct for an EXOR gate. & v. Which expression is correct for a NND gate. vi. Which expression is correct for an OR gate. 11