Logic gates

Logic gates are the basic building blocks of any digital system. It is an electronic circuit having one or more than one input and only one output. The relationship between the input and the output is based on a certain logic. Based on this, logic gates are named as AND gate, OR gate, NOT gate etc.

1. LOGIC GATES ELECTRONICS 2

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3. CONTENTS History Definition The OR gate The AND gate The NOT gate The NAND gate The NOR gate The XOR gate The XNOR gate Applications

4. HISTORY The binary number system was refined by Gottfried Wilhelm Leibniz (published in 1705 ). In an 1886 letter, Charles Sanders Peirce described how logical operations could be carried out by electrical switching circuits.

5. INTRODUCTION Logic gates perform basic logical functions and are the fundamental building blocks of digital integrated circuits . Most logic gates take an input of two binary values, and output a single value of a 1 or 0. LOGIC Formal logic is a branch of the mathematics that deals with true and false values instead of numbers. In the mid-19thcentury, George Bool developed many Logic ideas. Boolean logic deals with equations where the operators are “AND” or “OR” instead of “add” and “multiply”.

6. TYPES OF GATES AND - True if A and B are both True OR - True if either A or B is True NOT - Inverts value: True if input is False; False if input is True NAND - AND followed by NOT: False only if A and B both are True NOR - OR followed by NOT: True only if A and B both are False XOR -The XOR logic gate can be used as a one-bit adder that adds any two bits together to output one bit . XNOR - An XNOR Gate is a type of digital logic gate that receives two inputs and produces one output. Both inputs are treated with the same logic,

7. OR gate has two or more inputs and performs what is known as logical addition. The output of OR gate is low when all inputs are low otherwise all inputs are high. THE OR GATE OPERATION OF OR GATE

8. CIRCUIT DIAGRAM

9. An AND gate has two or more inputs and perform what is known as multiplication. The output of AND gate is high when all inputs are high otherwise all inputs are low. THE AND GATE

10. OPERATION OF AND GATE An AND gate produces a HIGH output only when all inputs are HIGH. When any of the input is LOW the output is LOW. Therefore the basic purpose of AND gate is to determine when certain conditions are simultaneously true, as indicated by HIGH levels on all of its input and produces a HIGH on its output.

11. CIRCUIT DIAGRAM

12. LOGICAL EXPRESSION If the logical expression is, X=A.B

13. This gate consists of three elements, a transistor and two resistors. It is called the NOT Gate. Its output is opposite to its input therefore it is also called an Inverter. It has one input and one output. Boolean expression of NOT Gate is: A = A NOT GATE TRUTH TABLE

14. SYMBOLIC REPRESENTATION

15. CIRCUIT DIAGRAM

16. A gate is a device that performs a basic operation on electrical signals Gates are combined into circuits to perform more complicated tasks There are three different, but equally powerful, notational methods for describing the behavior of gates and circuits Boolean expressions logic diagrams truth tables NOR GATE

17. CIRCUIT DIAGRAM

18. Logic diagram : a graphical representation of a circuit Each type of gate is represented by a specific graphical symbol. Boolean expressions:

19. LOGIC NAND GATE DEFINITION A combination of AND gate and NOT gate makes a NAND gate.Its symbolic representation is shown below. NAND GATE COMPO N E N T Two ideal p_n junction diode(D1 &D2) Ideal n-P-n transistor

20. THEORY AND C O N S T RUC TI ON If we connect the output y of AND gate to the input of a NOT gate the gate obtained is called NAND gate. The output y is voltage at c w.r.t earth In boolean expresion the NAND gate is expressed as y=A.B And is being read as A AND B negated

21. W O R K I N G The folowing inerference can be easily drawn from working of electrical circuit: If switch A &B open (A=0,B=0) hence Y=1. If switch Aopen B closed (A=0,B=1)hence Y=1 If switch A closed B OPEN(A=1,B=0)hence Y=1 If switch A&B are closed (A=1,B=1)hence Y=0 LOGIC SYMBOL AND TRUTH TABLE The graphic symbol for the NAND gate consists of an AND symbol with a bubble on the output, denoting that a complement operation is performed on the output of the AND gate.

22. XOR GATE What is an XOR Gate ? An XOR gate (also known as an EOR, or EXOR gate) – pronounced as Exclusive OR gate – is a digital logic gate that gives a true (i.e. a HIGH or 1) output when the number of true inputs is odd. An XOR gate implements an exclusive OR, i.e., a true output result occurs if one – and only one – of the gate’s inputs is true. If both inputs are false (i.e. LOW or 0) or both inputs are true, the output is false . XOR represents the inequality function, i.e. the output is true if the inputs are not alike; otherwise, the output is false. A common way to remember the XOR is “must have one or the other, but not both”. Another way to look at an XOR gate: a modulo sum of two variables in a binary system looks like this:

23. XOR Gate Truth Table XOR Gate Circuit Diagram Logical Symbol of XOR Gate

24. What is an XNOR Gate? The XNOR gate (also known as a XORN’T, ENOR, EXNOR or NXOR) – and pronounced as Exclusive NOR – is a digital logic gate whose function is the logical complement of the exclusive OR gate (XOR gate). Logically, an XNOR gate is a NOT gate followed by an XOR gate . XNOR Gate Truth Table XNOR GATE

25. XNOR Gate Circuit Diagram The symbol of the XNOR gate:

26. LOGIC GATES APPLICATIONS

27. OR GATE Industrial Plant Wherever the occurrence of any one or more than one event is needed to be detected or some actions are to be taken after their occurrence, in all those cases OR gates can be used. In an industrial plant if one or more than one parameter exceeds the safe value, some protective measure is needed to be done. OR gate is used to detect exceed of temperature or pressure and produce command signal for the system to take required actions. We are going to show this with the help of a diagram .

28. Microwave oven will only start if the ‘’start’’ button is pressed and the door is closed. If the ‘’start’’ button is pressed and door is open then the oven will not work and vice versa. Illustration is given by the diagram. AND GATE MICROWAVE OVEN

29. The NOT gate is also known as an inverter, simply because it changes the input to its opposite (inverts it). It is used in combination with other gates. The application discussed here is that of a door closing system of an automobile. A car needs to be so designed that the driver gets a visual indication if any of the doors of the car is open so that it helps to avoid accident and injury to the passengers. Assuming there are two doors (just for simplicity, it works for more doors as well) where this system is fitted, the circuit can be designed using a NAND gate as follows You can see from the figure that when any of the switches is open due to the door position, the NAND gate energies the lamp inside the car, hence warning the driver. NOT GATE Safety Reminder in Cars

30. Signal

31. When the thermistor ( sensor for temperature) is COLD its resistance is LARGE and the input to the NAND gate is high. Since the NAND gate is connected as an INVERTER the output is LOW. As the thermistor warms up its resistance decreases, the voltage across it falls and the input to the NAND gate falls. When it becomes low enough the output becomes HIGH and the buzzer sounds. NAND GATE Freezer warning buzzer

32. When the switch is closed (burglar steps on a pressure pad) one input of the NAND gate is LOW. When the LDR (Light Dependent Resistor) is in the light (the burglar's torch) the other input is LOW. This means that if either of these things happen, i.e. the switch is closed or the light is on one of the inputs is LOW, the output is HIGH and the buzzer sounds. BURGLAR ALARM NAND Gate

33. NOR GATE BLENDERS