2. ALLAH, IN WHOSE NAME I BEGIN,
THE MOST AFFECTIONATE,
THE MOST MERCIFUL.
2
3. Objective:
To understand the function of
Logic Gates.
•APPARATUS: Breadboard Diode
Transistor Connecting Wires
Resistor LED
3
4. A digital system deals with quantities or variables which have only
two discrete values or states.
Following are the examples of such quantities:
•a switch can be either closed or open.
•the answer of a question can be either yes or no.
•a certain statement can be either true or false.
•a bulb can be either off or on.
Digital Systems
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5. •We have mostly two inputs A, B & one output C.
•There are many different ways that C could depend upon A and B.
•The output C is a function, - a logic function - of the inputs A and B.
Operations on logic signals
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6. • DEFINITION:
• A logic gate is an elementary building block of a digital circuit. Most logic
gates have two inputs and one output. At any given moment, every
terminal is in one of the two binary conditions low (0) or high (1),
represented by different voltage levels. The logic state of a terminal can,
and generally does, change often, as the circuit processes data. In most
logic gates, the low state is approximately zero volts (0 V), while the high
state is approximately five volts positive (+5 V).
Logic Gates
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10. • Some gate symbols have a circle on their output which means that their function
includes inverting of the output. It is equivalent to feeding the output through a not
gate.
The inverting circle (o):
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11. •A DIAGRAM OF THE OUTPUTS FROM ALL POSSIBLE COMBINATIONS
OF INPUT
Truth Table
Input A Input B Output Q
0 0 0
0 1 0
1 0 0
1 1 1
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17. Mode Of Action
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1. When A is at 0V, diode D1 conducts. So the
point C is driven to 0V.
2. The same mechanism is repeated if B is at 0v,
then D2 conducts which switches the point like
0v.
3. When both A & B are at 0V, both the diodes
conduct and point C is at 0V.
4. When both the inputs A & B are at +5V, no
current will flow. The point C will be at 5V and
we get the logic answer 1.
19. 1. ELECTRONIC DOOR WILL ONLY OPEN IF IT DETECTS A PERSON AND THE SWITCH IS SET
TO UNLOCKED.
2. MICROWAVE WILL ONLY START IF THE START BUTTON IS PRESSED AND THE DOOR
CLOSE SWITCH IS CLOSED.
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General Application
25. Mode Of Action
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1. When A is at +5V, D1 is forward biased and
hence conduct. We get +5V at point C.
2. Same mechanism happens if B is at +5V, D2
Conducts, switching the point C at 5V.
3. When both A & B are at 5V, both the diode D1
and D2 conduct and the point C is again at
5V.
4. The point C will be at 0V if and only both A &
B are at 0V.
26. Application
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Front Doorbell Switch
Back Doorbell Switch
Doorbell
ON/OFF
ON/OFF
ON/OFF
If either the Front Doorbell Switch
OR the Back Doorbell Switch is
pressed then the Doorbell rings.
32. Mode Of Action
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1. When A is at 5V, the emitter based junction is
forward biased and the transistor is on and
maximum current flows through the emitter lead.
So point C will be at 0V.
2. When we connect A with 0V, the transistor stops
working and we get the point C at +5V.
33. Application
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Temperature Detector
(Above 20°C)
Central Heating
ON/OFF ON/OFF
If the temperature is above 20°C
then the Central Heating is switched
off.
If the temperature is below 20°C
then the Central Heating is switched
on.
34. 1. MICROWAVE WILL STOP IF THE DOOR IS NOT CLOSED.
2.TRAFFIC LIGHT WILL OPERATE NORMALLY IF IT'S NOT IN MAINTENANCE
MODE.
3.STREET LIGHTS ARE TURNED ON WHEN THERE IS NO SUNLIGHT OR JUST
WITH TWILIGHT.
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General Application
36. Definition
The NOR is a combination of an OR followed by a
NOT gate. The output is logical 1 when non of the
inputs are logical 0
NOR gates are so-called "universal gates" that can be
combined to form any other kind of logic gate.
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40. • NOR GATE CAN BE USED TO REALIZE THE BASIC LOGIC FUNCTION OF OR GATE, AND GATE
1. AS OR GATE:
THE OUTPUT FROM NOR GATE IS 𝑨 + 𝑩 BY USING ANOTHER INVERTER IN THE OUTPUT, THE
OUTPUT IS INVERTED AND IS GIVEN BY , C=A+B, WHICH IS LOGIC FUNCTION OF A SIMPLE OR
GATE.
2.AS AND GATE:
TWO INVERTERS WILL BE USED, ONE FOR EACH INPUT. THE INPUTS HAVE BEEN INVERTED
BEFORE THEY HAVE BEEN APPLIED TO THE NOR GATE. THE OUTPUT IS
𝑨 + 𝑩 = 𝑨. 𝑩
WHICH IS DEMORGAN’S THEOREM.
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NOR gate as Universal Gate
41. Mode Of Action
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1. The function of NOR gate is just the reverse
of OR gate function.
2. NOR gate will have an output 1 only when
both inputs A & B are at logic 0V
3. If any of the input is 1V, the output is 0V.
48. • NAND GATE CAN BE USED TO REALIZE THE BASIC LOGIC FUNCTION OF OR GATE, AND GATE
1. AS AND GATE:
THE OUTPUT FROM NAND GATE IS 𝑨. 𝑩 BY USING ANOTHER INVERTER IN THE
OUTPUT, THE OUTPUT IS INVERTED AND IS GIVEN BY , C=A.B , WHICH IS LOGIC
FUNCTION OF A SIMPLE AND GATE.
2.AS OR GATE:
TWO INVERTERS WILL BE USED, ONE FOR EACH INPUT. THE INPUTS HAVE BEEN
INVERTED BEFORE THEY HAVE BEEN APPLIED TO THE NANDGATE. THE OUTPUT IS
𝑨. 𝑩 = 𝑨 + 𝑩
WHICH IS DEMORGAN’S THEOREM.
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NAND gate as Universal Gate
49. Mode Of Action
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1. The function of NAND gate is just the reverse of
AND gate function.
2. NAND gate will have an output 1 only when both
inputs A & B are at logic 0V or any of the inputs is
0V.
3. If both the inputs are 1, the output will be 0V.
50. Application
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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 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.
