Digital Logic Gates

Digital systems are constructed using logic gates. Basic Logic gates are AND, OR, NOT, NAND, NOR, EXOR and EXNOR. The basic operations are described below with the aid of truth tables.

AND

A Logic AND Gate is a type of digital logic gate that has an output which is normally at logic level “0” and only goes “HIGH” to a logic level “1” when ALL of its inputs are at logic level “1”. The output state of a “Logic AND Gate” only returns “LOW” again when ANY of its inputs are at a logic level “0”.

The logic or Boolean expression given for a Digital Logic AND Gate is that for Logical Multiplication which is denoted by a single dot or full stop symbol, ( . ) giving us the Boolean expression of:  

 A.B = Q.

 

“If both A and B are true, then Q is true”

Symbol	Truth Table
2-input AND Gate	A	B	Q
	0	0	0
	0	1	0
	1	0	0
	1	1	1
Boolean Expression Q = A.B	Read as A AND B gives Q

OR

A Logic OR Gate or Inclusive-OR gate is a type of digital logic gate that has an output which is normally at logic level “0” and only goes “HIGH” to a logic level “1” when one or more of its inputs are at logic level “1”. The output, Q of a “Logic OR Gate” only returns “LOW” again when ALL of its inputs are at a logic level “0”.

The logic or Boolean expression given for a Digital Logic OR Gate is that for Logical Addition which is denoted by a plus sign, ( + ) giving us the Boolean expression of: 

 A+B = Q.

 “If either A or B is true, then Q is true”

Symbol	Truth Table
2-input OR Gate	A	B	Q
	0	0	0
	0	1	1
	1	0	1
	1	1	1
Boolean Expression Q = A+B	Read as A OR B gives Q

NOT

The digital Logic NOT Gate is the most basic of all the logical gates and is sometimes referred to as an Inverting Buffer or simply a Digital Inverter. It is a single input device which has an output level that is normally at logic level “1” and goes “LOW” to a logic level “0” when its single input is at logic level “1”.

The output from a NOT gate only returns “HIGH” again when its input is at logic level “0” giving us the Boolean expression of:   

A = Q.

“If A is NOT true, then Q is true”

Symbol	Truth Table
Inverter or NOT Gate	A	Q
	0	1
	1	0
Boolean Expression Q = not A or A	Read as inverse of A gives Q

NAND

 

The Logic NAND Gate is a combination of the digital logic AND gate with that of an inverter or NOT gate connected together in series. The NAND (Not – AND) gate has an output that is normally at logic level “1” and only goes “LOW” to logic level “0” when ALL of its inputs are at logic level “1”. The Logic NAND Gate is the reverse or “Complementary” form of the AND gate we have seen previously.

Logic NAND Gate Equivalence

The logic or Boolean expression given for a logic NAND gate is that for Logical Addition, which is the opposite to the AND gate, and which it performs on the complements of the inputs. The Boolean expression for a logic NAND gate is denoted by a single dot or full stop symbol, ( . ) with a line or Overline, ( ‾‾ ) over the expression to signify the NOT or logical negation of the NAND gate giving us the Boolean expression of:   
                                                                                                               ___

A.B = Q.

“If either A or B are NOT true, then Q is true”

Symbol	Truth Table
2-input NAND Gate	A	B	Q
	0	0	1
	0	1	1
	1	0	1
	1	1	0

NOR

The Logic NOR Gate or Inclusive-NOR gate is a combination of the digital logic OR gate with that of an inverter or NOT gate connected together in series. The NOR (Not – OR) gate has an output that is normally at logic level “1” and only goes “LOW” to logic level “0” when ANY of its inputs are at logic level “1”. The Logic NOR Gate is the reverse or “Complementary” form of the OR gate we have seen previously. 

Logic NOR Gate Equivalent

 

The logic or Boolean expression given for a logic NOR gate is that for Logical Multiplication which it performs on the complements of the inputs. The Boolean expression for a logic NOR gate is denoted by a plus sign, ( + ) with a line or Overline, ( ‾‾ ) over the expression to signify the NOT or logical negation of the NOR gate giving us the Boolean expression of:   
                                                                                                              ____

A+B = Q.

 “If both A and B are NOT true, then Q is true”

Symbol	Truth Table
2-input NOR Gate	A	B	Q
	0	0	1
	0	1	0
	1	0	0
	1	1	0

 

EXOR

The two-input “Exclusive-OR” gate is basically a modulo two adder, since it gives the sum of two binary numbers and as a result are more complex in design than other basic types of logic gate. The truth table, logic symbol and implementation of a 2-input Exclusive-OR gate is shown below.

 

Symbol	Truth Table
2-input Ex-OR Gate	A	B	Q
	0	0	0
	0	1	1
	1	0	1
	1	1	0

The truth table above shows that the output of an Exclusive-OR gate ONLY goes “HIGH” when both of its two input terminals are at “DIFFERENT” logic levels with respect to each other. If these two inputs, A and B are both at logic level “1” or both at logic level “0” the output is a “0” making the gate an “odd but not the even gate”. 

EXNOR

The Exclusive-NOR Gate function or Ex-NOR for short, is a digital logic gate that is the reverse or complementary form of the Exclusive-OR function. Basically the “Exclusive-NOR Gate” is a combination of the Exclusive-OR gate and the NOT gate but has a truth table similar to the standard NOR gate in that it has an output that is normally at logic level “1” and goes “LOW” to logic level “0” when ANY of its inputs are at logic level “1”.

Ex-NOR Gate Equivalent

 

Symbol	Truth Table
2-input Ex-NOR Gate	A	B	Q
	0	0	1
	0	1	0
	1	0	0
	1	1	1

Variable Voltage Regulators

Here is a simple variable voltage regulator circuit using LM317 and LM337 ICs

Components Required:

1) Breadboard

2) 1Amps Fuse

3) 15 - 0 - 15 center tapped Transformer

4) 1N4007 diode = 4

5) 2200uf capacitor = 2

6) 100nf capacitor = 2
7) 150 ohm resistor = 2
8) 3.9 kilo ohm Potentiometer (POT) = 2

9) LM317 Positive Voltage Regulator IC

10) LM337 Negative Voltage Regulator IC

Explanation for Dual Voltage Regulator:

• Connect the circuit as shown in the circuit diagram below.
• The output of the center tapped transformer is AC voltage.
• Voltage between the center and anyone of the other terminal gives +15Volt AC, Voltage between the two outer terminals gives +30Volt AC.
• Connecting in center tapped pattern gives +15Volt AC across terminals 1 & 2 and gives -15Volt AC across terminals 3 & 2.
• Four 1N4007 diode connected as shown below forms a bridge circuit which converts bidirectional AC into unidirectional DC.
• The converted signal is not pure DC, it still consists of some AC signals.
• The 2200uf capacitor is used as a filter, which allows only DC to pass through..
• LM317 Voltage Regulators are used for Adjustable Positive voltage values & LM337 Voltage Regulators are used for Adjustable Negative voltage values.
• LM317 and LM337 consists of three terminals namely regulated output voltage, adjustable terminal and unregulated input voltage.
• +15Volt DC input goes to LM317 and -15Volt DC input goes to Lm337, which gives +1.5 to +13Volt and -1.5 to -13Volt regulated DC output.
• Output of the regulator produces noise which can be removed using 100nf capacitor, which acts as a filter.

Circuit Diagram:

Figure 1: Variable Dual Voltage Regulator using LM317 & LM337 Circuit simulation made in Multisim

Explanation for Single Voltage Regulator:

• Connect the circuit as shown in the circuit diagram below.
• The output of the center tapped transformer is AC voltage.
• Voltage between the two outer terminals gives +30Volt AC.
• Four 1N4007 diode connected as shown below forms a bridge circuit which converts bidirectional AC into unidirectional DC.
• The converted signal is not pure DC, it still consists of some AC signals.
• The 2200uf capacitor is used as a filter, which allows only DC to pass through..
• LM317 Voltage Regulators are used for Adjustable Positive voltage values.
• LM317 consists of three terminals namely regulated output voltage, adjustable terminal and unregulated input voltage.
• +30Volt DC input goes to LM317, which gives +1.5 to +28Volt regulated DC output.
• Output of the regulator produces noise which can be removed using 100nf capacitor, which acts as a filter.

Circuit Diagram:

 

 Figure 2: Variable Voltage Regulator using LM317 Circuit simulation made in Multisim

Fixed Voltage Regulators

Here is a simple Fixed voltage regulator circuit using 7805 and 7905 voltage regulator ICs

Components Required:

1) Breadboard

2) 1Amps Fuse

3) 9 - 0 - 9 center tapped Transformer

4) 1N4007 diode = 4

5) 2200uf capacitor = 2

6) 100nf capacitor = 2

7) LM7805 Positive Voltage Regulator IC

8) LM7905 Negative Voltage Regulator IC

Explanation for Dual Voltage Regulator:

• Connect the circuit as shown in the circuit diagram below.
• The output of the center tapped transformer is AC voltage.
• Voltage between the center and anyone of the other terminal gives +9Volt AC, Voltage between the two outer terminals gives +18Volt AC.
• Connecting in center tapped pattern gives +9Volt AC across terminals 1 & 2 and gives -9Volt AC across terminals 3 & 2.
• Four 1N4007 diode connected as shown below forms a bridge circuit which converts bidirectional AC into unidirectional DC.
• The converted signal is not pure DC, it still consists of some AC signals.
• The 2200uf capacitor is used as a filter, which allows only DC to pass through..
• 78XX Voltage Regulators are used for Positive voltage values and 79XX Voltage Regulators are used for Negative voltage values.
• Here i have used +5Volt and -5Volt Voltage Regulator ICs ie) 7805 and 7905.
• 78XX and 79XX consists of three terminals namely unregulated input voltage, ground and regulated output voltage.
•  +9Volt DC input goes to 7805 and -9Volt DC input goes to 7905, which gives +5Volt and -5Volt regulated DC output.
• Output of the regulator produces noise which can be removed using 100nf capacitor, which acts as a filter.

Circuit Diagram:

 

Figure 1: Fixed Dual Voltage Regulator Circuit simulation made in Multisim

Explanation for Single Voltage Regulator:

• Connect the circuit as shown in the circuit diagram below.
• The output of the center tapped transformer is AC voltage.
• Voltage between the center and anyone of the other terminal gives +9Volt AC, Voltage between the two outer terminals gives +18Volt AC.
• Four 1N4007 diode connected as shown below forms a bridge circuit which converts bidirectional AC into unidirectional DC.
• The converted signal is not pure DC, it still consists of some AC signals.
• The 2200uf capacitor is used as a filter, which allows only DC to pass through..
• 78XX Voltage Regulators are used for Positive voltage values and 79XX Voltage Regulators are used for Negative voltage values.
• Here i have used +5Volt Voltage Regulator IC ie) 7805. Similarly we can replace 7805 with 7905 to get -5Volt regulated output.
• 78XX consists of three terminals namely unregulated input voltage, ground and regulated output voltage.
•  +9Volt DC input goes to 7805, which gives +5Volt regulated DC output.
• Output of the regulator produces noise which can be removed using 100nf capacitor, which acts as a filter.
• For other voltage values we can simply replace 7805 with the corresponding 78XX or 79XX Voltage Regulator ICs.

Circuit Diagram:

 

Figure 2: Fixed Single Voltage Regulator Circuit simulation made in Multisim

Regulated Power Supply

A regulated power supply converts unregulated AC into a constant DC with the help of a voltage rectifier. Its function is to supply a stable voltage (or less often current), to a circuit or device that must be operated within certain power supply limits. Any deviation from these rating can damage the devices.

Some DC power supplies use AC mains electricity as an energy source. Such power supplies will employ a transformer to convert the input voltage to a lower AC voltage. A rectifier is used to convert the transformer output voltage to a varying DC voltage, which in turn is passed through an electronic filter to convert it to an unregulated DC voltage. The filter removes most, but not all of the AC voltage variations; the remaining voltage variations are known as ripple.

Types:

• Fixed Voltage Power Supply
• Variable Voltage Power Supply

Fixed Voltage Power Supply

The unregulated DC output is fixed to a constant voltage level by using a voltage regulator. Commonly used fixed voltage values are 5,9,12,15 Volts.

Variable Voltage Power Supply

The unregulated DC output can be varied over a range of voltage levels by using a variable voltage regulators.

Simple Alarm

Here is the Simple Alarm circuit using SCR.

Components Required:

1) Breadboard

2) 1N4007 Diode

3) 470 ohm resistor

4) 100 kilo ohm resistor

5) LED (Any Color)

6) C106 SCR

7) Buzzer

8) DIP Switch = 2

9) 9Volt Battery

Procedure:

• Connect the circuit as shown in the circuit diagram below.
• Close the S1 switch and switch S2 is kept open therefore there is no gate current and the SCR remains OFF.
• Hence the LED and alarm connected to the circuit also remains OFF.
• Now close switch S2 also therefore gate current is applied which triggers the SCR to ON state and the gate terminal looses its control and cannot be used to switch OFF the SCR.
• Hence the LED and alarm connected to the circuit switches to ON state.
• To switch OFF the SCR, decrease the gate current below the holding current of the SCR or disconnect the gate current.

 Circuit Diagram:

Figure 1: Simple Alarm Circuit simulation made in Multisim (S1 Closed & S2 Open)

 

Figure 2: Simple Alarm Circuit simulation made in Multisim (S1 & S2 Closed)

Figure 3: Simple Alarm Circuit (S1 & S2 Closed)

SCR (Silicon-Controlled Rectifier)

A Silicon-Controlled Rectifier is a four layer solid state current controlling device. It forms PNPN structure and has three junctions, labeled anode, cathode and gate. Anode terminal is connected to the P-type material, cathode terminal is connected to the N-type material and the gate is connected to the P-type material nearest to the cathode. SCRs are unidirectional devices (i.e. can conduct current only in one direction)

Modes of Operation:

• Forward blocking mode (OFF state)
• Forward conduction mode (ON state)
• Reverse blocking mode (OFF state)

Forward blocking mode

In this mode of operation the anode is given a positive potential while the cathode is given a negative voltage keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward biased while J2 is reversed biased due to which only a small leakage current flows from the anode to the cathode until the applied voltage reaches its breakover value at which J2 undergoes avalanche breakdown and at this breakover voltage it starts conducting but below breakover voltage it offers very high resistance to the flow of current and is said to be in off state.

Forward conduction mode

SCR can be brought from blocking mode to conduction mode in two ways - either by increasing the voltage across anode to cathode beyond breakover voltage or by application of positive pulse at gate. Once it starts conducting no more gate voltage is required to maintain it in on state. There is one way to turn it off i.e. Reduce the current flowing through it below a minimum value called holding current.

Figure 1: Silicon-Controlled Rectifier (SCR) Symbol

 

Figure 2: Silicon-Controlled Rectifier (SCR) Schematic Diagram

Automatic Night Light using Phototransistor

Here is the simple Automatic Night Light circuit using Phototransistor.

Components Required:

1) Breadboard

2) 470 ohm resistor
3) 22 Kilo ohm resistor

4) LED (Any Color)
5) Phototransistor
6) 3904 NPN Transistor

6) 9Volt Battery 

Procedure:

• Connect the circuit as shown in the circuit diagram below.
• During day time the brightness will be more, which forward biases the phototransistor.
• Therefore the current flows through the collector to emitter of phototransistor.
• Hence there is no base current to forward bias the 3904 NPN transistor and the LED remains OFF.
• During night time the brightness goes down, which cannot forward bias the phototransistor.
• Therefore the current will not flow through the phototransistor.
• Hence there is enough base current to forward bias the 3904 NPN Transistor and the LED glows.

Circuit Diagram:

Figure 1: Automatic Night Light using Phototransistor Circuit simulation made in Multisim

Figure 2: Automatic Night Light using Phototransistor