Table of Contents

I. Introduction to Sequential Logic

1.1 Combinational vs Sequential Logic

1.2 What is a Sequential Circuit?

A sequential circuit is a digital circuit whose output depends not only on the current inputs but also on the previous state of the circuit. This memory capability is provided by flip-flops.

1.3 Block Diagram

1.4 Types of Sequential Circuits

1.5 Clock Signal

A clock is a periodic square wave that synchronizes state changes in sequential circuits. It has two important parameters:

• Frequency (f): Number of cycles per second (Hz)
• Period (T): Time for one complete cycle, \( T = 1/f \)

II. Latches — SR, D, JK

2.1 What is a Latch?

A latch is a bistable memory device that changes state based on input levels (not clock edges). It is the simplest form of sequential circuit.

2.2 SR Latch (NOR-based)

Built from two cross-coupled NOR gates. Has two inputs (S = Set, R = Reset) and two outputs (Q, Q̄).

2.3 Gated SR Latch

Adds an Enable (E) input to control when the latch can change state. Uses NAND gates.

2.4 D Latch (Data Latch)

Eliminates the invalid state of the SR latch by using a single data input D and an Enable input. When E = 1, Q follows D; when E = 0, Q holds its value.

2.5 JK Latch

A JK latch resolves the invalid state of the SR latch: when J = K = 1, the output toggles.

II-b. Latch Comparison & Examples

2.6 Comparison of Latches

2.7 Race-Around Condition

In a JK latch with J = K = 1, the output toggles repeatedly while the enable is high. This is called the race-around condition. It is solved by using edge-triggered flip-flops instead of latches.

2.8 Latch vs Flip-Flop

III. Flip-Flops — SR, D, JK, T

3.1 What is a Flip-Flop?

A flip-flop is an edge-triggered bistable memory device that changes state only at a clock edge (rising or falling). It is the fundamental building block of sequential circuits.

3.2 SR Flip-Flop

3.3 D Flip-Flop

Also called a delay flip-flop or data flip-flop. Output Q follows input D at the active clock edge.

3.4 JK Flip-Flop

The JK flip-flop eliminates the invalid state of the SR flip-flop. When J = K = 1, the output toggles.

3.5 T Flip-Flop (Toggle)

The T flip-flop is a single-input version of the JK flip-flop with J = K = T. When T = 1, output toggles; when T = 0, output holds.

III-b. Flip-Flop Comparison & Examples

3.6 Comparison of Flip-Flops

3.7 Characteristic Tables vs Excitation Tables

JK Excitation Table

D Excitation Table

IV. Triggering — Level & Edge

4.1 Level Triggering

In level triggering, the flip-flop responds to the input as long as the clock is at a particular level (high or low). This is characteristic of latches.

4.2 Edge Triggering

In edge triggering, the flip-flop responds only at the moment the clock transitions. This eliminates the race-around condition.

Positive Edge Triggering (Rising Edge)

Output changes when the clock transitions from 0 → 1.

Negative Edge Triggering (Falling Edge)

Output changes when the clock transitions from 1 → 0.

4.3 Master-Slave Configuration

Two flip-flops connected in cascade — the master is triggered on one edge and the slave on the opposite edge. This eliminates race-around and provides reliable edge-triggered behavior.

4.4 Triggering Comparison

V. Flip-Flop Conversions

5.1 Why Convert Flip-Flops?

Different flip-flop types are available in IC form. Being able to convert one type to another allows a designer to use the available IC efficiently. Conversion involves designing combinational logic that feeds the available flip-flop's inputs to mimic the desired flip-flop's behavior.

5.2 SR to JK Flip-Flop

We need to find S and R in terms of J, K, and Q:

5.3 SR to D Flip-Flop

5.4 JK to D Flip-Flop

5.5 JK to T Flip-Flop

5.6 D to JK Flip-Flop

VI. Shift Registers — SISO, SIPO, PISO, PIPO

6.1 What is a Shift Register?

A shift register is a cascade of flip-flops that shifts binary data one position per clock pulse. It is used for temporary data storage and serial/parallel data conversion.

6.2 SISO (Serial-In Serial-Out)

Data enters one bit at a time and exits one bit at a time. After 4 clock pulses, the input data appears at the output.

6.3 SIPO (Serial-In Parallel-Out)

Data enters serially but is available in parallel at the outputs of all flip-flops.

6.4 PISO (Parallel-In Serial-Out)

Data is loaded in parallel and shifted out serially. Requires additional logic for parallel loading.

6.5 PIPO (Parallel-In Parallel-Out)

Data is loaded in parallel and read out in parallel. Used as a temporary storage register.

VI-b. Shift Register Comparison & Examples

6.6 Comparison of Shift Registers

6.7 Shift Register Operations

6.8 Universal Shift Register (IC 74LS194)

A 4-bit bidirectional universal shift register that supports:

• Shift left
• Shift right
• Parallel load
• Hold

VII. Shift Register Applications

7.1 Serial-to-Parallel Conversion

A serial data stream is converted into parallel data. Used in communication receivers, where serial data arrives on a single line.

7.2 Parallel-to-Serial Conversion

Parallel data is converted into a serial stream for transmission. Used in communication transmitters, saving transmission lines.

7.3 Delay Line

A SISO shift register delays the input by \( n \) clock cycles, where \( n \) is the number of flip-flops.

7.4 Sequence Generator

A shift register with feedback can generate a repeating sequence of bits (e.g., pseudo-random binary sequences used in cryptography and testing).

7.5 Ring Counter

A ring counter is a shift register with the output of the last flip-flop fed back to the input of the first. It circulates a single 1 through the register.

7.6 Johnson Counter (Twisted Ring)

Similar to a ring counter, but the complemented output of the last flip-flop is fed back. A 4-bit Johnson counter has 8 states (2n).

7.7 Applications Summary

VIII. Counters — Asynchronous & Synchronous

8.1 What is a Counter?

A counter is a sequential circuit that counts clock pulses. It is built from flip-flops and produces a binary count sequence.

8.2 Asynchronous (Ripple) Counter

In an asynchronous counter, the clock is applied only to the first flip-flop. Each subsequent flip-flop is clocked by the output (Q) of the previous one. This creates a ripple effect.

4-bit Asynchronous UP Counter

8.3 Modulus of a Counter

The modulus (mod) of a counter is the number of unique states it goes through before repeating.

8.4 Synchronous Counter

In a synchronous counter, all flip-flops are clocked simultaneously by the same clock. Additional combinational logic determines the next state. This eliminates the ripple delay and allows higher-speed operation.

IX. Mod-N Counters & UP/DOWN Counters

9.1 Mod-N Counter Design

A Mod-N counter counts from 0 to N−1 and then resets. It is designed by decoding the state N and using it to reset the flip-flops.

Steps to Design a Mod-N Counter

1. Find the number of flip-flops: \( n = \lceil \log_2 N \rceil \)
2. Determine the reset state (binary equivalent of N).
3. Connect the flip-flop outputs corresponding to 1s in N to a NAND gate.
4. The NAND gate output clears all flip-flops (active-low reset).

9.2 Example: Mod-6 Counter

Design a Mod-6 counter (counts 0–5, then resets). Requires \( n = \lceil \log_2 6 \rceil = 3 \) flip-flops.

9.3 Mod-N Counter Table

9.4 UP/DOWN Counter

An UP/DOWN counter can count either upward (0, 1, 2, …) or downward (…, 2, 1, 0) based on a control input.

UP Counter (Count Up)

Each flip-flop toggles when all previous flip-flops are 1.

DOWN Counter (Count Down)

Each flip-flop toggles when all previous flip-flops are 0.

9.5 4-bit UP/DOWN Counter Truth Table (Partial)

X. Ring & Johnson Counters

10.1 Ring Counter

A ring counter is a shift register with the output of the last flip-flop fed back to the input of the first. A single 1 circulates through the register.

Ring Counter Properties

• Uses \( n \) flip-flops for an \( n \)-state counter (Mod-n).
• Only one flip-flop is 1 at any time (one-hot encoding).
• Requires initialization to a valid state (e.g., 1000).
• Speed is fast (only one flip-flop toggles per clock).

10.2 Johnson Counter (Twisted Ring)

A Johnson counter feeds back the complemented output of the last flip-flop. It has \( 2n \) states (twice as many as a ring counter).

10.3 Ring vs Johnson Counter

10.4 Applications of Counters

XI. Arduino Programming Basics

11.1 What is Arduino?

Arduino is an open-source electronics platform based on easy-to-use hardware and software. It consists of a microcontroller board (e.g., Arduino Uno with ATmega328P) and an Integrated Development Environment (IDE) for writing code.

11.2 Arduino Uno Specifications

11.3 Arduino IDE Structure

Every Arduino sketch has two required functions:

• setup() — runs once at the beginning; initializes pins, serial, etc.
• loop() — runs repeatedly after setup() finishes.

void setup() {
  // Runs once at startup
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  // Runs repeatedly
  digitalWrite(LED_BUILTIN, HIGH);
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW);
  delay(1000);
}

11.4 Data Types in Arduino

11.5 Common Arduino Functions

XII. Arduino Digital & Analog I/O

12.1 Digital I/O

Digital pins can be configured as either INPUT or OUTPUT. They read or write binary values (HIGH = 5 V, LOW = 0 V).

// Blink LED on pin 13
void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  digitalWrite(13, HIGH);  // LED ON
  delay(500);
  digitalWrite(13, LOW);   // LED OFF
  delay(500);
}

12.2 Reading a Push Button

const int BUTTON_PIN = 2;
const int LED_PIN = 13;

void setup() {
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  int state = digitalRead(BUTTON_PIN);
  if (state == LOW) {        // button pressed
    digitalWrite(LED_PIN, HIGH);
  } else {
    digitalWrite(LED_PIN, LOW);
  }
}

12.3 Analog Input (ADC)

Arduino Uno has a 10-bit ADC, giving values 0–1023. The reference voltage is typically 5 V.

// Read analog voltage from A0
void setup() {
  Serial.begin(9600);
}

void loop() {
  int adcValue = analogRead(A0);
  float voltage = (adcValue / 1023.0) * 5.0;
  Serial.print("ADC: ");
  Serial.print(adcValue);
  Serial.print("  Voltage: ");
  Serial.println(voltage);
  delay(500);
}

12.4 Analog Output (PWM)

Some digital pins support PWM (Pulse Width Modulation), which simulates analog output by rapidly switching the pin on and off.

// Fade LED on pin 9
void setup() {
  pinMode(9, OUTPUT);
}

void loop() {
  for (int i = 0; i <= 255; i++) {
    analogWrite(9, i);
    delay(10);
  }
  for (int i = 255; i >= 0; i--) {
    analogWrite(9, i);
    delay(10);
  }
}

XIII. DHT11 / DHT22 Sensor Interfacing

13.1 What is DHT11 / DHT22?

The DHT11 and DHT22 are low-cost digital temperature and humidity sensors. They output calibrated digital signals over a single-wire protocol.

13.2 Comparison of DHT11 and DHT22

13.3 Pin Configuration

13.4 Circuit Connection

13.5 Arduino Code for DHT11

#include <DHT.h>

#define DHTPIN 2
#define DHTTYPE DHT11   // or DHT22

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  dht.begin();
}

void loop() {
  float humidity = dht.readHumidity();
  float temperature = dht.readTemperature();

  if (isnan(humidity) || isnan(temperature)) {
    Serial.println("Failed to read from DHT sensor!");
    return;
  }

  Serial.print("Temperature: ");
  Serial.print(temperature);
  Serial.print(" °C  |  Humidity: ");
  Serial.print(humidity);
  Serial.println(" %");
  delay(2000);
}

13.6 Applications

• Weather stations
• HVAC (Heating, Ventilation, Air Conditioning) systems
• Greenhouse monitoring
• Smart home automation
• IoT environmental monitoring
• Food storage monitoring

XIV. Arduino-Based IoT Monitoring Systems

14.1 What is IoT?

The Internet of Things (IoT) refers to the network of physical devices embedded with sensors, software, and connectivity that enables them to collect and exchange data over the internet.

14.2 IoT Architecture

14.3 IoT Monitoring System Components

14.4 Example: IoT Temperature Monitoring

#include <DHT.h>
#include <ESP8266WiFi.h>

#define DHTPIN 2
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

const char* ssid = "YourWiFiSSID";
const char* password = "YourWiFiPassword";

void setup() {
  Serial.begin(115200);
  dht.begin();
  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("Connected!");
}

void loop() {
  float t = dht.readTemperature();
  float h = dht.readHumidity();

  // Send to ThingSpeak or other cloud platform
  Serial.print("Temp: ");
  Serial.print(t);
  Serial.print(" °C, Hum: ");
  Serial.println(h);

  delay(30000);  // Send every 30 seconds
}

14.5 Popular IoT Platforms

14.6 Applications of IoT Monitoring

• Smart home: temperature, lighting, security
• Agriculture: soil moisture, weather monitoring
• Healthcare: patient monitoring, wearables
• Industrial: machine health, predictive maintenance
• Smart cities: traffic, air quality, waste management
• Environmental: air/water pollution monitoring

XV. Smart Home Automation with Arduino

15.1 What is Smart Home Automation?

Smart home automation uses sensors, microcontrollers, and communication networks to automatically control home appliances such as lights, fans, air conditioners, and security systems.

15.2 Components of a Smart Home System

15.3 Example: Automatic Light Control

const int LDR_PIN = A0;
const int LED_PIN = 9;
const int THRESHOLD = 500;  // Adjust as needed

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int lightLevel = analogRead(LDR_PIN);
  Serial.print("Light: ");
  Serial.println(lightLevel);

  if (lightLevel < THRESHOLD) {
    digitalWrite(LED_PIN, HIGH);  // Dark → turn on
  } else {
    digitalWrite(LED_PIN, LOW);   // Bright → turn off
  }
  delay(200);
}

15.4 Example: Temperature-Based Fan Control

#include <DHT.h>

#define DHTPIN 2
#define DHTTYPE DHT11
#define FAN_PIN 9

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  dht.begin();
  pinMode(FAN_PIN, OUTPUT);
}

void loop() {
  float temp = dht.readTemperature();

  if (temp > 30.0) {
    digitalWrite(FAN_PIN, HIGH);  // Fan ON
    Serial.println("Fan ON (temp > 30°C)");
  } else {
    digitalWrite(FAN_PIN, LOW);   // Fan OFF
    Serial.println("Fan OFF");
  }
  delay(2000);
}

15.5 Advantages of Smart Home Automation

• Energy savings through automatic control
• Enhanced convenience and comfort
• Improved security (motion detection, smart locks)
• Remote monitoring and control
• Elderly and disabled assistance

XVI. Proteus Simulation & Circuit Design

16.1 What is Proteus?

Proteus is a circuit simulation and PCB design software. It allows students and engineers to design circuits, simulate behavior, and test code (including Arduino sketches) before building physical prototypes.

16.2 Key Features

16.3 Simulating Arduino in Proteus

1. Install Proteus and the Arduino library for Proteus.
2. Place the Arduino Uno board in the schematic.
3. Connect external components (LEDs, sensors, etc.).
4. Write the Arduino sketch in the Arduino IDE.
5. Compile the sketch to generate a .hex file.
6. Load the .hex file into the Arduino board in Proteus.
7. Run the simulation and observe the behavior.

16.4 Example: Blink LED in Proteus

16.5 Proteus Component Library

16.6 Advantages of Proteus Simulation

• No hardware required for testing
• Cost-effective for complex circuits
• Safe — no risk of damaging components
• Faster iteration cycles
• Detailed measurements and waveforms
• Can test code before hardware deployment

Summary & Formula Sheet

Flip-Flops

Flip-Flop Conversions

Shift Registers

Counters

Arduino Key Functions

Sensor Formulas

Exam Tips & Practice Questions

Practice Questions

Solutions to Practice Questions

#include <DHT.h>

#define DHTPIN 2
#define DHTTYPE DHT11
#define LED_PIN 9

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(9600);
  dht.begin();
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  float t = dht.readTemperature();
  if (isnan(t)) {
    Serial.println("Sensor error!");
    return;
  }
  if (t > 30.0) {
    digitalWrite(LED_PIN, HIGH);
  } else {
    digitalWrite(LED_PIN, LOW);
  }
  delay(2000);
}

References & CO Mapping

Textbooks

Relevant Websites

Audio-Visual Aids

Software

Key Takeaways

1. Sequential circuits have memory; outputs depend on present inputs and past state.
2. Latches are level-triggered; flip-flops are edge-triggered.
3. SR flip-flop has an invalid state when S = R = 1.
4. JK flip-flop eliminates the invalid state and toggles when J = K = 1.
5. D flip-flop: \( Q_{next} = D \); T flip-flop: \( Q_{next} = T \oplus Q \).
6. Shift registers shift data one bit per clock; types: SISO, SIPO, PISO, PIPO.
7. Asynchronous counters ripple; synchronous counters are clocked simultaneously.
8. Modulus of an n-bit counter is 2ⁿ; Mod-N counters reset at N.
9. Ring counter has n states; Johnson counter has 2n states.
10. Arduino: setup() runs once, loop() runs repeatedly.
11. DHT11/DHT22 measure temperature and humidity; use a 10kΩ pull-up resistor.
12. Proteus allows virtual simulation of Arduino-based circuits before hardware build.

CO Mapping