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Introduction to Optocouplers

Have you ever heard the word isolation, especially in electronics? As you might guess, isolation is a key factor when it comes to optocouplers. Isolation is sometimes mandatory and sometimes an extra feature in circuits. Optocouplers are used in many electronic devices, from mobile electronics to household electronics.

So, in this article, let's learn more about optocouplers along with their basics, types, working principles, simulation, hardware demonstration, and live application demonstration. For our demo purposes, we will be using the PC817, a commonly used transistor output optocoupler in electronics.

Starting with a brief explanation of the optocoupler, we begin our walkthrough.

Basics of Optocoupler

In the path of Exploring Optocoupler, let's dig deep into answering questions like WHAT, WHERE, WHY, and HOW.

What is an Optocoupler?

Let's understand the term Optocoupler. It can be separated as OPTO + COUPLER. So, technically, as per the name, it is used as a coupler with the help of some sort of optical technology. In brief, a light source is used as a link between two isolated circuits.

In terms of textual Representation: 

“An optocoupler, also known as an opto-isolator, is an electronic component that transfers electrical signals between two isolated circuits using light. It typically consists of an LED (light-emitting diode) and a photodetector, such as a phototransistor, housed within a single package. When the LED is energized by an input signal, it emits light that is detected by the photodetector, which then produces an output signal. This optical coupling allows the input and output circuits to remain electrically isolated from each other, providing protection against high voltages and electrical noise.”

Here, I would like to add a point that not only optical technology but also electromagnetic induction is used for isolation more commonly.

Where are the optocouplers used?

Commonly the optocouplers are used in the circuits where the isolation is required between any two regions.

Relay Module

For example, let’s consider that we are working on a project where an Arduino UNO-like microcontroller needs to control an AC tube light. In this case, the first thing that will come to mind is a Relay module. Of course, we use a relay module, but do you know exactly why we use a relay module when even a TRIAC can be used to do the same work? Yes, it’s isolation. In the case of a TRIAC, there is a chance of higher AC voltage entering the low-power DC network, which, of course, fries the ICs like chips. So, the relay makes a suitable choice. Yet our concern is not about the relay, it’s the optocoupler. 

A relay is the first level of protection, an optocoupler is also used between the microcontroller and the relay coil as a second level of protection. Being an electromechanical component, a relay could wear out over time. In that rare case, the AC power might touch the coil of the electromagnet inside the relay, which once again creates a path for AC to enter the DC network. This is where the optocoupler comes in handy and isolates both networks. 

Actually, apart from the relay, some types of Optocouplers can be used to switch a TRIAC directly.

Hope you understand the usage of optocouplers. Next, let's know why optocouplers are still preferred to do the job.

Why are Optocouplers Preferred Over Other Options?

The answer is simple. Unlike other options, there is no chance of electrical bonding between the separated regions even in the event of system failure. The possibility is very rare, such as if the potential is greater than the isolation voltage between the input and output of the optocoupler, which is about 5000 volts for the Optocoupler like PC817. That's why I said it's rare. There is no chance of placing such low-power electronics in such a high-voltage area. So, we trust optocouplers more than others.

Now you should have a clear understanding of optocouplers. Let's move the interesting part of how it works.

How Optocouplers Works?

There are numerous ways to understand the Working of the optocoupler. I like to make you to compare the wireless Remote with the optocoupler. Let's look at it in detail.

Optocoupler’s Working Explanation

In the above illustration, you can see the remote car [Output] setup along with the wireless remote [Input]. Each has a separate power source, so the remote needs to be charged separately, and similarly, the car needs to be charged as well. If neither is charged, there is no chance of driving the car. Even if there is some issue with the car, it won't affect the remote, and vice versa. This is because there is wireless transmission and reception technology in between. The overall working will only be affected if some other RF signal interferes with the existing system. So, that's the point I wanted to deliver.

In the Above Animated GIFs, you can see the working of the optocoupler. Like the remote control car, the optocoupler has an LED as an input and a phototransistor as an output. The LED transmits infrared rays, and the phototransistor receives the transmitted infrared waves at its base as a signal, which turns on the transistor. Similar to the remote control car, the functioning of the optocoupler can be disturbed by any external light sources. That’s why the optocoupler is completely sealed to avoid external light interference. Remember, This explanation Using the remote car is only for understanding the concept of Optocoupler.

You might wonder if there is a physical connection between the input and output internally, which may cause any trouble. Ha ha, don't worry; there is a term known as dielectric strength. Usually, the material used to isolate the LED and phototransistor is non-conductive epoxy resin, which has a dielectric strength of Vmax = 20kV/mm. So, let's assume there is a 0.25 mm gap in between, which might require nearly 5000 volts to start conducting. 

Hereby, the working of the optocoupler PC817 is completed.

Types of Optocouplers

Optocouplers can generally be classified into three categories: Based on their Input, Output, and Functions. Let's see each category in detail.

Types of Optocoupler

Types of Optocouplers Based on Input:

Optocouplers can be categorized based on their input types into two divisions: unidirectional input and bidirectional input, also known as DC input and AC input, respectively. The primary difference lies in the configuration of the LEDs within the optocoupler.

  • Unidirectional (DC) Input: This type has a single LED that responds to current flowing in one direction only.

  • Bidirectional (AC) Input: This type features two LEDs connected in opposite directions (one inverted), allowing it to respond to current flowing in either direction, making it suitable for AC input signals.

Types of Optocoupler based on their Input

Types of Optocouplers Based on Output:

Here the optocoupler can be classified based on the type of Output Device used. Some of the used output devices are Photodiode, Phototransistor, Photodarlington, MOSFET, SCR, and TRIAC.

Optocoupler with Photodiode Output:

In this type, the output is a direct photodiode. this optocoupler is widely used in proximity detection, Rotary encoders, and Photo Interrupter sensors.

Optocoupler with Photodiode Output

The above is the image and symbol of the photo-interrupter sensor used for measuring the speed of rotating motors and in many other applications.

Optocoupler with Phototransistor Output:

Phototransistor output optocouplers are widely used due to their simplicity and low cost. In this type of optocoupler, a phototransistor is integrated at the output, providing an easy way to draw output from the device using a load resistor.

Optocoupler with Phototransistor Output

https://components101.com/sites/default/files/component_datasheet/PC817%20Datasheet.pdf

The above is the image and symbol of the PC817, a commonly used optocoupler that has a phototransistor as its output device.

Optocoupler with Photodarlington Output:

Photodarlington output optocouplers are utilized when a higher current transfer ratio (CTR) is required. This type of optocoupler incorporates a Photodarlington transistor pair at the output.

Optocoupler with Photodarlington Output

https://www.vishay.com/docs/83617/il221at.pdf

Above You can see the image and Symbol of IL221AT, an Optocoupler with Photodarlington Output, Low Input Current, High Gain, and Base Connection.

Optocoupler with MOSFET Output:

MOSFET output optocouplers are used in applications that require high-speed and efficient power switching. These optocouplers incorporate a MOSFET at the output, providing several advantages over other types of optocouplers like High-speed Switching, Efficiency, and immunity to Noise

Optocoupler with MOSFET Output

https://www.farnell.com/datasheets/461023.pdf

In the above image, you can see the TLP222A, which consists of an infrared emitting diode optically coupled to a photo-MOSFET in a DIP package. It is suitable for use as on/Off control for high current.

Optocoupler with Triac & SCR Output:

Triac & SCR Output optocouplers are known for its requirement in higher power switching and capability of triggering thyristor and triac on its own. This comes in handy when we need to switch the AC appliance with Triac directly from a microcontroller.

Optocoupler with Triac & SCR Output

https://www.farnell.com/datasheets/3929882.pdf

The above is the image and symbol of the MOC301XM/MOC302XM, which contains a GaAs infrared emitting diode and a light-activated silicon bilateral switch, functioning like a triac. They are designed for interfacing between electronic controls and power TRIACs to control resistive and inductive loads.

Types of Optocouplers Based on Function:

Optocouplers based on Function are designed to perform specific tasks, often integrating multiple Blocks into a single device. There are eight primary types of function-based optocouplers, each tailored for distinct applications. These optocouplers have more complex internals compared to other types due to their specialized nature.  

The most common types are

  1. Logic Output Optocouplers (Eg: 4N35)

  2. High Linearity Optocouplers (Eg: IL300)

  3. High-Speed Optocouplers (Eg: 6N137)

  4. Galvanically Isolated Gate Drivers (Eg: ADuM3223)

  5. Optically Isolated Gate Drivers (Eg: HCPL3120)

  6. Optically Isolated Amplifiers (Eg: HCPL-7800A)

  7. Solid State Relays (SSR) (Eg: G3MB-202P-5VDC)

  8. Voltage and Current Sensors (Eg: ACPL_798J)

To know more about these, you can Explore its example links nearby.

And this might not be the end of the types of optocouplers. There are still many optocouplers out there, of which the above were our basic considerations. So out of these, let's consider the PC817 as an example optocoupler for our following simulations and practical demonstrations. 

Next, let's get introduced to the PC817.

Pinout of PC817 IC

Pinout of PC817 IC

The above image shows the pinout of the PC817, providing a clear explanation of each pin. Below is the pin description of the PC817, explained in the following table:

Pin NoPin NameDescription
1AnodeAnode Pin of Infrared Light Emitting Diode.
2CathodeCathode Pin of Infrared Light Emitting Diode.
3EmitterEmitter Pin of the Internal Photo Transistor.
4CollectorCollector Pin of the Internal Photo Transistor.

Let’s look at some of the important specifications of PC817.

Specifications of PC817

Here's the quick specification table for the PC817:

Specification of PC817

First, let’s look at the input parameters, starting from the anode and cathode side. Consider it as a simple LED. Like a light-emitting diode, it has a forward voltage (Vf) and forward current (If), as shown above. Using these, we can calculate the appropriate resistor to be used in series with the input side. Make sure you are mindful of polarity because the IR LED diode inside has a very low reverse voltage of around 6V, which can permanently damage the LED.

The output part, consisting of the emitter and collector, can be considered as a transistor. As a transistor, it has a maximum collector current of 50mA and a higher collector-emitter voltage range of 80V maximum. Another important factor to consider is the frequency, with a typical cutoff frequency of 80kHz. So, it too has its limitations.

Finally, the operating temperature ranges from -30 to +125 ˚C, and storage should be between -55 to +100 ˚C. While soldering, you can reach a maximum of 260˚C for up to 10 seconds on the pins of the PC817. If the conditions exceed these limitations, the PC817 will be damaged internally.

Next, we are moving to the Stimulation of PC817 Optocoupler.

Stimulation of PC817 Optocoupler in Proteus:

In this simulation section, we will delve deeper into the workings of the PC817, starting with a basic simple simulation of the PC817. 

In the above diagram, you can see the direct output method. Here, R1 is the current-limiting resistor for the IR LED inside the PC817, and a button is connected between R1 and the positive power supply. R2 is the load resistor, which allows you to control the voltage gain and frequency response directly by adjusting this resistor. The output is connected directly to the LED via R3, completing the circuit. When the push button is pressed, the output LED turns off.

Input StateOutput State
HIGHLOW
LOWHIGH

In the above table, you can see the logic state difference between the input and output for the direct method. Now, let’s move to the next method, the inverted output method.

In the inverted method, everything is the same except for Q1, which is a PNP transistor used to invert the output from the optocoupler, ensuring that the output state matches the input state. Below, you can see the output of the inverted method.

Input StateOutput State
HIGHHIGH
LOWLOW

As the signal is inverted by the PNP transistor Q1, the logic states of the input and output are directly proportional.

Next, we have a bonus simulation of the actual relay module available in the market.

Here, the inverted output from Q2 is connected to one side of the relay coil, and the other side is grounded. A diode is connected in parallel to the relay coil to protect the circuit from reverse EMF, and an LED is also connected in parallel to the output for indication.

At the output, the switch of the AC light bulb is connected to the Normally Open (NO) and Common (COM) terminals of the relay. So, when the push button is pressed, the relay turns on, along with the AC light, as shown in the above GIF.

Now let us Move towards the Hardware demonstration of the Optocoupler PC817.

Hardware Demonstration of PC817 Optocoupler:

Below You can see the hardware demonstration of PC817 Optocoupler.

In this hardware demonstration, the direct output method is applied. Choosing different power supplies helps you understand more about the working. Here, there are two different power supplies, one for the input side and another for the output side. You can see that both sides are perfectly isolated on the breadboard.

You might wonder about taking output directly from the optocoupler by driving the output in a source or sink drive method, which doesn’t invert the signal. Yes, it doesn't invert the signal, but this method is not recommended in the datasheet, even if it requires less current than the maximum collector-emitter current of 50mA. However, if you are confident about your circuit, you can proceed that way.

When you press the button, the LED goes off. This demonstrates the concept of direct output.

Let’s learn more about testing the PC817 Optocoupler.

How to Test Optocoupler?

Testing an optocoupler is very simple and easier than you might think. There are many ways to do that, which we will discuss next.

Test Circuit for Optocoupler:

This method is preferred for professionals who need to ensure that the component meets its specific requirements and operates correctly within the intended application. However, if you are a hobbyist, you can skip this section and move to our next method, where you only need a multimeter to carry out the process.

You can find the test circuit in the datasheet of the respective optocoupler you selected. In our case, it's the PC817. If you explore its datasheet, you will find two test circuits: one to check response time and another to check frequency response. These two test methods require a function generator and an oscilloscope.

Test Circuit For Optocoupler

The above is the test circuit for checking the response time of the optocoupler PC817. Here, a square wave of the desired frequency is passed as an input to the anode side of the optocoupler through a current-limiting resistor Rd. The input square wave is verified using the output received between the load resistor Rl and the collector of the optocoupler. This input and output wave is compared simultaneously using a two-channel oscilloscope, and the deflection in response time can be easily found and classified.

Test Circuit for Frequency Response

The above is the test circuit for checking the frequency response of the optocoupler. As you can see, the hardware setup is the same as above. The only difference is that the input signal’s frequency is adjusted, and you can use the above graph to verify the results. You can adjust the load resistance to set the gain to the required amount. That's how we can check the working using the test circuit provided in the datasheet.

Next, let's look at the easiest and most affordable method.

Using Multimeter For testing Optocoupler:

In this method, the concept is simple: you will consider the input side (anode and cathode) as a diode and the output side (collector and emitter) as a transistor. So, the next step is straightforward. Yes, we keep the multimeter in diode mode and check the optocoupler's input in both forward and reverse bias as follows.

Checking Procedure for Input Side of the Optocoupler

In the illustration above, you will get the following results. In forward bias, you should see a voltage of around 1V with an accepted tolerance of ±0.1V. In reverse bias, you should get no voltage, so "OL" should be displayed on the multimeter, indicating that no current is flowing. This verifies the input infrared LED. If there are any abnormalities, there might be an issue with the LED side.

Next, we need to determine the resistance value to connect to the anode of the optocoupler. You can use a free LED resistance calculator tool to find out the required resistance value. Check the specifications of the optocoupler you are using or use the data below for the PC817 to fill in the input spaces in the tool. Once you have the value, if you don't have that exact resistor, use a combination of series and parallel resistors to approximate it. A slightly higher value is acceptable.
[Screenshot of the Parameters used in our Online Led Resistor Calculator]

Online Led Resistor Calculator

In my case, it calculated a 190-ohm resistor, but I am using a 220-ohm resistor, which is close enough. Now, follow these steps:

Checking Procedure for Output Side of the Optocoupler

Forward Bias of the Collector-Emitter of the Optocoupler with Connected Input Power:

  • Power up the input side of the optocoupler by connecting the calculated resistance in series with the anode and providing 5V. Connect the cathode to the ground.

  • Set the multimeter to resistance mode. Connect the positive lead to the collector and the negative lead to the emitter. The measured resistance value should be below 100 ohms. In my case, it read 90 ohms. The read resistance is proportional to the power supplied to the infrared LED. For correct calculations, the value should be less than 100 ohms. If it exceeds 100 ohms and moves into the kilo-ohm range, there may be an issue.

Without Powering the Input Side:

  • The resistance should read "OL." If it shows values in the ohm or kilo-ohm range, there may be a short in the transistor part.

This completes the testing process, and you should now understand how to test an optocoupler using a multimeter.

Next, we see a few real-world applications of Optocoupler.

Application Of Optocoupler:

Let's see some of the applications where optocouplers play a crucial role in our DIY projects for a better understanding of the concept.

  1. Relay Modules - Here, the optocoupler PC817 is widely used for isolating the relay side from the main control circuitry.

  2. AC Light Dimmer using Arduino and TRIAC - This project uses two types of optocouplers: a transistor output optocoupler and a TRIAC output optocoupler. The transistor output optocoupler is used to detect the zero crossing of the AC signal, while the TRIAC output optocoupler is used to drive the TRIAC directly, enabling phase angle control using a microcontroller or other circuitry. This is crucial for applications like dimming AC lights and regulating power to AC equipment.

  3. AC Lights Flashing and Blink Control Circuit Using 555 Timer and TRIAC - Similar to the AC light dimmer project, this application also uses both transistor and TRIAC output optocouplers. The transistor output optocoupler finds the zero crossing of the AC signal, and the TRIAC output optocoupler drives the TRIAC for precise control, enabling the flashing and blinking of AC lights.

  4. Raspberry Pi Emergency Light with Darkness and AC Power Line Off Detector - In this project, a transistor output optocoupler is used to drive the MOSFET, which controls the brightness of multiple LEDs. This setup ensures that the emergency light activates in the absence of AC power or in low-light conditions, providing reliable illumination.

  5. Design and Build a Compact 3.3V/1.5A SMPS Circuit for Space Constraint Applications - In this application, the PC817 optocoupler provides feedback of the output to the internal SMPS IC in an isolated manner. This isolation is crucial for maintaining the stability and safety of the power supply, especially in space-constrained applications where efficient and compact design is essential.

Conclusion

I hope you understand this article about optocouplers in detail. Visit our site for more projects that use optocouplers and to gain a deeper understanding of their applications.

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Flip-Flop in Digital Electronics: Types, Truth Table, Logic Circuit and Practical Demonstration

A flip-flop in digital electronics is formally defined as "A bistable device with synchronous inputs that changes state only at specified transitions of a clock signal" (IEEE Standard 91/1984)

In general, flip-flops are fundamental components in digital electronics, capable of functioning as 1-bit memory storage devices. Unlike combinational logic gates, flip-flops have the unique ability to store state information (0 or 1, low or high) and maintain this stored state indefinitely until changed, all with synchronized operation using a clock signal.

If this sounds confusing right now, don't worry! By the end of this article, you'll understand everything clearly with simple explanations and real-world examples.

What is a flip-flop and how does it work?

A flip-flop is a fundamental digital circuit designed to store a single bit of data, serving as a critical memory element in electronics. It maintains one of two stable states (0 or 1) and transitions between them based on input signals and a clock pulse. There are many types of flip-flops, such as SR (Set-Reset), D (Data), JK, and T (Toggle) flip-flops and each has distinct input-output functionalities. We will get in-depth into each of them but for simple understanding, a D flip-flop captures the input value at the clock’s edge and holds it until the next cycle, ensuring precise data retention. Essential in sequential logic, flip-flops are integral to registers, counters, and memory units in microprocessors and digital systems, enabling synchronized and reliable operation.

Flip-flops operate by processing input signals through logic gates, with state changes synchronized by a clock signal to ensure stability and predictability. For example, in a D flip-flop, the data input is sampled only when the clock triggers, storing the value in a feedback loop until the next clock edge. Similarly, an SR flip-flop uses set and reset inputs to toggle states, while a JK flip-flop offers enhanced flexibility by resolving indeterminate states. This clock-driven mechanism prevents erratic behavior, making flip-flops ideal for applications requiring precise timing, such as in CPUs and memory modules. By maintaining and updating data in a controlled manner, flip-flops form the backbone of modern digital circuit design.

Difference between flip-flop and latch

When learning about flip-flops, it’s essential to mention latches as well—and there’s a good reason. The evolution of digital electronics typically follows this path:
Transistors → Logic Gates → Latches → Flip-Flops

This progression is why flip-flops and latches are often compared, even though they serve different purposes in circuit design.

Put simply, flip-flops are an advanced form of latches. Both can store a single bit of data, but,

  • Flip-flops are edge-triggered and change states based on a synchronous clock signal.

  • Latches are level-sensitive, meaning their output changes whenever the input signal is active.

The main challenge with using multiple latches is synchronization. Since latches respond immediately to input changes, a slight delay in signal arrival (like due to varying wire lengths) can lead to timing errors. In high-speed circuits, even nanosecond-level differences can cause problems.

Flip-flops solve this issue by ensuring all data changes happen only at the clock edge, making them perfect for synchronized operations.

FeatureLatchesFlip-Flops
Triggering TypeLevel-triggeredEdge-triggered
Control SignalControlled by the Enable or Gate signalControlled by a Clock signal (rising/falling edge)
Timing SensitivityChanges state as long as enable is activeChanges state only on the clock edge
Synchronous or AsynchronousAsynchronous (with respect to the clock)Synchronous
SpeedFaster switching due to level sensitivitySlightly slower but more predictable
Design SimplicitySimpler circuitsSlightly more complex
Data StabilityLess stable – may change unintentionallyMore stable – changes only at the clock edge
Usage in CircuitsSuitable for simple and small circuitsIdeal for synchronous sequential systems
Risk of GlitchesHigher, due to continuous sensitivityLower – updates only on clock edge
Power ConsumptionTypically lowerSlightly higher due to clocking circuits
Common Applications- Temporary storage
- Gating signals
- Simple memory elements		- Registers
- Counters
- Finite State Machines (FSMs)
- Pipeline stages
Practical ExampleModern car ignition switches (especially push-to-start ones)Digital Clock

With the above comparison chart, I’m sure you'll understand the difference between the two more clearly. With this, let's move on to the types of flip-flops, along with practical demonstrations.

Types of Flip-Flops in Digital Electronics

Although flip-flops evolved from latches, the Flip-Flop itself has some types to make it fit in certain applications. As I said, each single type was an evolution from another with a few improvements. So, without wasting any time, let's jump straight away into the types of flip-flops,

Here are the four primary types of Flip-Flops, which are

  1. SR Flip Flop (Set Reset)

  2. D Flip Flop(Data)

  3. JK Flip Flop

  4. T Flip Flop (Toggle) 

SR Flip-Flop

The SR (Set-Reset) Flip-Flop is the most basic and commonly used type of flip-flop, and it’s the foundation for more advanced types like D, JK, and T. It has two inputs: S (Set) and R (Reset). When S = 1, the output is set to 1. When R = 1, it resets to 0. If both S and R are 0, the output simply holds the previous state. But if S = R = 1, it enters an invalid state (also called the race around condition), which must be avoided in real-world circuits.

Unlike latches that respond to input levels, the SR Flip-Flop is edge-triggered, meaning it only reacts at the rising or falling edge of the clock signal. This makes it more reliable for timed operations in sequential circuits.

Also, keep in mind: most flip-flops (and logic ICs) are rising edge-triggered, while falling edge or dual-edge triggered types are rare and used in specific designs. That’s why we generally call them edge-triggered—because either edge could be used, depending on the application.

SR Flip-Flops are often used in basic control systems, like 

  • Turning a relay ON or OFF with logic signals, 

  • Simple memory storage where only a few states are needed, like in reset/start switches, push button debouncing.

  • Simple status flags.

Symbol & Truth Table of SR Flip-Flop

Symbol and Truth Table of SR Flip-Flop

Above, you can see the symbol, along with the truth table. After undergoing some detailed research, we arrived at this truth table.

To make this simple to understand, I’ve used straightforward terms here:

  • No Change (S = R = 0): The flip-flop retains its previous state.

  • Set (S = 1, R = 0): Output Q becomes '1' (Set).

  • Reset (S = 0, R = 1): Output Q becomes '0' (Reset).

  • Invalid State (S = R = 1): Both outputs (Q and Q̅) may become '1', leading to instability. This condition is prohibited in most designs.

SR Flip-Flop Circuit Working

One of the simplest ways to implement an SR flip-flop is using NAND gates.

SR Flip-Flop Circuit Diagram using NAND gates

Above, you can see the basic circuit diagram of an SR flip-flop made using a combination of NAND gates. You can use any NAND IC, and the result will be the same. Here, we are using four NAND gates to construct the SR flip-flop.

The working is simply a combinational output of the logic gates. If you spend some time mapping the values for each individual logic gate, you should be able to understand it clearly.

SR Flip-Flop Circuit Working 1

According to the truth table, if we initially keep both the Set and Reset inputs LOW, the output will remain in its last state. I would like to add a note that this behavior might vary depending on the setup, especially between simulation and real hardware implementation.

SR Flip-Flop Circuit Working 2

If I make the Set pin HIGH while keeping the Reset pin LOW, that will eventually make Q HIGH and Q̅ LOW. Conversely, if the Reset pin is kept HIGH and the Set pin LOW, you will get LOW at Q and HIGH at Q̅.

SR Flip-Flop Circuit Working 3

Furthermore, if we keep both Set and Reset HIGH, the output will be unpredictable. In more technical terms, a race-around condition will occur, which should be avoided. To overcome that issue, the SR flip-flop was upgraded to a D flip-flop.

Practical Demonstration of SR Flip-Flop

SR Flip Flop Demonstration

To learn the SR flip-flop in a more practical way, you can refer to the article SR Flip-Flop with NAND Gates: Circuit, Truth Table and Working, which covers the SR flip-flop concept, its truth table, a list of components required, the circuit diagram with explanation, and the working of the SR flip-flop using NAND gates.

How to make SR flip-flop using NAND gates?

An SR (Set-Reset) flip-flop can be effectively built using NAND gates, as shown in the provided circuit diagram, which utilizes four 7400-series NAND gates. The 7400 chip contains four 2-input NAND gates, each with inputs (e.g., pins 1 and 2) and an output (pin 3). In this setup, two NAND gates form the core latch, while the other two act as input conditioners with the clock signal. The inputs S (Set) and R (Reset) are fed into the first pair of NAND gates alongside the clock signal, ensuring that state changes occur only on clock pulses. The outputs of these gates connect to the second pair, which are cross-coupled to create the latch, producing the Q and Q' (complementary) outputs. This configuration ensures the flip-flop stores a bit of data, setting Q to 1 when S is active or resetting Q to 0 when R is active, all synchronized by the clock.

SR flip flop circuit with NAND gates

In the diagram, the clock signal enables the flip-flop to update its state only when active (typically high). When the clock is high, the first NAND gate (top left) processes the S input, and the second NAND gate (bottom left) processes the R input. These outputs feed into the cross-coupled NAND gates (right side), which form the latch. If S is 1 and R is 0, the latch sets Q to 1 and Q' to 0; if R is 1 and S is 0, it resets Q to 0 and Q' to 1. When both S and R are 0, the latch holds its previous state, and both being 1 is an invalid condition for an SR flip-flop, as it leads to instability. The use of NAND gates ensures that the circuit operates reliably, with the clock providing precise control over when the flip-flop updates, making it ideal for applications in sequential logic circuits like registers and counters.

D Flip-Flop

The D Flip-Flop is actually an upgrade of the SR Flip-Flop, specifically designed to overcome its major limitation—the invalid state. Unlike the SR Flip-Flop, which requires two inputs (Set and Reset) and may enter an undefined condition when both are high, the D Flip-Flop simplifies the design by using just one input, the D (Data) input.

The operation is straightforward, whatever value is applied to the D input gets stored and reflected at the output Q, but only at the triggering edge of the clock signal (usually the rising edge).

Because it cleanly passes the input to the output in a controlled and predictable way, the D Flip-Flop is widely used in digital electronics, especially in applications like Data storage registers, Pipeline registers in CPUs, Buffering systems, Clocked data transfer circuits

Symbol & Truth Table of D Flip-Flop

Symbol and Truth Table of D Flip-Flop

Above, you can see the symbol along with the truth table for the D flip-flop. It is self-explanatory and simple to understand.

In Simple Terms:

  • At Clock Edge (↑), the flip-flop copies the D input and holds it until the next tick. That is, if we give HIGH, then Q will be HIGH, and vice versa.

  • Otherwise, the output stays the same as the previous state.

D Flip-Flop Circuit Working

Among many options for implementing a D flip-flop, I’ll go with the NAND gate version to keep it simple.

Below, you can see the circuit diagram of the D flip-flop, which might look similar to the SR flip-flop.

Circuit Diagram of the D Flip Flop

The only difference is that we have used an additional NAND gate as a NOT gate and combined the Set and Reset inputs into a single D input.

D Flip-Flop Working

Here, as per the combinational logic circuit, Q will be equal to the D input value. If D is HIGH, Q will be HIGH, and Q̅ will be LOW. Similarly, when we provide LOW to the D input, Q will be LOW, and Q̅ will be HIGH. This all happens only at the rising or falling edge of the clock pulse.

If the clock pulse remains at the same logic level (either HIGH or LOW), then regardless of the D input, Q and Q̅ will remain the same as their last state.

Practical Uses of the D Flip-Flop

The D flip-flop is widely used in digital electronics for reliable data storage and synchronization. It forms the core of shift registers, enabling serial-to-parallel data conversion in communication systems. In microprocessors, D flip-flops are essential for building registers to hold temporary data during processing. They’re also used in counters, facilitating frequency division and event sequencing in clocks and timers. Additionally, D flip-flops ensure synchronized data transfer in memory units, preventing timing errors. Their ability to capture and hold data on clock edges makes them indispensable in sequential logic circuits.

Practical Demonstration of the D Flip-Flop

If you want to explore the D Flip-Flop in a more practical way, check out the article  D Type Flip-Flop: Circuit, Truth Table and Working. It covers the D Flip-Flop basics, truth table, logic gate representation, list of components required, circuit diagram with explanation, a practical demonstration, and even a video to help you understand better.

JK Flip-Flop 

The JK Flip-Flop comes with a bit of history behind its name. The most common explanation is that it was just a naming choice used by an engineer in his documentation, maybe based on initials or just random letters that stuck around. Whatever the reason, this flip-flop became popular because it fixed a major issue in the SR Flip-Flop.

It has two inputs, J and K, which function similarly to the Set and Reset inputs in an SR flip-flop, but with a smart upgrade. When both J and K are held HIGH, instead of entering an invalid or unstable state (as seen in the SR flip-flop), the JK flip-flop simply toggles its output. This means if Q is currently LOW, it becomes HIGH, and if Q is HIGH, it becomes LOW. This automatic flipping behavior is what makes the JK flip-flop particularly useful and reliable in sequential circuits.

Because of this, the JK Flip-Flop is perfect for things like counters, digital clocks, and frequency dividers, where the circuit needs to switch states on its own in a clean and timed manner.

Symbol and Truth Table of the JK Flip-Flop

Symbol and Truth Table of the JK Flip-Flop

With the above symbol and truth table, you can understand the concept behind the JK flip-flop.

Just like the SR flip-flop, the combinations LOW-LOW, HIGH-LOW, and LOW-HIGH behave the same way. However, if both the J and K inputs are set to HIGH, you will observe a continuous state toggle with each clock pulse. So, let’s move on to the working part to understand it more clearly.

Working Explanation of JK Flip Flop

Like SR and D, it is also possible to implement a JK flip-flop using NAND gates. And that will be appropriate to learn the behavior easily.

Circuit Diagram of JK Flip-Flop

Above, you can see the circuit diagram, but with a difference, it uses a 3-input NAND gate. So, it looks a little complex, but it’s actually quite easy.

If you're someone like me who tries to test the circuit using any simulation software like Proteus or Logisim, you might face difficulties in getting the proper output—I did too.

This flip-flop is difficult to simulate with its current combination. So, after some research, I found the circuit diagram below to work well for simulation. This was simulated using Proteus, and it replicates the same JK logic.

JK Flip Flop Stimulation Circuit Diagram

This is because, in real-world situations, there are pull-ups and pull-downs responsible for maintaining the active LOW or active HIGH state. But in simulation, it seems to lack such features. Adding them manually also didn’t work on my side.
With this, we can move to the explanation of the working.

JK Flip-Flop Working 1

Here, most of the logic is similar to the SR flip-flop, which means if you keep both inputs LOW, it will remain in the last state.

And if the J input is kept HIGH and the K is kept LOW, the output Q will be HIGH and Q̅ will be LOW.

Vice versa, if the J input is kept LOW and the K input is kept HIGH, the expected output will be Q LOW and Q̅ HIGH.

JK Flip-Flop Working 3

But in the case where both inputs are HIGH, the output will continuously toggle with each clock pulse.

Hope you got to know about the JK flip-flop. Now, in case you are looking to implement this in real-time hardware, we can still make it simple by using dedicated JK flip-flop ICs like 7473, which is often known as MC74HC73A, SN74LS73A, etc. By using this, it’s much easier to implement the JK flip-flop.

Simple explanation of JK flip-flop with Circuit

JK Flip Flop Demonstration

To explore the representation of JK Flip-Flop using logic gates, a list of components required, the circuit diagram with explanation, and a practical demonstration with working, you can check out the article: JK Flip-Flop: Circuit, Truth Table and Working.

T Flip-Flop

The T Flip-Flop, also known as the Toggle Flip-Flop, is actually a simplified version of the JK Flip-Flop. In fact, it’s like taking a JK Flip-Flop and tying both the J and K inputs together. So instead of two inputs, it has just one—called T (for Toggle).

The working is super simple,

  • When T = 0, nothing happens. The output stays the same.

  • When T = 1, the output continuously toggles. IE: If Q = 0, it becomes 1; if Q = 1, it becomes 0, and this cycle repeats on each Clock pulse.

That’s it. This flip-flop is all about flipping the state when told to do so.

Because of its simplicity, the T Flip-Flop is commonly used in binary counters, toggle switches, and control circuits, especially where you want to alternate states with each clock pulse. It’s a neat little component that turns a basic input into reliable, timed output transitions.

Symbol & Truth Table of the T Flip-Flop

Symbol and the Truth Table of T Flip-Flop

Above you can see the symbol and the truth table for the T Flip-Flop. The T Flip-Flop has just one input, T, and it works based on clock edges—usually the rising edge.

If T is set LOW, the output doesn’t change, no matter how many clock pulses come in. It just holds the previous state.

But when T is set HIGH, the magic happens—on every rising edge of the clock, the output continuously toggles. That means if the current output (Q) is LOW, it becomes HIGH, and if it’s HIGH, it becomes LOW.

T Flip-Flop Circuit Working

As you know, this T Flip-Flop is the extended version of the JK Flip-Flop. Simply tying the J and K inputs of the JK Flip-Flop to the T input makes the T Flip-Flop. Below you can see the circuit diagram of the T Flip-Flop.

Circuit Diagram for T Flip Flop

As we already discussed in the JK working section, simulating the same is not easy. I tried different combinations, but finally, it was only possible using the direct JK Flip-Flop IC. So, if you need to do a simulation, you can check it out using the 7473 JK Flip-Flop IC.

Above you can see the general working of the T Flip-Flop’s combinational logic circuit. As we already discussed, when T is LOW (0), the output stays in its last state, no matter how many clock pulses you give. But when T is HIGH (1), the output toggles on every rising edge of the clock—so if Q was 0, it becomes 1, and if it was 1, it becomes 0.

Just like the JK Flip-Flop, we can use the 7473 IC for demonstrating the T Flip-Flop.

Practical Demonstration of the Toggle Flip-Flop

T Flip Flop Demonstration

To dive deeper into the T Flip-Flop, check out the article T Flip-Flop: Circuit, Truth Table and Working. It explains the logic diagram, truth table, and excitation table, compares D and T flip-flops, shows how to convert between D, T, and JK flip-flops, lists the components required, and includes the circuit diagram, working explanation, a practical demo, and even a video to help you understand it clearly.

Comparison between the types of flip-flop

Hope you understand all the types of Flip-Flop Individually in the above sections. Now lets go through a direct brief comparison between each type of flip-flop.

Flip-Flop TypeInputsMain FunctionToggle SupportInvalid StateCommon Applications
SR Flip-FlopS (Set), R (Reset)Stores 1 (Set) or 0 (Reset)NoYes (S = R = HIGH)Simple control circuits, basic memory units
D Flip-FlopD (Data)Locks input at clock edgeNoNoRegisters, data latches, CPU pipelines
JK Flip-FlopJ, KImproved SR; adds toggle abilityYesNoCounters, frequency dividers, control logic
T Flip-FlopT (Toggle)Toggles output on every clock edgeYesNoBinary counters, toggle switches, blinking LEDs

Remember,

  • SR Flip-Flop is basic but can become unstable when both inputs are HIGH.

  • D Flip-Flop is ideal for precise data latching, what goes in comes out on clock.

  • JK Flip-Flop fixes SR's flaws and adds a toggle feature, great for counters.

  • T Flip-Flop is perfect for flipping states, used widely in frequency division.

Projects on Flip-Flop

These projects offer practical insights into flip-flop applications in digital electronics, helping to understand their use in memory storage, timing circuits, counters, and data synchronization in simple systems.

SR Flip-Flop with NAND Gates: Circuit, Truth Table and Working

SR Flip-Flop with NAND Gates: Circuit, Truth Table and Working

Learn how to design an SR flip-flop circuit using NAND gates. This article covers the circuit diagram, working principles, and truth table for a better understanding of SR flip-flops in digital electronics.

T Flip-Flop: Circuit, Truth Table and Working

T Flip-Flop: Circuit, Truth Table and Working

Learn about T Flip-Flop circuits, including their truth table, working principles, and applications. Explore how this type of flip-flop is used in digital electronics for toggling operations and building counters, with practical circuit examples and clear explanations.

JK Flip-Flop: Circuit, Truth Table and Working

JK Flip-Flop: Circuit, Truth Table and Working

Explore the JK Flip-Flop, its truth table, working principles, and circuit design. Explore its applications in digital electronics, including its use in counters and toggle circuits. Understand how this versatile flip-flop functions with practical examples and clear explanations.

D Type Flip-Flop: Circuit, Truth Table and Working

D Type Flip-Flop: Circuit, Truth Table and Working

Understand how the D Flip-Flop works, including its circuit design, truth table, and practical uses. Learn its importance in memory storage and data synchronization with simple explanations and diagrams.

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