Overview of LED and FND
Before presenting the experimental results, this section provides an overview of the operational principles of LEDs and FNDs.
In modern society, we interact closely with a myriad of electronic devices in our daily lives, such as televisions, smartphones, smartwatches, and tablet PCs. As the primary user interface for these devices, display technology that visually communicates information is an indispensable component. Even the most advanced, high-resolution displays of today are fundamentally rooted in the operating principles of basic optoelectronic components, such as Light-Emitting Diodes (LEDs) and Seven-Segment Displays. Therefore, this experiment aims to build a foundational understanding of modern display systems by practically implementing and analyzing the electrical characteristics and driving mechanisms of LEDs and Seven-Segment Displays, which form the cornerstone of display technology.
1. Operating Principle of LEDs
First, an LED (Light Emitting Diode) is a semiconductor device that emits light when an electric current flows through it. As a type of diode, it operates only when current flows in a specific direction (forward bias). When current flows, electrons recombine with holes to release energy, which is emitted in the form of light (photons).
Since LEDs share the same PN junction structure as standard diodes, the color of the emitted light depends on the energy band gap of the semiconductor material. A larger energy difference results in shorter wavelengths (e.g., blue), while a smaller energy difference results in longer wavelengths (e.g., red).
Forward Voltage vs. Cut-in Voltage
The forward voltage of an LED refers to the minimum voltage required for light emission, which differs slightly from the standard diode "cut-in" voltage. While it varies by device, the forward voltage typically ranges from 1.8V to 3.3V. In contrast, the cut-in voltage refers to the threshold where the diode begins to conduct significant current, leading to a rapid exponential increase in current flow.
Circuit Protection and Resistance
When designing circuits with LEDs, a current-limiting resistor is essential because LEDs are highly sensitive to current changes. If the current exceeds the rated maximum, the device can be easily damaged. Therefore, an appropriate resistor value must be calculated based on the supply voltage and the LED's forward voltage to ensure safe operation and accurate measurement.
Reverse Bias and Protection
Since LEDs are PN junction diodes, they are subject to breakdown phenomena. However, LEDs typically have a very low reverse breakdown voltage (often below 5V) and can be damaged before or immediately upon reaching this threshold. For this reason, extreme caution is required when applying reverse bias, or a standard rectifier diode should be connected in parallel for protection.
Why is a parallel connection necessary?
A series connection can control the direction of the current, but it cannot absorb the reverse voltage itself. Conversely, a parallel connection protects the LED by bypassing the current (providing an alternative path) when a reverse voltage or transient spike occurs.
2. Overview of FND (7-Segment Display)
The FND (Flexible Numeric Display), commonly referred to as a 7-Segment Display, is an electronic display component capable of representing decimal numerals (0 through 9) and specific alphabetic characters by illuminating combinations of seven distinct segments.
Structure and Configuration The device consists of the following key elements
Seven LED Segments : The bars that form the digits.
Decimal Point (DP) - A dot-shaped LED used to indicate a decimal point.
Depending on the internal wiring, 7-segment displays are classified into two types:
Common Anode - All anodes of the LEDs are connected together. A specific segment lights up when a LOW signal (logic 0) is applied to its cathode.
Common Cathode - All cathodes of the LEDs are connected together. A specific segment lights up when a HIGH signal (logic 1) is applied to its anode.
Driving Methods There are two primary methods for driving 7-segment displays
Direct Drive (Static Drive)
Each segment is connected to an individual I/O pin.
Pros/Cons - While the circuit design is simple, the required number of I/O pins increases significantly as the number of digits increases.
Multiplexing (Dynamic Drive)
The common terminals of multiple digits are selected sequentially. Although only one digit is lit at a time, high-speed scanning creates the illusion that all digits are lit simultaneously due to the persistence of vision.
Pros/Cons
This method conserves I/O pins. Although brightness may decrease due to the duty cycle, this can be compensated for by adjusting the resistance values and the scanning frequency.
Definition of I/O Pins I/O (Input/Output) pins are terminals designed to exchange data between digital circuits (such as microcontrollers) and external devices.
Input : Reads digital signals from external sensors, switches, or buttons.
Output : Sends processed signals from the microcontroller to external devices like LEDs.
Measurement Data
We constructed a circuit using Red and Yellow LEDs and measured the voltage drop across the LED and the current flowing through the circuit while increasing the power supply voltage from 0.5V to 6.0V.
Analysis and Observations
Based on the measured data, the following characteristics were observed
Light Emission Threshold
The LEDs remained off at low voltages. Light emission began only after the voltage exceeded a specific threshold (approximately 1.8V to 2.0V). The brightness increased as the voltage rose, reaching its maximum intensity at around 5V.
Voltage Clamping
As shown in the tables, when the input voltage was low (below 1.5V), the measured voltage across the LED was nearly identical to the input voltage, indicating an open circuit state. However, once the input exceeded 2.0V ~ 2.5V, the voltage across the LED did not increase significantly, remaining clamped between 1.9V and 2.3V, even as the input voltage continued to rise.
Current Surge
In contrast to the voltage, the current remained negligible until the voltage approached the LED's forward voltage (approx. 1.8V). Beyond 2.0V, the current increased rapidly, demonstrating the exponential nature of the diode's I-V characteristic.
Discussion
Current-Controlled Brightness
The experiment confirms that an LED is a current-controlled device. Although the voltage across the LED remained relatively constant after turning on, the brightness continued to increase. This indicates that the intensity of light emission is determined by the magnitude of the current flowing through the device, not the voltage across it.
Non-Ohmic Behavior
Unlike resistors that follow Ohm's Law linearly, the LED exhibited non-linear (Non-Ohmic) behavior. While the current increased drastically from roughly 1mA to 37mA, the voltage across the LED only changed slightly (approx. 0.2V ~ 0.3V). This implies that the dynamic resistance of the LED decreases significantly in the forward-biased region.
Safety Consideration (Over-Current)
It is important to note that at a 6V input, the current reached approximately 37mA. Since standard 5mm LEDs typically have a maximum rated current of 20mA, driving the LED at 37mA exceeds its safe operating range. Prolonged operation at this level can lead to overheating and permanent damage. Therefore, in practical circuit design, a current-limiting resistor with a higher resistance value must be selected to maintain the current within a safe range (typically 10mA ~ 20mA).
Conclusion
Through this experiment, we verified the operational characteristics of LEDs and 7-segment displays. We confirmed that LEDs require a minimum forward voltage to conduct and that their brightness is controlled by current. Additionally, the experimental data highlighted the importance of using appropriate resistors to protect the semiconductor elements from over-current damage due to their non-linear I-V characteristics.
LED Serial & parallel measurement & conclusion
In this experiment, the illumination of four light-emitting diodes (LEDs) was evaluated by configuring them in both parallel and series circuits with an applied source voltage of 3.5V. As observed in the visual data, the LEDs successfully illuminated in the parallel configuration but failed to emit light in the series configuration. This discrepancy can be attributed to Kirchhoff's circuit laws and the forward voltage (cut-in voltage) characteristics of the LEDs.
Based on previous experiments, the forward voltage required to activate a single LED is approximately 2V. In a series configuration, the applied voltage is divided across each component. Consequently, applying 3.5V across four LEDs results in an insufficient voltage drop per LED (less than 1V), which is well below the 2V threshold required for activation. Therefore, to successfully illuminate the four LEDs in a series circuit, a total supply voltage of approximately 7V to 8V is necessary. Conversely, in a parallel configuration, the full 3.5V source voltage is applied uniformly across each parallel branch. Because this applied voltage exceeds the 2V forward voltage requirement, all four LEDs illuminate successfully.
FND measurement & conclusion
In this experiment, numeric characters were implemented using a Seven-Segment Display. Among the two primary configurations—common anode and common cathode—the common anode type was utilized first. In a common anode display, all segment anodes are tied together to a positive voltage supply ($V_{CC}$), while the individual segments are activated by connecting their respective cathode terminals to ground.
Although only a single display unit was used, rendering specific numbers required a substantial number of jumper wires. While the theoretical principle of displaying alphanumeric characters is straight forward once the pin configuration is understood, the practical implementation proved to be complex the dense cluster of jumper wires significantly increased the probability of wiring errors. Through these consecutive LED and Seven-Segment Display experiments, the fundamental principles of basic display implementation were successfully comprehended.
Furthermore, the complexity and error-proneness experienced during manual wiring highlight the critical limitations of directly driving displays in practical circuits. Relying solely on jumper wires is inefficient and unscalable for multi-digit displays. To mitigate these issues in real-world applications, it is essential to utilize dedicated decoder/driver ICs (e.g., BCD-to-Seven-Segment decoders) or to implement multiplexing techniques. Ultimately, this foundational experiment not only demonstrates basic display operation but also provides practical insight into why digital logic controllers and driver circuits are indispensable in modern electronic display systems.
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