Introduction

Seven segment displays are a basic component found in many electronic devices to display numerical information. They get their name from their appearance – seven individual LED segments arranged in the shape of an 8 to form each digit from 0 to 9. While simple, seven segment displays remain popular due to their low cost, ease of use, and versatility. This article will provide an in-depth look at the internal structure and functioning of these ubiquitous devices.

What is an IC Seven Segment Display?

An IC seven segment display contains multiple light emitting diodes (LEDs) arranged in a seven segment configuration along with support electronics integrated into a single package. The term IC stands for “integrated circuit”.

Common seven segment display layout with each segment labeled

The seven segments – labeled a through g as shown above – can be individually powered on or off to form the representations of numerals 0 through 9. Additional segments like the decimal point are also included on most devices.

By controlling which LEDs are illuminated, the integrated driver circuitry displays easy-to-read alphanumerical information. Single IC packages frequently include multiple digits, minimizing circuit complexity.

Internal Structure

Inside an IC seven segment display is a series of LEDs and integrated control circuitry. Let’s look at the internal components and operation.

LED Segments

The visible light emitting portion of the display is composed of multiple LEDs. There are seven LEDs arranged in distinct geometrical segments, plus often an LED for the decimal point.

Early displays used plain incandescent bulbs. LEDs require less power, are smaller, and have far greater longevity, making them ideal for modern devices. The LEDs are colored – most commonly red, but other colors like green and yellow are widespread.

The LEDs themselves have a typical forward voltage drop around 2V. This must be accounted for in the power supply and limiting resistor values.

Drive Circuitry

Support electronics control which LED segments are powered on or off to form the desired characters. This circuitry is integrated behind the actual display elements in the same overall package.

The driver contains multiple switching transistors, one for each display segment. Applying a voltage to the transistor base activates it, allowing current to flow and illuminate the attached LED.

Other components like resistors may be incorporated to protect the delicate LEDs. The driver electronics run off a higher supply voltage, usually 5V, that must be reduced to a safe value for the LEDs.

Digit Multiplexing

For displays with multiple numeric digits, the driver circuitry manages powering on the correct digit at a given time using a technique called multiplexing. Individual digits are turned on and off very quickly in sequence, relying on our persistence of vision to make the display appear continuously illuminated.

This crucial technique greatly reduces the number of I/O pins required. A 4-digit display for example needs only 7 segment pins and 4 digit pins instead of 28 individual pins. Multiplexing also lessens power consumption.

Interfacing Seven Segment Displays

To leverage a seven segment display in an electronic design, the proper connections must be made between the integrated driver circuitry and a microcontroller or logic circuits providing the digit data.

Supply Power

A regulated 5V supply is typical, though other voltages are sometimes used. This higher voltage is required by the internal driver circuitry, separate from the ~2V needed by the LEDs themselves. Make sure to use at least a 100mA supply, up to several amps for larger displays.

Digital Control Signals

Data lines from the controlling electronics must connect to the appropriate pins on the display to map each internal LED segment. Refer to datasheets for the correct pinout diagram.

Control lines for multiplexed displays must also be connected to digit selector pins. The microcontroller cycles through digit enable patterns to turn on the proper digits sequentially.

Current Limiting Resistors

Since LEDs require a specific forward voltage and current, each display segment must have a current limiting resistor. Size resistors according to your supply voltage and LED specs, typically 150-300 Ohms.

Resistors can be placed externally, though many displays integrate resistors internally to simplify circuit design.

Other Considerations

Pay close attention to a few other aspects when incorporating a seven segment display:

• Logic voltage levels – Many devices operate at 5V logic while modern microcontrollers use 3.3V or lower. Be sure to account for any logic level differences.
• Cathode vs. anode – LEDs have two terminals. Connect the cathode pins correctly based on whether the display uses a common cathode or common anode design.
• Brightness – Adjust current limiting resistors to set the LED brightness as desired. There is often a trade-off with power consumption.

Seven Segment Display Types

While sharing the same general seven segment structure, these displays come in a variety of form factors optimized for different applications.

Discrete LED Displays

Discrete LED displays use individual LEDs placed in a seven segment arrangement without integrated driver electronics. This provides the most flexibility for advanced circuits but requires the most external components.

Discrete seven segment display with LEDs arranged by hand

Multi-digit Numeric Displays

These cost-effective displays pack multiple digits together with integrated electronics to display numeric information like calculator outputs, clocks, and measurements.

Common sizes include single digit (1″), two digit (0.5″), four digit (0.4″), and eight digit (0.3″). Heights around 0.4″ are readable in most cases. High brightness versions are also available.

4 digit seven segment display in a compact package

Alphanumeric Displays

More advanced displays can show a full alphanumeric character set in addition to digits. This is accomplished by addressing additional LED segments. 14-segment and 16-segment designs are common.

Text displays provide versatility but have higher complexity and cost. They require microcontrollers with significant memory and processing capabilities to generate appropriate segment patterns.

14 segment alphanumeric display with extra horizontal and vertical segments

Dot Matrix Displays

Dot matrix displays use a grid of hundreds of tiny LEDs that can be turned on or off to generate custom shapes, graphics, and text instead of being limited to segments. Resolution ranges from 5×7 pixels up to high density 128×64 graphics displays.

Dot matrix LED display

These allow great versatility but are more expensive and complex to interface than segmented displays.

Other Display Types

• LCD displays allow low power alphanumeric text and primitive graphics.
• Vacuum fluorescent displays provide a high brightness/low power alphanumeric option.
• OLED displays deliver crisp high contrast graphics and text in slim form factors.

Applications of Seven Segment Displays

Thanks to their simplicity, low cost, and bright readable output, seven segment displays are ubiquitous:

• Digital clocks
• Electronic calculators
• Multimeters
• Appliances like microwaves, ovens, washers
• Industrial electronics and test equipment
• Automotive dashboard displays
• Set top boxes like cable boxes
• LED “smart” billboards and signs

Nearly any device that needs to display numeric information will likely contain a seven segment display. Their versatility ensures they will remain popular into the future across both hobbyist projects and commercial systems.

Conclusion

While the concept of seven segment displays has existed for over a century, LED-based devices provide an optimal modern way to display alphanumeric information. Their internal structure with individual LEDs and integrated driving circuitry creates a compact module ideal for integration into complex products. By following interfacing guidelines and selecting appropriate display types, seven segment LEDs can enable projects both big and small.

Frequently Asked Questions

What are the advantages of seven segment displays?

Some key advantages include:

• Inexpensive compared to other display types
• Easy to interface and control, even with simple logic circuits
• Bright and clearly readable alphanumeric output
• Widely available in many sizes and configurations
• Proven technology that is well understood

How do you control multiple digits on a multi-digit display?

Multi-digit displays use a technique called multiplexing. This cycles through and enables each digit one at a time in rapid succession. By strobing digits faster than our eye can detect, the display appears constantly illuminated. Multiplexing greatly reduces pin count.

How do you choose current limiting resistors for LED segments?

Use Ohm’s law. Determine the supply voltage, LED forward voltage from its datasheet (usually around 2V), and desired LED current (2-20mA is common). Then use R = (Supply – LED Vf) / LED current to calculate the needed resistor value.

Can seven segment displays show alphabetic characters?

Yes, alphanumeric displays have extra segments that allow showing a full uppercase character set along with numbers. This requires more complex driver circuitry and microcontroller programming but enables text displays.

Are there seven segment displays that can show graphics and images?

Yes, dot matrix graphic displays use a grid of LED pixels that can be individually addressed to create custom shapes, graphics, text, and images. Resolution ranges from 5×7 up to high density 128×64. They are more complex than segmented displays but very versatile.