What is a 7 Segment Display?

A 7 segment display is an device containing seven illuminating segments arranged in a rectangular fashion, as illustrated below. Typically the segments are LEDs (light-emitting diodes), but other technologies like LCDs, VFDs, and even incandescent bulbs have been used in the past.

 _       _   _       _   _   _   _   _  
| |   |  _|  _| |_| |_  |_    | |_| |_|
|_|   | |_   _|   |  _| |_|   | |_|  _|                               

By illuminating different combinations of these seven segments (labeled a through g), the display can represent the numerals 0-9. Some letters (like A, b, c, d, E, F, etc.) can also be displayed by a 7-Segment Display.

How Does a 7 Segment Display Work?

Each of the seven LEDs in the display is given a positional segment with one of its connection pins being brought straight out of the rectangular plastic package. These LEDs are strategically positioned and labeled from a to g, with the eighth LED being a dot (used to display a decimal point).

So by forward biasing the appropriate pins of the LED segments in a particular order, some segments will illuminate and others stay dim, displaying the corresponding character pattern.

Common Cathode vs Common Anode

There are two types of LED 7-segment display connections: Common Cathode (CC) and Common Anode (CA).

In a common cathode display, all the cathode connections of the LED segments are joined together to logic “0” or ground. The individual segments are illuminated by a “HIGH” signal applied to the anode through a current limiting resistor.

In a common anode display, the anodes of all the LED segments are joined together to logic “1”. The individual segments are illuminated by applying a ground, logic “0” or “LOW” signal to the cathode of the particular segment.

Here’s a table summarizing the differences:

Characteristic	Common Cathode	Common Anode
Common Pin	Cathode	Anode
Active Segment Pin	Anode	Cathode
Active Segment Voltage	High	Low

Driving a 7 Segment Display

A single 7-segment display can be controlled with just eight output pins from a microcontroller. However, there are some dedicated driver ICs available like MAX7219, MAX7221, TM1637, 74HC595, etc. that can control multiple 7-segment modules and greatly simplify the wiring.

These driver ICs handle all the complexities like multiplexing, current limiting, and communication required to drive multiple 7-segment displays based on a few serial data lines from a microcontroller.

Some popular ways to control a 7-segment display are:

1. Direct Drive: Control each segment directly via a microcontroller output pin. Simplest but requires many I/O pins.

2. BCD to 7-Segment Decoder: Use a dedicated BCD to 7-segment decoder IC like 7447 or 4511. The microcontroller sends a 4-bit BCD (binary coded decimal) value to the decoder, which translates it into the appropriate 7-segment code. Requires fewer microcontroller pins.

3. Multiplexed Display: Use a driver IC like MAX7219 that can multiplex multiple displays. The microcontroller sends serial data to the driver IC indicating what to display. Requires the fewest microcontroller pins.

Advantages and Disadvantages

Some key advantages of 7 segment displays are:

• Low power consumption
• High brightness and good contrast
• Fast switching capability
• Wide viewing angle
• Low cost and easy availability

Some limitations are:

• Limited to displaying numbers and a few letters
• Requires external driver circuitry to operate
• Can be hard to read in direct sunlight
• Larger than newer display technologies like OLEDs or LCDs

Applications of 7-Segment Displays

Some common applications of 7-segment displays include:

• Digital clocks and watches
• Calculators
• Microwave ovens
• Measuring instruments like digital multimeters
• Audio and video equipment
• Elevator floor indicators
• Parking meters
• Speedometers and odometers
• Stadium scoreboards
• Stock price tickers

Frequently Asked Questions (FAQ)

1. How do I know if my display is common anode or common cathode?

Check the datasheet or look for “common anode” or “common cathode” written on the display itself. If there’s a pin labeled “com” or “gnd”, it’s likely a common cathode. If there’s a pin labeled “vcc” or “+”, it’s likely a common anode.

2. Can a 7-segment display show all letters?

No, a standard 7-segment display is limited to numbers and a few letters like A, b, c, d, E, F, H, L, N, P, r, U, etc. To display the full alphabet, a matrix display or 14-segment display is required.

3. What’s the difference between a 7-segment and 16-segment display?

A 16-segment display has more segments (16 total – including diagonal segments) which allows it to display more characters and symbols compared to a 7-segment display. However, 16-segment displays are less common.

4. How do I limit the current through the LED segments?

Each LED segment needs a series current limiting resistor to avoid drawing excessive current. The resistor value depends on the LED’s forward voltage and the supply voltage. A typical value is 330Ω for a 5V supply.

5. Can I control a 7-segment display directly from an Arduino?

Yes, you can wire the segment pins directly to the Arduino’s digital I/O pins. However, this uses up many pins. It’s more efficient to use a driver chip like the MAX7219 or 74HC595 to control the display while using fewer Arduino pins.

Conclusion

The 7-segment display is a classic and widely used electronic display device found in countless applications from household appliances to industrial equipment. While newer display technologies like OLEDs and LCDs are replacing it in some areas, the 7-segment display remains popular due to its simplicity, reliability, and low cost.

By understanding how 7-segment displays work at a fundamental level, you can effectively use them in your own projects. Whether driving them directly from a microcontroller or via a dedicated driver IC, 7-segment displays are a proven way to integrate numeric displays into your designs.