Seven segment displays are a ubiquitous part of electronic devices, enabling the display of numerical information in a simple and efficient manner. The seven segments refer to the seven elements that make up each display digit – two vertical lines, three horizontal lines, and two diagonals. By illuminating certain segments, all numerical digits from 0 to 9 can be represented.
Driving these seven segment displays requires special integrated circuits (ICs) that can properly power and control each segment. In this guide, we’ll explore the inner workings and key specifications of these seven segment driver ICs.
How Seven Segment Display ICs Work
The basic architecture of a seven segment IC consists of:
- Display outputs – these output pins connect to each of the seven segments of the LED or LCD display.
- Digit outputs – these outputs control which digit is currently being displayed. For single digit displays, only one is needed. For multi-digit displays, multiple are required.
- Data inputs – these inputs receive the binary coded decimal (BCD) data for the number to be displayed.
- Power inputs – these provide power to the IC. Both low voltage and high voltage inputs may be required depending on the display.
Internally, the IC contains logic to decode the BCD input data and properly illuminate the corresponding segments. The digit outputs are used to multiplex between digits one at a time very rapidly so that they appear illuminated simultaneously.
Here is a diagram of a typical 7-segment IC block diagram:
Block | Description |
---|---|
BCD to 7-segment decoder | Decodes BCD inputs to appropriate segment outputs |
Multiplexing logic | Turns digit outputs on/off sequentially |
Output drivers | Drives display segments with sufficient current |
Timing and control logic | Generates internal timing signals |
Input registers | Store BCD input data |
Power regulation | Provides stable internal operating voltages |
To display a number, the BCD equivalent is loaded into the input registers. The decoder activates the proper segments, while the multiplexing logic cycles through each digit output to display the full value. This refresh process repeats constantly at a rate faster than the human eye can perceive, typically 100-1000 Hz.
Key Specifications
There are several key specifications to consider when selecting a seven segment display driver IC:
- Supply voltage – typically 3-5V for CMOS and TTL compatibility. Some displays require higher voltages.
- Output drive current – the max current each output can provide to illuminate the display. LCDs may require less than LEDs.
- Number of digits – single digit, dual digit, 4-digit and 8-digit devices are common.
- Multiplexing rate – the refresh rate; 500-1000Hz is standard.
- Logic family – Most ICs support TTL logic levels. Some support CMOS.
- Package type – Common options are DIP, SOIC, and SMT.
- Temperature range – Industrial and military-spec devices can operate from -40°C to +85°C or more.
- Features – Some provide built-in decoding from hexadecimal data or support dual-color displays.
Here are a few examples of common 7-segment ICs:
IC | Description |
---|---|
MAX7219 | 8-digit LED display driver with serial interface |
LM92608 | 4-digit CMOS LED driver with free-run refresher |
CD4511B | Single BCD-to-7-segment latch/decoder/driver |
MC14511B | CMOS single-digit BCD decoder driver for LCDs |
Typical Connection Diagram
Here is a typical schematic diagram for wiring a 7-segment IC to a display:
The anodes of each LED segment are connected to the cathode drivers of the IC. The cathodes are tied together and connected to the low voltage supply. Digit driver outputs control the common anodes for each digit. Simple resistors limit the current through each LED segment.
Sample Applications
Seven segment ICs are used in a wide variety of applications:
- Digital clocks
- Electronic meters
- Basic numeric displays and readouts
- Embedded systems status displays
- Automotive dashboard displays
- Low-cost calculating devices
- Home appliances with numeric displays
- Handheld electronics like multimeters
- Test and measurement equipment
With the simplicity and low cost of 7-segment displays and driver ICs, they are ideal for displaying basic numeric information in all types of electronic devices.
Some examples include:
- Digital clock – A real-time clock IC provides time data to a 7-segment driver, which refreshes a 4-digit 7-segment display once per second.
- Frequency counter – A frequency counter circuit uses a 7-segment display to show the frequency being measured. The input frequency is conditioned, measured by a microcontroller, and displayed on the 7-segment readout.
- Multimeter – A digital multimeter may use a 3.5 digit 7-segment display to show the measured voltage or current. The meter input circuitry and ADC convert the signal to digits which are sent to the display driver.
Design Considerations
Here are some important factors to consider when incorporating a 7-segment display IC into your design:
- Number of digits – Determine how many digits are required for your display. This dictates the number of digit outputs needed from the IC.
- Display brightness – LED displays are current-driven devices so sufficient drive current (10-30mA typ.) must be provided for adequate brightness.
- Multiplexing rate – Fast multiplexing (>500 Hz) avoids visible flicker. Slow rates may be acceptable for small LCDs.
- Input data format – Most ICs accept BCD, but some support binary, hexadecimal, or ASCII.
- Microcontroller interface – Many microcontrollers have dedicated 7-segment display controller hardware that can directly interface with the driver ICs.
- Power requirements – The IC and display may require 5V or other voltage levels. Regulators may be needed for stable operation.
- ESD protection – Protect inputs with resistors and clamping diodes where needed.
Paying attention to these factors will help ensure your 7-segment display operates reliably. Consult IC datasheets and application notes for additional design guidelines.
7-Segment IC Variants
While standard 7-segment ICs drive simple numeric readouts, some more advanced variants exist:
- 14-segment and 16-segment displays – Allow display of alphanumeric characters. Require ICs with additional segment outputs.
- Multi-color displays – ICs with redundant outputs for each segment to control multi-color Red/Green/Yellow displays.
- Dot matrix displays – More flexible alphanumeric displays arranged in a pixel grid. Require matrix display driver ICs.
- LCD displays – Require compatible waveforms and voltages to drive liquid crystal segments.
- I2C and SPI display drivers – Support serial interfaces for communicating with microcontrollers.
So while the standard 7-segment IC offers basic numeric display functionality, enhanced ICs are available when greater flexibility is required.
Conclusion
In summary, seven segment display ICs are essential for interfacing microprocessors and other logic systems to numeric LED or LCD displays. Their simplicity and low cost make them popular for all types of digital readout applications. By understanding the internal operation and key specifications of these devices, engineers can readily incorporate them into new designs. With the wide availability of 7-segment ICs offering various digits, interfaces, and features, they will continue to be a versatile display solution.
Frequently Asked Questions
What is the difference between a 7-segment display and a dot matrix display?
The main difference is that 7-segment displays are limited to displaying numeric digits, while dot matrix displays can be used to display alphanumeric characters in a pixel grid by selectively illuminating an array of LEDs or LCD elements.
How do I know how much current an LED 7-segment display requires?
Check the datasheet of your particular LED display – typical current values range from 10-30mA per segment, but can vary between part numbers and manufacturers. The display driver IC must be able to source or sink this much current.
Can a 7-segment IC work with a 4-digit display?
Most likely yes – you would just not use 4 of the digit outputs on an 8-digit driver IC. A 4-digit dedicated driver IC may be a more efficient and cost-effective choice however.
How do I interface a microcontroller to a 7-segment display IC?
Many microcontrollers have dedicated hardware modules that simplify interfacing. Otherwise, bit-bang the BCD data from GPIOs to the inputs of the display driver. Be sure to account for voltage level differences.
Why are extra resistors sometimes added in series with the LED segments?
The resistors help limit the current through each LED segment to prevent damage. The IC provides voltage but the resistors set the current based on Ohm’s law. Value depends on your supply voltage and LED forward voltage.