Introduction to 7 Segment Displays
Seven segment displays are a common component in electronic devices that need to display numerical information. They contain 7 LED segments arranged in the shape of an 8, which can be illuminated to display the numbers 0-9. To control these displays, dedicated ICs called 7 segment controllers are used.
Types of 7 Segment Controller ICs
There are two main types of ICs used to drive 7 segment displays:
BCD to 7 Segment Decoders
These accept Binary Coded Decimal (BCD) input and decode it to drive the 7 segments directly. Common examples include:
|7447||BCD to 7 segment decoder/driver with open collector outputs|
|4511||BCD to 7 segment decoder/driver with active high outputs|
LED Display Drivers
These are more advanced and can handle multiple displays. They have built-in features like brightness control, scrolling, and interfacing. Common examples include:
|MAX7219||LED display driver that can drive 8 digits with scrolling capabilities|
|MAX7221||Similar to MAX7219 but can cascade to drive larger displays|
Interfacing 7 Segment Displays with Microcontrollers
To use 7 segment displays in a microcontroller based system, the controller needs to be interfaced with a suitable driver IC. Here are some common techniques:
- Use a BCD decoder like 7447 and send BCD data from the MCU to display digits
- Use a driver like MAX7219, connect it to the MCU via SPI interface to send digit data and control the display
- Use transistors/LED drivers between MCU and display, and activate each segment by sending signals from GPIO pins
- Use the MCU’s built-in peripherals like timer modules to generate signals to drive the 7 segments directly
Sample Circuit with Arduino and MAX7219
Here an Arduino is interfaced to a MAX7219 driver via SPI. The MAX7219 takes care of scanning the display and controlling the brightness while the Arduino sends the digit data.
- 7 segment displays are a versatile way to display numeric information
- Dedicated ICs help interface them easily to microcontrollers and other systems
- BCD decoders and LED driver chips provide different levels of control over the display
- With appropriate interfacing, 7 segment displays can be used in embedded projects to display parameters, counts, etc.
Frequently Asked Questions
What is the difference between common cathode and common anode displays?
Common cathode displays have all the cathodes of the 7 LEDs connected together to ground. The anodes for each segment are separate. For common anode, the anodes are connected to +VCC while the cathodes are separate. The interface circuitry has to be designed accordingly.
Can 7 segment displays be multiplexed/interfaced with shift registers?
Yes, multiplexing is commonly used with 7 segment displays to control multiple digits while minimizing the number of IO pins required from the controller. Shift registers like 74HC595 can similarly help reduce the IO count.
How do I choose the right current limiting resistors for a 7 segment display?
Check the display’s datasheet for the maximum DC current per segment, usually around 10-20mA. Then use Ohm’s law to calculate the resistor value required for limiting the current through each segment to this desired value.
Can 7 segments display alphanumeric characters?
Standard 7 segments can only display numeric digits. To display alphabetical letters, alphanumeric displays with either 14 segments or 16 segments are required. These have additional horizontal and vertical segments.
How do dot matrix displays compare to seven segment ones?
Dot matrix displays use a matrix of LED dots to display alphanumeric characters and custom symbols. They offer flexibility but are more complex to interface compared to seven segments. They are used when textual display is required.