LED Dot Matrix Display

A dot matrix is a two dimensional array of dots used to represent characters, symbols and messages. Dot matrix is used in displays. It is a display device used to display information on many devices like machines, clocks, railway departure indicators etc.

An LED dot matrix consists of an array of LED’s which are connected such that the anode of each LED are connected together in the same column and the cathode of each LED are connected together in the same row or vice versa. An LED dot matrix display can also come with multiple LEDs of varying colors behind each dot in the matrix like red, green, blue etc.

Here each dot represents circular lenses in front of LEDs. This is done to minimize the number of pins required to drive them. For example an 8X8 matrix of LEDs would need 64 I/O pins, one for each LED pixel. By connecting all the anodes of LEDs together in a column and all the cathodes together in row, the required number of input and output pins reduced to 16. Each LED will be addressed by its row and column number.

                        Diagram of 8X8 LED Matrix using 16 I/O pins

Controlling the LED matrix:

Since all the LEDs in a matrix share their positive and negative terminals in each row and column, it is not possible controlling of each LED at the same time. The matrix controlled through each row very quickly by triggering the correct column pins to light the desired LED’s for that particular row. If the switching done with a fixed rate, humans can’t see the displaying message, because human eye can’t detect the images with in the milliseconds of time. Thus the displaying of a message on LED matrix must be controlled, with the rows being scanned sequentially at a rate greater than 40 MHz while sending out the column data at the exact same rate. This kind of controlling can be done my interfacing the LED matrix display with the microcontroller.

Interfacing the LED matrix display with Microcontroller:

Choosing a microcontroller for interfacing with LED matrix display which is to be controlled is depends on the number of input and output pins needed for controlling all the LEDs in the given matrix display, the amount of current that each pin can source and sink and the speed at which the microcontroller can send out control signals. With all these specifications, interfacing can be done for LED matrix display with a microcontroller.

           Using 12 I/O pins controlling the Matrix display of 32 LEDs

In the above diagram each seven segment display is having 8 LEDs. Hence the total number of LEDs is 32. For controlling all the 32 LEDs 8 information lines and 4 control lines are needed i.e. for displaying message on the matrix of 32 LEDs, 12 lines are needed when they are connected in matrix notation. Using the microcontroller instructions can be converted into signals which turn ON or OFF lights in the matrix. Then the required message can be displayed. By controlling with the microcontroller, we can change which color LEDs are lit at even intervals.

There are several options for choosing microcontroller and LED matrix. The easiest way is first choosing the LED dot matrix and then selecting a microcontroller which needs the requirements of LEDs to be controlled. Once these selections are completed, a major part is lies in programming to scan the columns and feed the rows with appropriate values for the LED matrix to display different patterns for displaying required message.