LED Scrolling Display Board This article explains the steps involved in designing a scrolling message LED Display. The following features of the display are explained in the article. Framing Alpha-Numeric characters and special characters Multiplexing the matrices Serial mode of data distribution using shift registers Static text display Scrolling text display Mode selection between static and scrolling Increasing and decreasing the speed of scrolling in real-time Scrolling LED Sign – Output Video Demo LED Scrolling Display Board – Block Diagram LED Dot Matrix – Internal circuit Diagram The name itself infers that the LEDs are connected in a matrix format i.e., in rows and columns. The matrix can be formed with the desired number of rows and columns. However, 7X5 (5X7) and 8X8 dot matrix displays are readily available as modules. The designer can use these modules or can build an array by using individual LEDs. As the LEDs have two terminals, the matrix orientations are of two types. The row terminals are of Anode type and the column terminals are of Cathode type vice-versa or the row terminals are of Cathode type and the column terminals are of Anode type. Below are the images of the Matrix displays for the common anode and common cathode types. Here is a typical data sheet of Dot matrix displays with the above images. Refreshing the Matrix Display From the circuit diagram of the matrix displays, it is clear that any LED can be controlled only by the Row and Column terminals. So, in order to show a symbol or character through the LEDs, it is not possible to continuously turn ON all the LEDs corresponding to the character all the time. The LEDs should be addressed and controlled using individual pixels of the matrix. The matrix displays are similar to the Matrix keypads and are interfaced in the same way, the only change is that both the rows and columns are viewed as Output terminals. So, to display a character on the matrix, the information should be sent to the matrix either column by column or row by row. This is known as refreshing. Below video shows digit zero at a slower refresh rate. Let’s discuss the column by column type of interface for an 8X8 Matrix display. Let the columns be named from C1 to C8 and the rows from R1 to R8. In order to display a character, C1 is enabled and the rows that are required to show the character are enabled/turned ON. In the above video the digit zero is shown on the matrix. So, when C1 is enabled, R3 to R7 should be turned ON. Similarly, when C2 is enabled R2 and R8 should be turned ON. Remember that, before enabling C2 the C1 should be disabled and all the rows must be turned OFF (Rows is Optional). This is done by decade counter in our circuit. Framing the Characters Array for LED Scrolling Display For the refreshing of the display as discussed above, the characters are framed as two-dimensional arrays. The first dimension is used to address the character and its length or size is equal to the number of characters. The second dimension contains the information of rows corresponding to the enabled column or the information of columns corresponding to the enabled row. The array containing alpha-numeric and special characters is provided below. This data corresponds to Column Cathode type dot matrix display. Below is the image showing a character using asterisks. Download - Code of Characters Array Multiplexing the Matrices An 8X8 matrix contains 16-Pins, 8 pins for 8 rows... read more
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