LED dot matrix driving topologies and panelling

Abstract

Different LED dot matrix driving topologies and concurrent data transfer methods were investigated in this dissertation. The LED dot matrix driving topologies was first implemented and then tested. The designs tested were: the constant voltage, constant current, bi-polar and use of an existing LED dot matrix display driving integrated circuit (IC). All four driving topologies were evaluated to determine the brightest design. It was found that the constant current design performed the best, while the existing driving IC did not function at all. The bi-polar was a very close second while the constant voltage design performed the worst. The improvements in the bi-polar design with respect to the refresh rate were not enough to warrant the use of the bi-polar design in future permutations of the hardware. It was decided to improve the constant current driving topology and use this as the backbone for the LED dot matrix displays. Since the size of LED dot matrix displays are increasing, a smarter driving algorithm in conjunction with hardware needed to be developed. It was therefore decided to subdivide a large LED dot matrix display into smaller sections called panels. These panels can be connected in a horizontal, vertical, or a combination of the two to suit the user requirements. The panels were connected to adjacent panels to aid in determining the display size and configuration. Display graphics data was then sent to all the panels concurrently even though each panel still updated serially. This improved the brightness and refresh rate of the entire LED dot matrix display. Controlling this display required the use of a powerful processor as the algorithm had to subdivide graphics and distribute these to the panels to be displayed. Making use of pseudo-addressing, each panel connected to the system was assigned a temporary address. A Raspberry Pi computer was used to implement and execute the display algorithm. For connecting the Raspberry Pi computer to the LED dot matrix display panels, an interface card was developed. This interface card was only activated once the Raspberry Pi was connected. The on-board microcontroller on the interface card controlled the LED dot matrix display brightness. This was done by measuring the ambient light and adjusting the digital resistors placed on the LED dot matrix panels. The value of these resistors in conjunction with the current sources used in the hardware design determined the brightness of the display.