LED Dot Matrix Display

A nifty 32×7 dot matrix display module, programmable via an RS232 serial port. There are preset inputs to display preset messages or simply control directly from a PC or laptop.
The principle of the display is based on our persistence of vision, the same thing that enables us to watch movies without seeing the flickering changes of frame. This display has 32 LEDs horizontally by 7 vertically. At any one time there is only ever one column of LEDs lit. The on’s and off’s are presented to the anode connections while the columns are enabled one by one. In this way a dot display of characters can be generated and when the speed is increased sufficiently, we stop seeing the flickering and see it as a steady display of dots.
The circuit function is fairly straight forward and is best shown by the block diagram. The PIC presents the data for each column and clocks the shift registers at the same time. During the initial pulse out of every 32 pulses, the DATA goes high to set the first output on, after which it returns low. This “1” is then clocked in serial manner through the shift register, driving the Darling tons to provide a “sink” for whatever LEDs are lit in that particular column. This is repeated every millisecond to provide a flicker-free display.
LED Dot Matrix Display

A major drawback of course is that the brightness of the LEDs appears very much reduced, after all, any one LED is only on for 1/32 of the time. The answer is to drive the LEDs much harder than normal, at about 200mA which would damage them for any longer than a few milliseconds.
The PIC is the heart of the circuit managing all serial communication and display. IC3-6 are shift registers with the 8th output also connected to the DATA input of the next register. The Darlington drivers are ULN2803A octal drivers capable of at least 500mA per output. T1-8 provide the row drive and T9 buffers the serial input from +/-12v to 5v.

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