Large LED Dot Matrix Display

This project started with me buying 72 red LED dot matrix displays from ebay for £15 with each display having 5 x 7 LED's which seemed ideal for displaying characters (letters and numbers), especially as the displays have an unused area top and bottom which would make it difficult to display a large, continuous graphic. A message board seemed an obvious choice, to display the status of the Underground trains and the weather, by pulling information from online.

The displays I will be using for this project have the model number CSM57121D (written on the side) which has common cathode columns and measure 22mm x 38mm. On the back of the display it has the text 'CS57121' and the pin at the bottom right is marked with a '1'. If we use the following numbering:

14 13 12 11 10 9 8

CS57121

7 6 5 4 3 2 1

Then we have this pinout:

1 - R6

2 - C5

3 - R4

4 - C3

5 - C2

6 - R7

7 - R5

8 - R3

9 - R1

10 - C4

11 - C3

12 - R4

13 - C1

14 - R2

Where Cx is the column number and Rx is the row number. Note that R4 and C3 are duplicated.

I did find the datasheet for the display, which also covers similar displays, and it can be viewed at:

http://www.icpdf.com/icpdf_datasheet_1_datasheet/CSM57121D_pdf_1662669/

The datasheet does specify the forward current of 20mA and a forward voltage of about 2V.

In total there will be 2,520 LED's and you may wonder not only how we can handle the connections of such a large number of LED's but also how we can update them. To address the LED's we can use a technique that involves scanning the columns at high speed (turning them on and off in turn) and setting the required rows for each column, or we can scan the rows at high speed and set the required columns each time. If we do this fast enough it will look to the viewer as if all the LED's are lit at once even though they are actually being turned on and off very fast. The more LED's we have on at once the more current will be drawn so it is difficult to get a good balance between the number of rows and columns, and having all LED's evenly lit and bright while keeping power consumption low.

This project went through many changes and I looked into many different types of chips but in the end I settled on using an Arduino Mega to communicate with 12 MM5451 IC's which drive the columns, with each MM5451 providing constant current sink to 6 display each. The MM5451 not only helps to keep the LED's bright and without the need of limiting resistors it also cuts down on I/O as each MM5451 only needs to 2 inputs for communication: clk and data. By connecting all of the clk lines together we can update all of the MM5451 IC's at the same time, which helps to reduce flicker. Although a MM5451 has 35 outputs only 30 of each chip is used as we would have left over outputs even if we used all 35 and with only 30 used from each IC that simplifies the wiring as each MM5451 handles 6 displays which works out well as each of the circuit boards holds 6 displays across (see diagram below).

As well as talking to the MM5451 chips, the Arduino Mega also drives the display rows but it does so through a number of TD62783 transistor driver array, equivalent of UDN2981. The TD62783 acts as source drivers and gives a boost in current as the quicker we update the MM5451 chips the less current each LED will get and unfortunately the more LED's we have on at once the dimmer the LED's will be. We will use only 7 of the 8 drivers in each TD62783 just to keep the wiring the same for each circuit board but you may use individual transistors if you like, however, you would have to use a base resistor for each transistor whereas the TD62783 has built-in base resistors.

The arrangement of the displays is 18 x 4 which gives us a total of 72 displays (see image below), which should allow us to display a reasonable amount of information. As for generating the information to be displayed, an Asus Tinker Board (you could use a Raspberry Pi) pulls the information from online and breaks it down into rows for the Arduino Mega.