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LED Displays configured as Common Anode be used with Micros using HC595 or HT16K33 LED Driver  


Don_Z
(@don_z)
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Hi, Everyone, I am working with some of my students on a project where they came across some LED displays they are trying to use in one of their projects. After they had done some basic research, they have concluded that most Display that are common cathode is the easiest and least components to build and configure. I was wondering if someone has a circuit they built that would use HC595 or better HT16K33 LED driver display that works with Common Anode displays. I am asking since one of my students parents electronic stores closed their shop they had a lot of these displays on hand and was wondering if we could use these devices to display info about these things they are doing in the First Lego actually it is the older kids that are building their own robots. I suggested,  that reversing the logic on the common anode display to that of a common cathode would get them in the right direction. I have looked to see if there are project boards or circuits that can do this but to no avail nothing that works. I am tapping into this forum hoping the experience and intellects here, might have such a thing or know where to look for such a circuit that exists and would be willing to share?


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magicChristian
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The HT16K33 is a versatile LED driver for Common Cathode Leds.
Pleas give more detail for the LED display´s which are to be used.
You can find circuit examples within the datasheet of Holtek HT16K33.
The chip does the multiplexing of the LED matrix with a period of about 11 ms and also reads some keys if necessary. The currents are quite limited. You need a µC to send commands over a I2C bus what is confortable and need only 2 signal wires.


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Don_Z
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Thank you magicChristian, the displays are Red and Green Dot Matrix with the Rows as Common Anode and Each of column is cathode for each of the leds.

 I have attached the LED Spec Sheet. I can make the HT16K33 work with just either red or green by switching the ROWs as Columns and Columns as Rows in respect to the HT16K33. But, I like to use the Bi color that would require common cathode for the ROWs. I have heard where if you can invert the Anode to act like the Cathode and Vice Versa with the Columns inverted to act like Anodes it would work. But the ULN2803 and UDN2981A are just buffers not inverters.


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magicChristian
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Hi Don_Z,    The  HT16K33  is not fitting your Display.  You would need 8 Outputs with high current sourcing capability  and 16 outputs with lower sinking capability;  the Holtek IC is vice versa and the currents needed are not possible.
For a good visibility you need a mean curren per LED of 10 mA. Because of multiplexing for 1/8th of time you need a peak current of 80 mA per Column for 16 outputs  and for the 8 row outputs 8 x 80 = 640 mA, calculating that no green and red LED dot is lit at the same time.
Using shift register HC595 has also not the current cabability we need.  You could use 3 of HC595 and add 8 driver transistors for Anodes and use some busdriver for the column with 80 mA sinking cabability.

But there are IC´s from MAXIM which are able to drive 8 x 8 Led Arrays :  MAX6963
I am not familar with those.  Look if it seems like a solution - we can discuss further details later.

MAX6963


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Don_Z
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@magicchristian{ Thanks for the tip on the chip. I was trying to use what we have on hand. I have like 20 of these ht16k33. After looking at the Specs and looking at how some have use the HC595 shift register. I see that what I had to do was put a 74ACT04 on all of the output pins. This would make the Anode out act like Cathode and the Cathode pins like Anodes, Then to drive the leds I needed to add 2 ULN2803 on the 16 pins that come from the A0 - A15 and UDN2981A on the Common Pins from the 74ACT04 fed from the HT16k33 pin. The display matrix works but, The sample program does not work as expected cause of the logic change from the output. So this was a good test and lesson learn for the kids that wanted to give it a shot at making it work.


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