Greetings everyone,
First off, I am a complete amateur who is learning on his own. So if I'm making an obvious mistake, I'm glad to be told so and I apologize for any oversight. This may also affect my terminology usage.
I am working on a project to help me learn as I'm a hands-on learner. I am building a 24x8 multiplexed LED array which I intend to use for displaying simple text etc. I intend to drive the LEDs with shift registers which in turn are run from an Arduino Nano. I have 4 shift registers. The first one has its 8 outputs connected to NPN2222A transistors which act as switches for enabling 5V, 1000amp power into the individual rows of LEDs. The remaining three shift registers also have their outputs connected to NPN2222A transistors, but these ones act as switches to enable the ground connections for the columns. The four shift registers are linked together with common clocks and the data is passed on from shift register 1 to 2 to 3 to 4, etc.
I am actually running into an issue which I can "reproduce" without having the shift registers and arduino in play, so we can ignore that complexity. I'm providing it simply for clarity. Based on the behavior, I believe it has something to do with how I've setup my transistors, but I believe it looks correct, hence I'm confused and reaching out for help.
A few more clarifying details.
First off, I am a complete amateur who is learning on his own. So if I'm making an obvious mistake, I'm glad to be told so and I apologize for any oversight. This may also affect my terminology usage.
I am working on a project to help me learn as I'm a hands-on learner. I am building a 24x8 multiplexed LED array which I intend to use for displaying simple text etc. I intend to drive the LEDs with shift registers which in turn are run from an Arduino Nano. I have 4 shift registers. The first one has its 8 outputs connected to NPN2222A transistors which act as switches for enabling 5V, 1000amp power into the individual rows of LEDs. The remaining three shift registers also have their outputs connected to NPN2222A transistors, but these ones act as switches to enable the ground connections for the columns. The four shift registers are linked together with common clocks and the data is passed on from shift register 1 to 2 to 3 to 4, etc.
I am actually running into an issue which I can "reproduce" without having the shift registers and arduino in play, so we can ignore that complexity. I'm providing it simply for clarity. Based on the behavior, I believe it has something to do with how I've setup my transistors, but I believe it looks correct, hence I'm confused and reaching out for help.
A few more clarifying details.
- The first 8 transistors have their collectors connected to the same 5V, 1000amp current that is feeding all other components. Their base pins are connected to the shift register outputs, with a 430ohm resistor between the shift register output pins and the base pin. Their emitters are connected to one of 8 rows of 24 LEDs (LED anodes).
- The remaining 24 transistors have their collectors connected to the LED columns (LED cathodes). Their base pins are connected to the shift register outputs with a 430ohm resistor between the shift register and the base pin. Their emitters are connected to the same ground line that all other components are sharing.
- A PDF of my schematic is attached. I apologize for any lack of skill that is apparent in how I've drawn it.
- Basically in my testing, I've gotten very irregular behavior from the grid that seems relatively unpredictable. This has led me to testing without the shift registers, etc to find where the problem lies. I know that the multiplexed grid itself works as expected. If I apply 5V to a given row and then ground a given column (no transistors, just direct to the wires), the expected LED lights up. I only start having issues at the point where I've introduced the transistors and their base pin resistors.
- If I manually apply 5V to a transistor that is controlling a row (which in turn switches on 5V to that row), and then apply 5V to a transistor that is controlling a column (which in turn switches on the ground for that column), I get no LEDs lit up.
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