I am currently working on a project which involves controlling a matrix of 64 LED's I am using the MAX7219 multiplexing chip. I have managed to program it correctly and have now started the task of wiring up a breadboard to plug in the 64 LED's that I want to use. I was not getting enough power from the chip itself so have decided to use an external power supply and some transistors to allow me to use some brighter LED's. I am using the 2N2222A transistors with these LED's:
Luminous Intensity: 25000mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical:3.2 V Max: 3.6V
DC Forward Current:20mA
when i wire up the first row of the matrix it all works fine, but as soon as i wire in the second row none of the led's will light up. I have tried so many variations but i just do not know enough about electronics at the moment. I am convinced it is something to do with the GND. I have the MX7219 GND and the + coming off the arduino and the LED's getting their + and GND off an external power supply.
the way i actually plugged in the transistors at the moment is contrary to the way I have seen it online but it works well. however it is this problem of plugging in more that one row of the matrix that i have come up against.
I have included a little schematic below which shows how i have wired it up currently (the arduino + and GND is the SEG and DIG coming out of the chip)
I have plugged in the GND from the external power supply into the base pin alongside the Arduino +. I know it must be wrong but weirdly it works in terms of lighting up the correct led.
does anyone have any experience wiring up the MX7219 chip with transistors to run 64 LED's - any help would be greatly appreciated as i am working against a very tight deadline.