I have recently been working on a small project involving RGB LED's and multiplexing.
It is based on two videos I found on youtube:
I have made some changes and additions to the design, but I'm not completly sure it is correct. I intend to drive it using SPI so I have used two 74HC595N shift registers to essentially control the multiplex. I have a few queries about the pull up/down resistors:
I want the system to be disabled till I'm ready so I have pulled up the OE pin to disable the register outputs. On the anode multiplex of 2n3904 transistors I have a 10K pulling the bases LOW,
when the register is active and outputing a high, will the connection go through the 100K transistor base or to the 10K pull down?
I'm pretty sure the pull down resistors need to be something like 200K or 1M, I just didn't notice this until I did up the schematic.
And the same question for the cathodes, just inverted.
My other concern involves the base resistors on the anode transistors: Will the current limiting resistors on the LED's further limit the current set by the base resistors causing a voltage drop too low?
Or do the base reisitors only set a maximum current flow, and if so, do I actually need the limiting resistors, or inversly keep the limiting resistors and remove the base resistors?
I used this page to work out the base resistors: http://www.rason.org/Projects/transwit/transwit.htm
Hopefully my calculations were correct, It worked out near 96k, so I used 100k resistors that I had handy.
I also replaced the videos NPN transistors on the cathodes with PNP using this page.
And to get 20mA at the LED's I calculated my current limiting resistors using 5v. Is this correct or do I need to use resistors for 3.3v supply?
My LED info:
Forward Voltage ( @ 20mA )
R = 2.1v
G = 3.0v
B = 3.0v
R = 145 Ohms ( using 150 )
G & B = 100 Ohms
Any advice would be greatly appreciated, the LED's were mildly expensive so I'm quite reluctant to turn it on without some input from a knowledgeable few.