I'm planning to put together a simple shift-register driven array of LEDs as a first project, something akin to this: But using a 74HC595 shift register (two, eventually) and a transistor to drive each individual LED alone.
It's been twenty years since my (limited) formal electronics education, and I have a few questions:
- I believe understand why the resistors in line with the LEDs are necessary, because an LED essentially behaves like a short circuit, and will draw a lot of current from the register, right up until either it dies or the register dies. Is this correct?
- Why the "1K or 2.2K" resistor on the base of the transistor? Will I need one on each of the bases? Is this to limit the current between the base and emitter of the transistors? How do I know what value to use?
- Should I use 2N2222 transistors or should I use something like a 2N3904? What is the difference, other than the 2N2222 seems to have a much higher collector-emitter capacity? Does that affect the choice of resistor I need?