I'm building a circuit based on the 74HC595 shift register and I believe I've run into a problem with the current available from this device. The code appears to work as desired thus far, so I'm hesitant to change to another device such as a TPIC6595 and I'd like to make this one work. To that end, I believe that I can use a set of transistors on the output side of the shift register to drive the LEDs.
The LEDs are 20 ma (2v forward voltage) each segment and there are two sets of 7-segment displays, which indicates 280 ma total current draw. I can confirm that the rest of the circuit fails when the LED displays are connected and works when bypassed/disconnected. Voltage is 5v for the LED but can be 12v, as the 5v is coming from a voltage regulator drawing from 12v anyway. The LED modules are common cathode.
I've not picked a transistor because I'm lost when it comes to the calculations I've been presented. I believe that I should have an NPN type transistor, so I used that in the schematic above and I hope I've referenced the wiring connections properly. The resistor value for the LED is a little high but not by much I hope. Some of the research I've been doing suggest that I need a resistor between the base and the 74HC595 pin, some suggest that no resistor is needed.
I would appreciate a confirmation that the circuit is correct. If so, I would also appreciate a confirmation that a resistor is not needed between the shift register and transistor or if one is needed, what value to use. What transistor should I use in this circuit? I've found a 2N3904 listed as one option during my research, but it was not specified for a circuit of this type, more like a generic solution. I'm okay with generic.