Controlling SMD-LEDs using a 74HC595 & NPN-transistor as switch

Question

I want to control several SMD-LEDs (0603) 3.5V - 25mA, using a voltage source of 4V. Each LED needs a resistor of ~20 Ω to work fine, like shown in the circuit below:

^{simulate this circuit – Schematic created using CircuitLab}

But I would like to switch and control the LEDs pairwise. So in this example LED1 & LED2 should light together and in a next step LED1, LED2, LED3 & LED4 should all light together. In my real project I have 16 LEDs in total. I got to know that the 74HC595 must not exceed 70mA, but each LED needs 25, so I need to have a separate source for all LEDs and an NPN-transistor for each state to be able to switch my LEDs using the 74HC595.

1. Is that correct?
2. Will the following circuit work? (Note: I simplified the 74HC595 in this circuit)

Please be fair, I'm a beginner in electrical engineering.

Answer 1

If you use NPN or n-channel MOSFETs your circuit will get much simpler. By raising the LED voltage supply you can put them in series reducing the number of resistors and the heat dissipated. Since the 74HC595 sinks and sources you can connect the gate of the mosfet directly to the 74HC595 output, the source to ground and the drain to the series string of LEDs. If you use a transistor yo will need to add a resistor to each base. Size the MOSFET or transistor to be able to supply all the current the LED string requires. More current capacity with the MOSFET will allow it to run cooler as the voltage drop across it will be less.