I'm trying to construct a time fountain with an Arduino.
The programming is simple, and the pump can easily be not controlled by the Arduino (Though I will have the pump controlled by the Arduino later)
For now, i need to control 20 LEDs with the Arduino, and I've devised this schematic:
I have two questions: is how the external power arranged correct, and is R1 the proper value?
You have only 9mA through each diode, most LEDs can take 20mA. So you could afford to drop the LED resistor value to about 150R.
However, that would put the total load up to 400mA. The transistor will take this.
In fact it is not a simple transistor but a darlington pair. This means that the base will sit at two Vbes above ground therefore you could dispense with R1 if you want.
I would put a 100uF or so capacitor across the external supply close as you can to the diodes.