I have a very basic LED circuit:
D# are 3Vx0.02A LED. In order for this circuit to work, we need a 25Ω resistor for each LED quartet:
Total LEDs current = 0.02A*4 = 0.08A Voltage drop = 5V - 3V = 2V Resistor value = 2V/0.08A = 25Ω
I didn't have a 25Ω resistor, so I put a 27Ω resistor instead.
Now, I want to add an Arduino in order be able to power on and off each LED quartet programmatically. I don't want to overload the Arduino, so I thought it would be ok to have an external power supply which will give the LEDs all the current they need to work, without worrying about burning the Arduino itself.
I thought I could either use transistors or MOSFETs as logical on/off switches, but I chose transistor because they are way less expensive than MOSFETs.
I thought about getting a
2N3904, because it supports up to 200mA and the circuit requires 0.08A per LED quartet.
So far I know the circuit should look something like this:
I'm definitely not an electronic expert, I assumed it should be like that because I looked a site which talked about Arduino controlled LEDs via transistor used as on/off switches. It didn't explain a thing, it just had the circuit so what I want to ask is: why the 10kΩ resistor? Is the value right? Do I need a 1/4W 10kΩ resistor or a 1/2W one? The 27Ω resistor should remain 27Ω or should be changed when a transistor is introduced?