Thank you for your help.
I really need your expertise. Any advice on my plan on this project?
I really don't know if it will work or not.
I'im planning to use an external power battery.
Thank you for displaying the image. I'm a newbie here 
The diagram doesn’t really say much about what you are doing - are those loads that will be on the board .
I would as a general good practice it is not a good idea to load the UNO board , your devices should be powered from a different source . It’s ok to drive the odd led , but that’s about it .
Yes they are the load with corresponds operating current. Maybe should I use 3 arduino nano instead of Arduino uno? or Arduino mega.
The amount of information you provided in this post is the real disaster here (;
Also note that A4/A5 are the I2C pins and directly connect to SCL/SDA.
Output voltage drops (high output) or rises (low output) as current increases on an output pin. So 30mA, 50mA, is not good planning. Current flow across the Rds of the output MOSFET creates voltage (V = I*R), so the output becomes 5V - IR, or 0V + IR.
The stepper motor pins will be buffered (via ULN2003 for example) to drive the stepper motor?
These notes under Table 30-2 explains more:
Notes:
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“Max” means the highest value where the pin is ensured to be read as low
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“Min.” means the lowest value where the pin is ensured to be read as high
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Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
1] The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150mA.
2] The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150mA.
If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not ensured to source current greater than the listed test condition.
(I out high, V out high: high current flow leads to voltage drop) -
Although each I/O port can sink more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
1] The sum of all IOL, for ports C0 - C5, ADC7, ADC6 should not exceed 100mA.
2] The sum of all IOL, for ports B0 - B5, D5 - D7, XTAL1, XTAL2 should not exceed 100mA.
3] The sum of all IOL, for ports D0 - D4, RESET should not exceed 100mA.
If IOL exceeds the test condition, VOL may exceed the related specification. Pins are not ensured to sink current greater than the listed test condition.
(I out low, V out low: high current flow leads to voltage rise)
"Maybe should I use 3 arduino nano instead of Arduino uno? or Arduino mega."
Maybe you should look at adding some transistors to source or sink the high current loads.
iamphysics:
I'im planning to use an external power battery.
This is also possible problem. What battery? How connected?
Reading some of the post in this forum makes me think that I'm such far behind with you guys. I'm not an EE graduate nor a student. But, I really want to learn this.
I'm planning to add 3 AA batteries as my power supply and connect to VIN and GND.
I'm going to separate some of the sensors to NANO. Maybe, 3 NANO will do.
iamphysics:
Reading some of the post in this forum makes me think that I'm such far behind with you guys. I'm not an EE graduate nor a student.
There's no shame in that and people here are more than willing to help you get going.
It would be helpful if you described the overall project, what sensors you're planning to use, and the objective of the system rather than some small part of the solution.
3AA bateries provide about 4.5 V - it may work but connect them directly to 5V. But I am not sure how that will work with the stepper motor.
I'm planning to add 3 AA batteries as my power supply and connect to VIN and GND.
That won't work. Vin has to be 7V minimum for the Uno.
You can connect 3xAA to the 5V input/output, but that will power only the Arduino and a couple of LEDs.
Always use a separate power supply for motors and servos, and be sure to connect the grounds.