How to power 2 DC motors, UNO, HC-SR04, and motor shield

I'm trying to get my first basic project off the ground, writing the code has been relatively easy but I've hit a blocker with how this thing should be powered.

It's a pretty simple project just to get me learning how this stuff works, a robot that runs around and detects walls - nothing original.

The parts are:

So I read that there's like a few ways to power this thing:

  • Single power source = Power to the Arduino DC jack, insert the power jumper on the motor shield
  • Multiple power sources = Power the Arduino DC jack, remove power jumper on motor shield, power the PWR_EXT block on the motor shield

I think #2 is recommended in order to reduce noise and problems through the system.

So, how do I calculate the power requirements of my project from here?

I'd like to use AA Batteries, but I don't know how many, or how to split them, if I'm going for option #2 then for example do I need something like:
2x AA batteries to the Arduino DC Jack, and then 4x AA batteries to the motor shield PWR_EXT block?

The specs of the motors are below, but I don't know enough about this stuff yet to make sense of it

For Motor: (Voltage, Current, No-load Speed, Load Speed, Torque, )
DC 3V, 160mA, 120 rpm/min, 100 rpm/min, 0.45;
DC 6V, 220mA, 200 rpm/min, 175 rpm/min, 1.0;
DC 7.2V, 250mA, 250 rpm/min, 210 rpm/min, 1.5
Reduction Ratio: 1:48
Noise: less than equal 65dB
Motor Size: 65mm x 18mm x 22mm(LWH)
For Smart Car:
Voltage: 3~9V
Drive Mode: 2WD
Current: 600mA

2x 1.5v into the jack will not work. It need 7.5v or more (12v top is recommended).

I would probably use 9v bank connected to each board. Make sure you use decoupling capacitors.


How do you know that the DC jack on the Arduino requires that?

I guess my real question here is - how do you calculate the power requirements of your project?

Is there a calculator or resource that would be beneficial to read in order to understand how to calculate this project and future project for myself?

The power jack feeds a 5v regulator that requires 7.5v or more to work, IE, give 5v out. If you plug in 3v, you will probably get 1v out. The regulator can supply up to 500mA but this is too high for continuous output.

The Arduino draws a current depending on what it is doing. It can be uA or 100mA.

Use Ohm's law, look up specifications or measure the current in your load.


Ok cool, thanks for that.

Do you know of any tools or resources I should use so that I can learn how to do the calculation myself?

I don't know how to measure current, I'm not sure I know the difference between current and voltage, or how to figure what the current and resistance of the project is in order to calculate the voltage

The usual analogy uses water. If you have a big tank of water the pressure is similar to voltage. If the tank is is many metres wide, you can take out a lot of water (current) at a time. If the tank is same height but only a few centimetres wide, you have the same pressure (voltage) but if you try to draw too much water at once, the pressure (voltage) will fall quickly.

This is why a car battery will last a long time where a 9v battery will drain very quickly if you try to run an Arduino.

There are many resources on the web for basic electronics. Others may be able to give specific links.