Why can't I use 3V output of arduino pro mini? I have an Arduino pro mini and I tested digital write of an pin and it is 3.2V and then I connected it to a 3V motor, but when I connected the motor to it, The voltage drops under 0.2 Volt, Can any one describe the science behind voltage drop ?

I know I've better use a transistor and separate the motor power but, what you should do when you have a single 3V power source? and I know there are some motor driver chips, but is it possible to keep it simple?
NEVER drive a motor directly from an Arduino IO pin.  You will kill the Arduino.

It's not a question of voltage, it's a question of current, and that is where the transistor comes in.  If you have a 3.3V Arduino (as it sounds like you have) then you may be able to drive the motor using the 3.3V output of the Arduino (VCC pin? Not sure with that model) and a transistor. That is the simplest arrangement.







simulate this circuit &ndash; Schematic created using CircuitLab