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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 – Schematic created using CircuitLab
You can drive the motor from the same 3v supply that powers your Arduino (assuming the supply can deliver enough current for both). What you can't do is power the motor through the Arduino, neither from a pin, nor from its on-board voltage-regulator. And especially don't try to use a 5v Arduino's 3v pin to drive it. Their 3v regulators provide only a limited amount of current.
If you try to drive a motor through the Arduino and it needs too much current, you risk frying the Arduino. If you supply the motor from the same power supply but not through the Arduino, and the motor takes too much current, the worst that will happen is the supply voltage will fall and Arduino will shut down.
