Lets turn on and off low powered fan running at low (<20V) DC voltages. We shall use a MOSFET to achieve this. MOSFETs have three terminals Drain (D), Source (S) and Gate (G), where source is connected to ground and the +12V or +5V along with load (Fan) is connected to drain. The MOSFET is on when gate voltage is higher than 1.7V and turned off when gate voltage is 0V.
This signal can be given out from a micro-controller as 3.3V/5V or 0V signals.