Driving high current devices Using Arduino

The maximum current capacity of Arduino digital pin is 20 mA, what if we wanted to power the devices or control the devices using Arduino which consume the current more than 20 mA?
The Basic Alternative is using Transistor to provide a Power supply to the high current/voltage devices.
A transistor will allow you to switch the Motor On and Off safely, as well as to control speed using Pulse Width Modulation techniques
NPN Transistor as a voltage controlled Switch.


Current flows through collector and out of emitter.
By controlling the voltage at the base pin we can control the flow if current in the transistor
The circuit to control the motor using arduino is



When sufficient voltage is applied at the base pin i.e arduino pin Digital High the flow of current is allowed through transistor.
When no voltage is applied at the base the transistor is act like a open switch. There is no current flow between emitter and collector.
In the above circuit the transistor is essentially connecting and disconnecting one terminal of Motor from ground and determining when a complete circuit is made with battery.
We must connect both power supply grounds together to ensure common reference point between voltage levels in two parts of the circuit.