This project adds a 2N2222A NPN transistor to the Arduino Blink project.   I set VCCA to 5V but you can use any voltage within the spec of the 2N2222A.   This is just a switch controlled by pin 6 powering the base to engage the transistor.   On the collector side I have 5V – LED – and 220Ω resistor.  The emitter side goes to ground.   When the base energizes the 2N2222A, the Collector load goes to the emitter ground completing the circuit.