So i'm building a simple circuit using arduino's PWM and a potentiometer to control a 12V pump(powered by an external source). I tested the circuit with an LED and it worked just fine. I connected the circuit to a uln2803 darlington transistor pair and again tested the circuit with an LED powered by an external source through a darlington pair. The LED lit up but I could not control its brightness. The LED is connected to the output of the IC and the + of a 5V source. A simple diagram is shown below.
Any idea's as to why it's not working?