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.
"the source and the arduino do not share a common ground"
That would be a problem. As far as the ULN2803 is concerned the base voltage is just floating.
Need the grounds connected so all signal levels are properly referenced to something.