PWM 12V pump control


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?


what about ground ? both the microcontroller (arduino) and external power has to have common ground

your diagram shows that only external power is connected to the ground pin on the ULN... what about arduino ?

Currently, the source and the arduino do not share a common ground.

"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.

there is your problem


both arduino and power source ' - ' have to be connected to gnd pin on the ULN chip

Now it works just dandy. Thanks for the help