Arduino and DC Motor control board I've this PWM Module - 6V-90V 15A Pulse Width PWM DC Motor Speed Controller Switch which I use to control a peristaltic pump (ordinary DC motor), now I want to replace the potentiometer with an Arduino in order to create a soft-start for the pump by pressing a button. 
The control is a 0-5V input from the potentiometer, and the description says "you can external drive" and "It could use of PLC controls speed of motor", unfortunately I cannot provide datasheets or schematics.  

Is it possible? Or I need a digital pot?    

How should I wire it? The 3 cables from the board to the pot are: ground, 0-5V and 5V
Or you may exchange the whole controller with a power mosfet module connected to your arduino. And use pwm directly from your arduino.
http://store.arduino.cc/product/T010020