I am tring to drive a 12V DC motor for 10 to 15 seconds. My thought was to charge and then discharge a capacitor. I dont know what size capacitor would be required for that time and voltage? Or should I just use a resistor of xx ohms to limit the amount discharged?

 

That depends quite a bit on the motor, but it is doubtful that you would be able to store enough energy in any reasonable capacitor to run a motor for that amount of time. How about using a circuit that when triggered will just supply power to the motor for 10-15 seconds? That is much easier to do.

 

How do you make a timer or allow the current to flow for xx seconds?

 

Before that question is answered, we need to know the stall current of the motor, i.e., if you lock the shaft and apply 12v, how much current flows?