Help with pump motor control circuit from ArduPilot Mega 2.5

[mysticboon]

Hi. I'm currently trying to build a circuit in order to control a small pump motor from an ardupilot mega 2.5 microcontroller. The pump is rated at 12 v and 3.6 w but I only need to give it about 9 v for operation. I'm using a basic supply of 9 v battery or 2 in series to give the power necessary. I've designed a basic bjt circuit that outputs the correct voltage I need in simulation and theory, but when I connect it to the microcontroller output pin, the ardu cannot sync enough voltage/current to give me the output I need to turn on the pump. I've tried buffering the input with an op amp and several different configurations on my bjt circuit. Can anyone please help me out with this?
This is a rough schematic of the circuit I'm currently using:

Thank you for any help.

 

[schip666!]

Does the pump actually run when you connect it directly to the battery? 12v 3.6w is .3amps which is maybe a bit much for a small battery. There will be a voltage drop across the transistor so you won't get the whole 9v at the pump. Also you may not have enough gain with a simple transistor to switch fully on, you might need a darlington like the TIP111.

I presume the green resistor squiggle on your drawing is your pump, if so your circuit looks ok although I would use a reverse-bias diode for protection across any inductive load (a diode on the two pins of the green squiggle, pointing to the right). Make sure you have the ground at your negative battery terminal connected to the power supply ground of the micro-controller.

I'm also not sure what the square wave source that seems to be labeled 40Hz is for? If you are pulsing the transistor like that you are basically doing PWM speed control, and the pump motor may not be switched on long enough on each cycle. Try manually switching the transistor by disconnecting the left side of the red resistor and touching it to the battery + terminal. That should make it turn on and run the pump.