The turret is built completely out of lego pieces. It uses a structural design to support the airsoft gun and keep it stable when moving. The sideways motion is controlled by a motor connected to a turn-table through a complex gearing scheme.

A separate module on top the turn-table supports the gun and allows it to move up and down. The up and down motion is controlled using four pneumatic cylinders. A circuit of pneumatic tubes connects the four cylinders to the pneumatic control module. The pneumatic control module is the box connected to the NXT seen in the image. Its purpose is to use a single motor to control the airflow direction to and from the cylinder as well as pump the air to drive the cylinders. I came up with a special design to achieve this which can be seen in pneumatic control module post. A motor connected to the trigger allows it to be precisely pulled and released using a quadrature optical encoder in the motor. The trigger motor, pneumatic motor, and rotational motor are all connected to the NXT which serves as the brain. Also connected to the NXT is a ultrasonic sensor for detecting the distance to objects in front of it. The NXT’s firmware was replaced with leJOS, a JAVA virtual machine that runs on the NXT. The NXT has built in blue-tooth that allows it to connect to a blue-tooth dongle plugged into my computer. I downloaded a package for JAVA called iCommand, which allows me to control the NXT via bluetooth. All of the JAVA code runs on my computer which sends commands to the NXT when a key press event is triggered. This allows me to assign certain keys to specific functions. For example, I assigned the “F” key to the trigger pull like in most FPS.

Click here to download the java source that runs on the NXT. (This is not the version that runs on the computer and communicates with the NXT via bluetooth)

Above is a video of the airsoft sentry in use. The video was taken of an early version. I did not develop the iCommand code at this point and had a java program running directly on the NXT (why I used the NXT as the controller). This video also shows the first version of the pneumatic control module. The module has been updated to pump air faster so the gun currently moves up and down faster than seen in the video. The airsoft gun seen in this video broke shortly after filming as it is a cheap gun I bought on ebay. It has been replaced with a full metal MP5 as seen in the images above.

I still have much work to do on the airsoft sentry. I plan on revisiting this project and posting on it soon. Stay tuned!