Parallax has available a simple passive infra-red (PIR) motion sensor that is compatible with VEX.  This is perfect for sentinel ‘bots and Halloween effects that need to be triggered by an approaching person.

It has a 90° field-of-view (approx), and it will trigger when a warm object moves across the area it is watching.  It is very sensitive, and will trigger with just a hand movement. 

You connect it to a Digital I/O port or an Interrupt Port on the VEX Microcontroller using a standard servo extension cable.  The port should be programmed as a digital input, and it will read ‘0’ while nothing is moving.  The port will change to a ‘1’ when motion is detected, and will return to ‘0’ a few seconds after motion in the field-of-view stops.

One very import thing to consider is that the robot must be stationary while the PIR sensor is being used.  The programming must ignore any movement signal coming from the PIR sensor while the robot is in motion, since from the sensor’s perspective, the whole world is moving!  After the robot stops, you should wait a few seconds for everything to stabilize before you start reading values from the PIR sensor port.

The sensor has a pair of very small mounting holes, one on either side of the lens.  They won’t fit VEX fasteners, so you will need to come up with other arrangements to secure this to your robot.  The holes are approx 0.08” in diameter, which is suitable to fit a #1 machine screw.  Further, they are close to traces so you can’t enlarge them and you should use non-conductive (nylon) screws if possible.  The holes are on 1.1” centers.

The overall board size is about 1” x 1.25”, and it is

an inch tall (taller once you attach a cable).

The PIR sensor is available from Radio Shack

and Parallax for about ten (US) dollars.

See also: Related VexForum thread

                Parallax Spec Sheet (pdf)