January 2006, Issue 186

Electronic Scarecrow

SENSING TROUBLE

When I started this project, my goal was to build a simple yet reliable system for detecting a large animal. The best way seemed to involve using numerous types of sensors, several of which must be activated within a short amount of time in order to indicate a valid trigger. The sensors on the Freescale Wireless Design Challenge breakaway board enabled me to achieve this goal.

Freescale’s MMA1260D and MMA6261Q accelerometers detect motion. When setting up your system, you can mount an accelerometer on a piece of plywood and post it on a path where animals travel. You can also attach an accelerometer to one of a series of flexible garden stakes connected by a wire. The wire will move when an animal climbs over it or slides under it.

Another idea is to connect a Freescale MC33794 e-field sensor to an insulated wire on the garden stakes. The MC33794 will pick up a slight capacitance change as an animal brushes up against the wire. You can connect the pressure sensor to a bicycle tire or thin-walled flexible tubing placed under a sheet of plywood to act as a weight-activated switch.

Lastly, you can connect the switch expander to several IR optical beam-breaking sensors. You can place the sensors between fence posts or any other fixed objects to establish a perimeter.