PIR petsafe devices retail for around R500. There are multiple patents driving up their cost which we must infringe to mass produce a PIR for R50 per unit. Attach hundreds of PIR devices to trees inside a bush around a farm. A SerialCameras sends back a picture on a suitable RadioModems device. Serial cameras are digital and probably to expensive for mass deployment. The technology used for monitoring the environment are called "Motes". See SmartDust. The PIR triggers the camera to send back a snapshot of what triggered the PIR. All farmworkers must be tracked in real-time with GpsAndGprs.
In most cases it wont be necessary to send back a real-time picture since the outdoor-PIR are petsafe triggering only above 40kg weight. On any passive triggering dispatch a UAV MannedSystems to the intrusion spot.
The analogue option is cheaper but uses lots of bandwidth. Since not all 100's of PIR devices will trigger at the same time, a brief high RF pulse to send back an analogue snapshot to the base on 433Mhz or 500Mhz will work.
PacketRadio can be used to send a digital message on the 200Mhz band, which means extended kilometers of bush can be monitored.
Patents leads directly to farmers being killed, were it not for patents PIR would be far less cheaper than R500/unit. Patents = death.
Build own PIR devices
Buy the PIR focusing lenses from OpticalManufacturers and hack the design off a PIR circuit reverse engineered, infringing on the patents using a FrontingCompany. PIR lenses are cheap when purchased in bulk and we can't manufacture it ourselves , it needs specialized equipment.