These are two-piece systems where a thin infrared beam is passed between a transmitter and a receiver. Usually these two units are placed on either side of a path or known animal track. These active monitors are best if you are trying to determine movements of a particular type of animal. They can be set to a certain height to trigger an event only when taller animals pass through. For example, with the beam a 4ft high a badger would not trigger it but a deer probably would. They can also be adjusted to only record an event when the beam is broken for a certain length of time.

These are one-piece systems that emit a wedge of infrared that covers a much wider area than the active beam. If you just want to know the amount of activity in an area such as a woodland clearing, regardless of the type of animal, then a passive system is the answer. The monitor sends out pulses of infrared. Choosing a number of 'pulse windows' or zones to be interrupted within a certain time in order to trigger an event gives you some degree of selection of the types of animals recorded. Add a video to the TM700v and you can record the activity in the area onto film. You cannot directly use an active system to trigger a video camera.

Which model of passive or active monitor you require depends on the amount of information you wish to obtain. The basic models (TM300 - passive and TM1050 - active) just record the time and date of the event and cannot be used with any accessories. The other models allow various accessories to be used suchas cameras, to record the exact cause of an event, and dataloggers that allow easy retrieval of the event data recorded.