How do camera trap sensors work?

A closer look at what makes a trail camera trigger...

The use of camera traps in both research and as a fun hobby has increased immensely in the last few years.

In scientific studies, they are mostly used to monitor mammals and birds because reptiles and amphibians are harder to capture.

This is because of the type of sensor that the cameras use; called a Passive Infrared (PIR) Sensor. PIR sensors detect a difference between ambient background temperature and the rapid change in heat caused by an animal’s presence.

This means that PIR sensors detect heat from the surface of objects and air temperature itself should not affect it.

There are two main components in a PIR sensor; the Fresnel lens and the pyroelectric sensor. The Fresnel lens focuses incoming electromagnetic radiation onto the pyroelectric sensor which then triggers the camera to capture an image or to start recording. Camera traps are triggered when the electrical current in the pyroeletric sensor reaches a certain threshold.

As different manufacturers use different Fresnel lenses and pyroeletric sensors cameras have varying efficiency when it comes to capturing different species as the ambient surface temperature of species vary.

Camera traps also contain an infrared filter as part of the sensor which restricts the light that can reach the pyroelectric elements to wavelengths of approximately 6–14 μm. This limits false triggering of the cameras as these wavelengths are usually the range within which animal's surface temperatures occur.

A student positions her camera trap...

Moving objects that are warmer or cooler than background object, for example plants or even spiders' webs, may cause the camera to trigger if they are too close to the camera. The same is true for cameras facing directly into the sun.

It is therefore very important to mount your camera on sturdy poles or trees with a clear line of sight (and ideally sheltered from the sun) to minimise false triggers. This will save your batteries and space on your SD card for when the animals do arrive!

Patchy sunlight in forests can trigger the camera due to a rapid change in temperature, coupled with moving vegetation

Batteries & power

Battery choice can be extremely important to get the most from your camera trap.

A surge of energy is needed from the batteries to turn the camera on and take a picture or video. The IR LEDs particularly are the biggest power-draw on a camera trap, and poor batteries may mean your camera can only trigger during the day as it does not have the required power to trigger at night.

Battery choice is vital in any camera trap

A photo will also take less energy from the batteries than a video. For the best results you should use the maximum amount of batteries the camera allows and when possible use lithium rather than alkaline batteries.

We recommend you avoid Duracell batteries particularly because they have high internal resistance and some variations have a low energy capacity which makes them less effective in camera traps.

Lithium batteries can have a big difference - their capacity is approx 2900mAh (sometimes higher), whereas alkaline batteries can vary from 1700-2850mAh. These variations can also sometimes be unpredictable, and a camera trap generally needs 2500mAh to function well, particularly if it has a high amount of LEDs.

Knowing a little more about what makes a camera trap tick can hopefully help you can the most of out of yours!

Article by Tomos Williams