Sensor type

Pros, cons. considerations

Cat’s microchip

Cat’s microchips are passive RFID: These components can be read at no more than 10~15 centimetres of distance, not a reliable option for this application.

These sensors can be reliably used in small and obliged passages, like the cat flap doors.

We cannot modify the door, due to building’s policy, so these cat’s flap doors weren’t an option for us.

PIR

(Passive Infra-Red) sensors are largely influenced by surrounding temperature.

I haven’t tried, but I think it will be difficult to find a setting working both in winter and in summer; Furthermore, our door points to west, so often the sun hits it theoretically reducing the PIR sensors’ reliability.

Camera

I like computer vision, but I haven’t even consider it as a viable option.

This system must especially work in winter, where in The Netherlands there is little light; A camera based, will also require massive training to distinguish our cats from the many others in this large garden, under very different light conditions.

I feared this way would turn into an endless project.

BLE tag

(Bluetooth Low Energy) are small, battery-powered devices using Bluetooth to transmit data wirelessly. This is perfect to distinguish our cats from the others, but determining the distance from the sensor is not reliable.

24GHz FMCW radar

FMCW (Frequency Modulated Continuous Wave) can determine the distance of reflective objects (metal and water); It isn’t influenced much by light or temperature.

Another positive aspect is that it can be place inside, as the 24GHz can still pass through (non-metal) materials.

This technology has become very popular to sense human presence, leading to a good option of choices and rather low cost.

The radar...