The Sensing Trifecta

One of the primary goals in my research into occupancy is to meet The Trifecta; low cost, low power consumption, easy assembly. This is proving to be a goal not easily achieved.

A paper recommended to me shows prior work in a system called ThermoSense. ThermoSense uses a Phillips Grid-EYE, which provides an 8x8 grid of temperature measurements that when mounted on a ceiling allow the measurement of the temperature in different sections of a room, and the extrapolation of occupancy therein.


From the research I have done thusfar, the Grid-EYE would meet the cost requirements, with it being only around $30USD from DigiKey, however, it would require the manual development of a PCB to solder it on to, which would present significant difficulty given its size.

DigiKey offers an evaluation board, but this board is significantly more expensive.

An alternative that could potentially be breadboarded unlike the Grid-EYE is the MLX90620, but it is both more expensive (around $85USD) and in an inconvenient arrangement of 16x4 instead of the Grid-EYE’s 8x8.

Moving out of thermal completely, some interesting research has also been done into using ultrasonic ranging modules like the HC-SR04, which are much cheaper, but also would require a very different design.

Power consumption

Another concern that must be addressed is reducing the power consumption of the Raspberry Pi if it was to be used in this arrangement; a battery powered device of this nature should last at least a couple of weeks.

The RPi website suggests that the model B consumes between 700-1000mA @ 5V, which with a reasonably sized battery would lead to a very poor lifespan if left on continuously.

One possible solution to this was the Sleepy Pi, which uses a low powered Arduino board acting as a sort of watchdog to sleep and wake the RPi as necessary. With such an arrangement, the RPi could wake up at relatively long intervals (5-30 minutes), make measurements, and then sleep again. The Arduino could also be programmed to wake the RPi based information gleaned from a passive infrared sensor

However, using the Sleepy Pi goes against the concept of the occupancy determination being accessible in a RESTful way.

In a different vein, a system could be devised in which an low powered sensor on a battery (or energy harvesting device) reported back raw measurements on an energy efficient wireless link to a wall-socket-powered RPi, which could be responsible for the interpretation and RESTful serving of this data on a powered link.

Ideally, this could be the Mosquino, but the lack of manufacturing presents a difficulty in using that board specifically. This also brings back issues with integrating appropriate sensors (the Grid-EYE evaluation board requires USB support).

Finally, energy harvesting sensors like those provided by EnOcean could be used in conjunction with the EnOcean Pi module in a similar vein as the Arduino/RPi model, but with further power savings.

More research is needed.

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