PLD (Part 3)

Part 123456

The name PLD stands for “Place and Leave Device”. The original design is big enough to encapsulate its power reserve, the radio module, an arduino ™ microcontroller which will run your own code, and connectors for programming, debugging or plugging sensors. The radio module features the HL2 stack which handles data in terms of optimization of the payload and protection of its content.

Here is an example of use: the PLD drives a DS28b20 temperature sensor that I put in my refrigerator.

Temperature mesurement

From my own experience, leaving the sensor on its own exposed to the “ambiant” air inside the fridge drives to unstable readings. A better practice is to immerse the sensor in a half-liter bottle filled with water. Water will damp the fast temperature changes which translates in smoother temperature profiles. This method is also more realistic as we are interested in monitoring the temperature of the food and goods inside the refrigerator.

Statistics

The principle of IoT prevents you from uploading a googol of data bytes over the LPWAN networks. On the other hand, you do not want to miss critical events. The in between consists in using a relatively high sapling rate (e.g. one sample per minute) and a relatively slower upload rate (e.g. one upload per hour). This means that the device must be able to compute some statistics which will be chosen for their ability to describe to the user what happened between two uploads. After few experiments, we made our minds and decided to adopt the following statistics: median, quartiles and inter-quartiles. These statistics are much easier to understand than standard deviation and sigmas. The HL2 stack allows the upload of these 5 statistics and the battery level in only one frame thanks to the segmentation function !

Using the H2 PLD, it takes less than hour to configure, program and install a temperature monitoring device where ever you like and get the data from HL2 Panorama from any Internet navigator.

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