The hexfile for flashing the LoRIS-Base can be found at https://de.elv.com/elv-temp-hum1-applikationsmodul-temperatur-und-luftfeuchte-elv-am-th1-157134 under Downloads. To flash the code you need the flasher tool for the LoRIS-Base. The tool and necessary drivers can be found at https://de.elv.com/elv-lw-base-experimentierplattform-fuer-lorawan-elv-bm-trx1-156514 under Downloads.
Most of the parts come from the company ELV. The components can be easily linked together by stacking them on top of each other. At the top is the solar module. Below that is the harvester module and a temp/hum sensor. At the very bottom is a LoRIS base as a microcontroller.
The sensor is powered by a solar cell and supercaps. The solar power is fed into the harvester module and the module charges the supercap up to 4.5 V. The supercap is then used to power the sensor. To start the sensor, a voltage of 3.6 V is required at the supercap. At maximum the supercap can be discharged down to 2.8 V until the sensor is switched off. These threshold values can be set via the configuration pins of the harvester module. At the moment the values just mentioned were taken, because they fit best to supercaps. The sensor sends the temperature, humidity and voltage every 5 min.
Transmitting is done via LoraWAN, which is a Media Access Control (MAC), layer protocol. Designed for large public networks with a single operator, it is based on Semtech's LoRa modulation scheme. TTN (https://www.thethingsnetwork.org/) is used to receive and transmit messages.
TagoIO (https://tago.io/) was used to display the data. With this one can create diagrams to track the values. For example, on the picture you can see the voltage curve, how it falls and rises again.
The approximate lifetime of the sensor without sun can be calculated by the capacity of the supercaps. Using the formula
the stored energy in the supercap can be determined. The capacity of the supercap is 1.5 F. The maximum voltage is 4.5 V and the supercap can be discharged to 2.8 V. This results in
1 J corresponds to 1 Ws. Thus 9.3 J = 9.3 Ws. Dividing the result by 3600 and multiplying by 1000 gives approximately 2.589 mWh. Assuming an LDO of 3.3 V, we can assume a capacity of
The sensor consumes about 20 µA on average. On the pictures in the gallery you can see the current flow during measuring, sending and resting. The quiescent current is about 1.7 µA. During measuring and sending approx. 678 µA are consumed. Since the sending only takes place every 5 min, the average current with the LoRIS-Base and the Temp-/Hum-Module is 15 µA. The Supercap itself still has a leakage current of 5 µA. This results in a total current consumption of approx. 20 µA.
With this the runtime can be determined, which the sensor can endure without sun.
The lifetime can be increased by either increasing the transmission interval or by using more supercaps to increase the capacity.