The Sun as an Energy Source for IoT Sensors

Posted on: 14/04/2019 | By: DSS

What makes IoT sensors so special is that they can monitor conditions anytime and anywhere without an external power source. The IoT SoundSensor™ uses the sun as its power source, giving it an infinite service life. SensorTeam has designed the sensor case with this in mind and partners with a U.S.-based manufacturer of high-quality solar panels.

The solar panel fitted to the IoT SoundSensor™ delivers sufficient electricity to both power the PCB and charge the internal battery. The battery, a 6V Yuasa lead-acid battery, makes sure the sound sensor receives sufficient power at night or on days with little sunlight. In fact, the sensor can run on battery power alone and uninterrupted for as long as 12 days. And when the most energy-efficient mode is selected, it can run on battery power for nearly a month.

Battery voltage

Battery voltage provides an accurate indication of a battery’s charge level. By taking a voltage reading, we are able to check the battery’s charge level with great precision. The PCB controller monitors battery voltage continuously and with a high level of accuracy, calculating the average voltage. Along with the GPS location, the calculated voltage value is sent to the cloud by the sensor through a LoRa message, providing a real-time voltage reading on the dashboard. This produces a stable indication of the sound sensor’s service life, performance, and energy management.

Yuasa NP12-6 lead-acid battery

Based on this information, we check if the sound sensor still performs at the highest level and is positioned correctly. The solar panel, for example, needs to face south. And the information tells us whether the panel is dirty and may be delivering less power. Although the sensor case comes fitted with anti-bird spikes, there is still a risk of bird droppings ending up on the panel.

The high-capacity solar panel can have the battery recharged in no time. And we keep a fully charged battery at a charge voltage of 6.825V, which is called the ‘float charge current.’ And yet the battery voltage may drop to a value that is low enough for the sound sensor’s primary process – to send sound level data – to be jeopardized. When the voltage drops excessively, the sensor will go into energy-saving mode, switching off GPS communication, and sending data less frequently. Even on December 21st, midwinter, the day the sun is directly over the Tropic of Cancer and North West Europe gets the least sunlight, the sound sensor needs to remain fully operational.

Lead-acid battery charge table
The table shows at what percentage a battery is empty at a certain voltage. The values in the table are not absolute values and are intended solely to give an impression of how a battery (such as a semi-traction, AGM semi-traction, gel battery etc.)

Audio signal interference

SensorTeam has integrated a high-end charging chip into the PCB, which not only speeds up battery charging, but also prevents interference on the sound monitoring audio signal. Tests showed that certain types of charging chips cause interference (noise) from the moment the sun comes out.

We use this lead-acid battery because it is easy to procure anywhere in the world and because it is affordable. The battery will last 3 to 5 years and maintains excellent performance at sub-zero temperatures. Even though lead-acid batteries perform best at room temperature, they are also suited for use in winter. Changing the battery inside the case could not be any easier. After unscrewing the cover, it takes only a couple of maneuvers to swap batteries.

365 days a year

One important use of the sound sensor is for noise level monitoring in urban areas. Most of the world’s cities are in the Northern Hemisphere.

Using SensorTeam’s state-of-the-art technology, we ensure that the IoT SoundSensor™ will work autonomously 24 hours a day, 365 days a year, even in areas that get little sunlight.

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