These tiny battery-free sensor devices can float on the wind

The device is 30 times heavier than a one milligram dandelion seed but is still capable of travelling up to 100 metres in a moderate breeze. Due to the onboard electronics, it was challenging for the team to make it as light as an actual dandelion. The device can hold at least four sensors and uses solar panels to power the onboard electronics. Data can be shared up to 60 metres away.

“We show that you can use off-the-shelf components to create tiny things. Our prototype suggests that you could use a drone to release thousands of these devices in a single drop. They’ll all be carried by the wind a little differently, and basically you can create a 1,000-device network with this one drop,” said senior author Shyam Gollakota, a UW professor in the Paul G. Allen School of Computer Science & Engineering. “This is amazing and transformational for the field of deploying sensors, because right now it could take months to manually deploy this many sensors.”

75 different designs were tested by the researchers to find the best shape that would allow the devices to be thrown around in the breeze. Tests were conducted to find the smallest ‘terminal velocity’ or the maximum speed a device would have as it fell through the air.

“The way dandelion seed structures work is that they have a central point and these little bristles sticking out to slow down their fall. We took a 2D projection of that to create the base design for our structures,” said lead author Vikram Iyer, a UW assistant professor in the Allen School. “As we added weight, our bristles started to bend inwards. We added a ring structure to make it more stiff and take up more area to help slow it down.”

To help keep the design light, solar panels are used to power the electronics instead of batteries. However, without a battery the device is reliant on the sun as a power source so, after sunset, the device stops working and, in the morning, it requires a little bit of energy before getting started.

“The challenge is that most chips will draw slightly more power for a short time when you first turn them on,” Iyer said. “They’ll check to make sure everything is working properly before they start executing the code that you wrote. This happens when you turn on your phone or your laptop, too, but of course they have a battery.”

The team designed the electronics to include a capacitor, a device that can store some charge overnight.

“Then we’ve got this little circuit that will measure how much energy we’ve stored up and, once the sun is up and there is more energy coming in, it will trigger the rest of the system to turn on because it senses that it’s above some threshold,” Iyer said.

To transmit data wirelessly, the devices use backscatter – a method that involves sending information by reflecting transmitted signals. In tests, the device sent data until sunset, when it turned off.

Tests were also carried out to determine how far the device could travel in the wind by dropping it from different heights.

Hans Gaensbauer, who completed this research as a UW undergraduate majoring in electrical and computer engineering and is now an engineer at Gridware, is also a co-author.