Next-Generation Sensor Increases Smartwatch Battery Life by Five Times
Nearly 50 million smartwatches were sold worldwide in 2016, and this number is expected to grow to 150 million by 2021. Features like a step counter, altimeter, and GPS require sensors with varying energy needs. Nearly 80% of the battery power in smartwatches is used up by the heart-rate monitor. Now, a new generation of 'dynamic photodiode' sensors developed by EPFL startup ActLight can measure the wearer's pulse with the same precision while consuming only one-fifth of the energy. They have been tested and calibrated, and are now ready to be mass-produced for use in new models. The secret behind the energy-saving sensors lies in signal processing.For most sensors in smartwatches, two diodes are located on the back of the device -nestled against the wrist - which emit light that penetrates the upper layers of the skin, and blood flow determines how much light is reflected back. A sensor placed between the diodes detects these light waves and transforms the information into electrical current, which is then translated into the pulse displayed on the watch. Instead of converting the light into a current and then measuring the current's amplitude, ActLight's dynamic photodiode sensors turn the current back into light. The sensors use the pulse of light to identify the moment at which the current is triggered.