New Innovative Sensor Improves Smartwatch Battery Life Five Times

The heart-rate monitor in smartwatches consumes almost 80% of the battery power. But a new generation of sensors built by EPFL startup ActLight uses five times lesser energy. They have been tested and calibrated, and are currently ready to be mass-manufactured for use in new models.

The smartwatch market is flourishing. According to the International Data Corporation,* approximately 50 million smartwatches were sold globally in 2016, and this number is expected to increase to 150 million by 2021. The ever-increasing range of features – such as a step counter, GPS, and altimeter– requires sensors with varying energy requirements. The most power-hungry function – the heart-rate monitor – consumes nearly 80% of battery power.

The new generation of sensors by ActLight, which is based in EPFL Innovation Park, can measure the wearer's pulse with the same precision while using only one-fifth of the energy. These sensors have been analyzed by Maher Kayal’s laboratory as part of a Commission for Technology and Innovation (CTI) project and are presently ready to be employed in smartwatches. “The longer battery life certainly makes things easier for the user, but it also offers major savings in terms of electricity consumption,” said Kayal. Due to this innovation, ActLight was awarded the title of one of the best Swiss cleantech startups in 2015.

Time as a signal

Signal processing is the secret behind the energy-saving sensors. So as to understand how the company’s exceptional ‘dynamic photodiode’ sensor works, the workings of existing sensors in most smartwatches must be understood: Two diodes placed on the back of the device – cushioned against the wrist – release light that penetrates the upper layers of the skin, and blood flow establishes how much light is reflected back. A sensor positioned between the diodes detects these light waves and converts the information into electrical current, which is then translated into the pulse shown on the watch.

Rather than converting the light into a current and then measuring the current’s amplitude, ActLight’s dynamic photodiode sensors covert the current into time. The sensors use the pulse of light to detect the moment at which the current is activated. The result is a small reduction in energy consumption with every heartbeat, but repeated over 50,000 times per day it adds up to substantial energy savings.

Presently, ActLight is negotiating with leading semiconductor makers and is about to sell the rights to mass manufacture its innovative sensors. The new sensors are also appealing in terms of cost savings: “They are less expensive to produce because they do not require as much silicon for the same level of manufacturing,” said Sergei Okhonin, the company’s CEO. Many other applications are also being targeted, such as gesture control for video gamers.

* A US company that conducts market studies in information and communication technology and consumer electronics.

Credit: EPFL