With the addition of a tiny circuit that consumes little more than the power of a human cell, the batteries inside sensors could last for years sniffing out chemicals, listening for voice control keywords, or feeling for earthquakes.

The circuit, recently unveiled by researchers at the University of Bristol in England, is a new spin on the voltage detector, a device that monitors the health of power supplies inside things like cars and factory equipment. But while these devices normally chew through battery power to work, the new circuit can remain asleep until woken up by an electrical pinch from sensors.

The invention of the circuit is one of the latest attempts to improve sensors so that they can run for years without needing to be frequently recalibrated or recharged. That has become a major priority as companies deploy wireless sensors with an eye toward making everything from factories to farms more efficient.

The UB20M, as the patent-pending circuit is called, only wakes up an electronic device when something like an accelerometer feels movement or a microphone hears a sound, generating a voltage. That allows a sensor to keep watching its surroundings, consuming so little standby power it might as well be asleep, the researchers said.

“This is because it is able to respond to minute quantities of power from unpowered sensors,” said Bernard Stark, one of the researchers at the Bristol Electrical Energy Management Research Group, in a statement. “No battery or other power is needed for the device to stay alive and listening.”

Although sensors have grown cheaper and smaller over the years, it has proven difficult to make them more independent. Many companies are trying to make them more power efficient by using simplified wireless networks, energy harvesting chips that refuel batteries using light and vibrations, and designs that make readings less prone to drift.

But the University of Bristol researchers are tackling the fact that the energy to keep sensors alive and listening can far outweigh the energy used to actually do sensing. For certain types of wireless sensors, like security cameras or volcanic activity monitors, the battery expends lots of power listening for movements or short tremors. 

The UB20M circuit only consumes a few trillionths of a watt and wakes up devices using a thousandth of the energy spent by existing detectors, providing what Stark described as “sensing that is continuous and free.” The result is battery life extended from months to years, the researchers said.

These benefits have not been lost on many other researchers and companies. A start-up called Vesper Technologies sells microphones that perform "quiescent sensing," in which the device turns itself on when it hears a sound. Matt Crowley, Vesper's chief executive, says that is indispensable for a new generation of devices, like the Amazon Echo and Google Home, which can be controlled with your voice.

There are also potential military applications. For the Defense Advanced Research Projects Agency, that ability for sensors to be “asleep-yet-aware” could help soldiers identify improvised explosives. In 2015, the agency launched a program to design wireless sensors that could be sprinkled around a battlefield to listen for the particular radio signals that activate the explosives.

“It is the waiting for a specific event or activity that constrains mission life and drains the battery energy of these essential electronics,” said Troy Olsson, the N-Zero’s project manager, in a statement. “Our goal is to use the right signal itself to wake up the sensor, which would improve sensors’ effectiveness and warfighters’ situational awareness.”

The agency is aiming to complete the Near Zero Power Radio Frequency and Sensor Operations, or N-Zero, project by 2018.

The University of Bristol researchers are looking for commercial partners to use the circuit in connected door locks, forest fire sensors, and other applications. Another natural fit would be in fitness bands, as the circuit was developed as part of Sphere, a research program aiming to develop sensors for home health monitoring.

Correction: February 15th, 2017. An image caption was added to an article about a new circuit whose inventors claim that it can vastly improve the battery life of wireless sensors. The article was also updated to include the part's patent-pending status.