Kidde 900-0146 Battery operated carbon monoxide (CO) alarm clock uses the patented Nighthawk electrochemical CO sensor to protect you and your family from the dangers of CO Easy to install, the digital display is 900 -0146 backlit displays CO levels in parts per million and updates every 15 seconds. When the unit is working properly and no dangerous levels of CO are detected, the green LED flashes every 30 seconds. In an emergency situation, the CO, the red LED pulses and a 85 dB horn sounds at specific intervals until the threat of CO is eliminated. Because it is powered by 3 AA batteries, the 900-0146 is able to provide continuous monitoring of CO levels, even during power outages. And unity is an alarm that indicates when the bacteria