The Size of Smoke Detector Carbon Monoxide Detector Combo: Two Sensors: Carbon monoxide detector smoke alarm has a 2-in-1 dual sensors, photoelectrical and electrochemical sensors. They work separately, no matter which one reaches their dangerous concentraion, combination smoke carbon monoxide alarm detector will go off with red led flashing Readable Digital Display:The screen of smoke detector carbon monoxide detector combo will display the CO or smoke concentration and illuminate when it goes off. This smoke detector has a standby mode, its screen will not be on and no need to worry about being disturbed under a standby mode. Under a standby mode, green led blinks about every 60s Loud Beep: This combination smoke carbon monoxide alarm detector has a louder than 85 decibels beep with red LED flashing. It is loud enough to let you know the danger and take ation to avoid it quickly. It is recommended to install more than one in your house for a maximum protection Red or Green Led function: This smoke alarm and carbon monoxide detector has a low battery warning, it will go off and blink red led. When dangerous concentration happens,it also goes off and blinks red led. Green led with beep is for test button to detect this fire detector and carbon monoxide detector's different working status Green Led Too Bright? : If you feel that the green led is too bright, it is recommended to cover with a transparent pvc until you feel good. This smoke detector is easy to install. Both wall and ceiling are suitable for installation