The 4-Pack from ELRO FS4610 consists of 4 extra-thin smoke detectors with a long-lasting battery. They feature a built-in lithium battery with a lifespan of 10 years. Thanks to their slim design, you can install the smoke detectors anywhere without them being too obtrusive. The LED light on the detectors even turns off in the dark, so you won't be disturbed by unnecessary light in your bedroom. When the optical sensor detects smoke or fire, the detectors emit a loud signal of 85 dB. With the battery indicator, you will know exactly when the battery is running low. Replace it in time and regularly test whether the smoke detector is still functioning properly.