The Fire Sentry (Model i9040) smoke alarm is a battery operated smoke alarm that utilizes ionization sensing technology. The four inch (4″) unit is smaller in diameter, giving it discreet design with the same quality you would expect in a Kidde manufactured product. Fire Sentry (Model i9040) is also powered by a 9V battery, providing continuous protection, even during power outages. Furthermore, this easy to install unit includes a test button to verify the detection electronics are working as intended.