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