Amazon.com has the Kidde Smoke Detector Alarm | Battery Operated | Model # i9050 for $6.43.
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The Kidde P9040 smoke alarm is a battery operated smoke alarm that utilizes photoelectric sensing technology. The four (4) inch unit is smaller in diameter giving discreet design with the same quality you would expect in a Kidde manufactured product. The P9040 is powered by a 9V 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. The alarm also includes the Hush feature, which temporarily silences nuisance alarms. This alarm uses photoelectric sensing technology. Leading authorities recommend that both ionization and photoelectric smoke alarms be installed to help ensure maximum detection of the various types of fires that can occur within the home. Ionization sensing alarms may detect invisible fire particles (associated with fast flaming fires) sooner than photoelectric alarms. Photoelectric sensing alarms may detect visible fire particles (associated with slow smoldering fires) sooner than ionization alarms.