Kidde PI2010 Smoke Alarm Dual Sensor with Battery Backup for $21.97. Shipping is free on orders over $35. If needed, use this Filler Tool to pad the order for free shipping. Here is a screenshot for your reference. It features dual sensor with both a photoelectric and ionization sensor, 120V hardwire alarm with battery backup, and Red LED flashes every 30-40 seconds to indicate that the smoke alarm is operating properly.