Kidde KN-COPP-B Front Load Battery-Operated Carbon Monoxide Alarm for $25.97 shipped. Here is a screenshot for your reference. Its digital display shows the level of carbon monoxide the unit is sensing and the peak level memory displays the highest CO concentration measured since the last reset, and the battery compartment is accessible from the front so there is no need to remove it from the wall to replace the battery unit. Most places have this carbon monoxide alarm for over $50.