Patent ID: 8256133

Claim:
An alarm for detecting obstructions in a dryer exhaust duct and alerting a user to the obstruction, the alarm comprising: an enclosure having a first duct interface and a second duct interface, the first and second duct interfaces configured to securely joint to the exhaust duct so that the enclosure sits inline with airflow through the exhaust duct, an expected direction of airflow being from the first interface to the second interface; a freely rotating axle installed substantially near the top of the enclosure and oriented perpendicular to the expected direction of airflow; a single bell-shaped lever constructed of material having a thickness between one-half and three millimeters, and affixed to the axle so that motion of the lever is translated to the axle, the lever positioned to substantially intersect the expected director of airflow, the lever affixed to the axle so that the initial position of the lever in the absence of airflow is substantially vertical; and bell-shaped lever having a curvature between 120 degrees and 150 degrees, the curvature oriented toward the expected direction of airflow; an angular displacement sensor configured to detect angular displacement of the axle and to provide a displacement signal, the angular displacement sensor comprising a metallic bar projecting perpendicular from the axle, and a magnetic detector configured to detect proximity of the metallic bar; a dryer operation sensor configured to detect operation of the dryer, and provide a dryer operation signal when the dryer is in an operational state, the dryer operation sensor comprising a piezoelectric or capacitive microphone positioned substantially on the lower part of the enclosure; an audible alarm configured to operate upon receipt of an alarm signal; a logic device communicatively coupled to the angular displacement sensor, the dryer operation sensor, and the audible alarm, the logic device configured to: receive the dryer operation signal; upon receiving the dryer operation signal; receive the angular displacement signal; compare the angular displacement signal to a threshold, the threshold being 25 degrees past vertical; upon detecting that the angular displacement is below the threshold, provide an alarm signal to the audible alarm; whereby an obstruction in the exhaust duct causes an airflow back current, which current engages the curvature of the lever, driving the lever to an angular position below the threshold, and the logic device is enabled to detect the obstruction.