Abstract: An air filter monitoring device that includes an attachment mechanism for attaching the monitoring device to the cover grate of an air return of an air conditioning system and a first and a second air flow actuated switch wired electrically in series with an alerting device, such as a light emitting diode and/or an audible alarm, each having an air resistance plate positioned within an air flow chamber. The air flow chamber is formed within a housing having an adjustable air intake inlet in connection with the air flow chamber. Air flowing through the air flow chamber within a range between a first and a second predetermined value causes the first and second air flow actuated switches to activate the alerting device.