Abstract: An attachment unit (28) to an existing central air conditioning thermostat controller (24) like the ones found in most homes and offices; which provides two independent modes of saving energy, first by turning on a light to indicate when to change the filter and secondly by extending the fan operation for a few minutes without disruption. Uniquely conceived, smaller than the typical thermostat controller, automatic, maintenance free, no batteries or auxiliary power of any kind. Has one button (32) to reset the light (30) after changing the filter and one switch (34) to select the optional extended fan operation.

Abstract: A low-pass filter for digital bearing data from an aircraft navigation receiver in which the filter time constant is set to a high value when the receiver bearing data exhibits characteristics of a noisy signal received from a distant station where the bearing from the aircraft to the station can change only at a slow rate and in which the filter time constant is set to lower values for less filter delay when the receiver bearing data exhibits characteristics of a less noisy signal received from a nearer station where the bearing from the aircraft to the station can change at a higher rate. Receiver bearing data for a current sampling interval is compared to filtered bearing data for a previous sampling interval to determine change in magnitude of the difference. Such difference for a current sampling interval is compared with a similar difference for a previous sampling interval for change in algebraic sign.