Abstract: A dimming circuit (10) for use with an illuminated dual display (12) of an aircraft instrument is provided. A voltage divider (14) divides a variable supply voltage, V.sub.S, produces an input voltage, V.sub.IN, and applies V.sub.IN to one input of a switch (16). A threshold voltage circuit (15) receives the V.sub.S voltage and produces a reference voltage, V.sub.REF, and a lower magnitude supply voltage, V'.sub.S. A reference comparator (OA3) compares the V.sub.REF and V'.sub.S voltages and produces a threshold voltage, V.sub.T. The V.sub.T voltage is applied to a second input of the switch (16). A first comparator (OA1) in the switch (16) produces a low output voltage when the V.sub.T voltage exceeds the V.sub.IN voltage and causes a first lamp (L1) to brightly illuminate a first indicia (24). A second comparator (OA2) in the switch (16) produces a low output voltage when the V.sub.T voltage is less than the V.sub.IN voltage, which causes a second lamp (L2) to dimly illuminate a second indicia (26).

Abstract: A switch mechanism (11) comprising a red LED light source (13), a filter assembly (15) and a photodetector assembly (17) is disclosed. The photodetector assembly (17) includes two filtered photodetectors (33 and 35) that are aligned with the light source (13) either directly or via a light canal (19). The filtered photodetectors (33 and 35) are designed to detect light in different narrow wavelength bands lying within the band of energy produced by the light source (13). The filter assembly (15) includes a two-position movable element (23) located between the light source (13) and the photodetector assembly (17). The movable element (23) includes two sections (25 and 27) having different light-passing abilities. In one of the movable element positions one section (25) is located between the light source and both filtered photodetectors and in the other movable element position the other section (27) is located therebetween.