A window detector is a specialized comparator circuit designed to detect the presence of a voltage between two prescribed limits; that is, within a voltage window. This circuit is implemented by logically combining the outputs of two single-ended comparators by the IN914 diodes. When the input voltage is between the upper limit, VuL. and the lower limit, VLL, the output voltage is zero; otherwise it equals a logic high level. The output of this circuit can be used to drive a logic gate, LED driver, or relay driver circuit.
Window-comparator - schematic

The circuit shown in Fig. 16-1 shows a 2N2222 npn transistor being driven by the window comparator. When the input voltage to the window comparator is outside the range set by the VuL and I"LL inputs, the output changes to positive, which turns on the transistor and lights the LED indicator.

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