A simple window comparator - ontaining only one opamp!

7-60 - elektor july/august 1982 A window comparator, also called window discriminator, examines whether a voltage is situated in the range ("window") between two given reference points. In this way, a window comparator can be used for various kinds of control circuits. For example, it can be used to indicate the oil temperature of an engine; The window comparator can show whether the temperature is in the tolerated (green) range or not, (after the oil temperature has been converted into a DC voltage). Normally two comparators, an AND gate and at least two opamps are needed to construct a window discriminator. However, the circuit shown in figure 1 only requires one opamp! A reference voltage is set by means of the trimming potentiometer P1. D2 will conduct and D1 will be cutoff as long as the input voltage is below this reference voltage (set by P1). The voltage at the inverting input of the opamp is more positive than the non- inverting input, therefore the output of the compar...