This circuit de-energizes a normally energized relay if the input voltage goes above or below two individually set voltages. The transistor driving the relay is normally turned on by R4, so the relay is normally energized. Ifthe cathode of Dl or D2 is taken negative, Ql will turn off and the relay will de-energize. The IC is a 72747 dual op amp used without feedback, so the full gain of about lOOdB is available. The amplifier output will thus swing from full positive to full· negative for a few m V change at the input.
Window-detector - schematic

The relay is therefore only energized if llN is between VUL and VLL. The two limits can be set anywhere between the supply rails, but obviously VuL must be more positive than "LL. If Vrn can go outside the supply rails, DS, D6, and RS should be added to prevent damage to IC1. If VuL and "LL are required to be outside the supply rails, llN can be reduced by RV3. The supplies can be any value, providing that the voltage across them is not more than 30 V.

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