Analogue Electronic Key

This circuit uses two comparator that are combined in what is called a window comparator, i.e. resistors R2, R5, and R10 determine a voltage window within which the voltage applied to the junction of D2 and D6 must lie in order for the outputs of IC2.A and IC2.B to both be high at the same time. Given the value used for these resistors, this window is from 10/21 to 11/21 of the comparator supply rail (5 V).