PURPOSE: To surely transmit the upper and lower limit at automatic research, by obtaining an output only when an input voltage is respectively equal to each of two set-voltages.

CONSTITUTION: An input voltage starts dropping from a voltage between V1 and V2, where the V1 and V2 are voltage set with volume controls VR1 and VR2 respectively, inputs S and R for a flip-flop (FF) consisting of two NOR gates NOR1 and NOR2 are both H up to the immediately before the V1 and outputs Q and -Q are both at L. When the input voltage V is V1, the input S of the FF goes to L, but the R keeps H. In this case, the output Q keeps L, but -Q keeps H. A charging current flows to a C1 at this moment, a TR1 turns on and an output O1 keeping H just before goes to L. But an output O2 remains H. When the input V is smaller than the V1, -Q keeps H, but the TR1 is again turned off since the C1 is charged and the output O1 again goes to H. Similarly, when the input voltage V is V2, the output O2 goes to L.