Abstract: A method of and circuit for monitoring the amplitude of a sine wave-shaped electrical signal in a defined range of a signal period, wherein the signal is derived from scanning an object which is to be measured or controlled. A comparison circuit is provided which compares the signal being monitored to a reference signal and generates a first impulse signal which is applied to a logic gate. A second impulse signal generating network generates a second impulse signal determining the range of the signal period by adding at least one signal derived from additional signals contained in the signal spectrum to the signal being monitored. This impulse is also applied to the logic circuit. The logic circuit produces a switching signal for operating an indicator or control device.