Abstract: An alarm unit for a humidity sensor activates an alarm circuit when the relative humidity rises above a certain value or under certain malfunction conditions such as a missing sensor or a cut cable, includes a first circuit for establishing a reference signal, and a second circuit adapted to be connected to a humidity sensor for responding thereto and for generating a humidity out-of-limits signal. A third circuit responds to the reference signal and to the humidity out-of-limits signal to generate an alarm signal. The third circuit includes a level detector, and the first circuit comprises an impedance device which cooperates with the second circuit which in turn connects the sensor to the input of the level detector, the sensor being a variable impedance device.