Frequency monitoring circuit for monitoring an arbitrary AC-input signal, the circuit comprising: • an AC signal input for providing the input signal to be monitored; • a resettable circuit (2, 2') comprising: ∘ a capacitor connected to ground, ∘ a reference power supply (4, 4') for charging the capacitor, ∘ a discharge switch connected in parallel with the capacitor to control charging and discharging of the capacitor, • a switch control for controlling the discharge switch, the switch control being electrically connected between the AC signal input and the discharge switch, wherein the switch control is adapted to detect a predetermined edge of the AC-input signal and to control opening and closing of the discharge switch in dependence of the detected predetermined edge; • a comparator with at least one comparator output (14a, 14b), the comparator being electrically connected after the resettable circuit (2, 2'), wherein the comparator is adapted for comparing the capacitor voltage with at least one threshold limiting a predetermined allowed frequency range, and for providing a comparator output signal at its comparator output (14a, 14b) indicating whether the capacitor voltage complies with the threshold; and • a memory element comprising: ∘ at least one signal input (16, 17) connected to the comparator output for receiving the comparator output signal (14a, 14b), ∘ a signal output for outputting a constant frequency-ok-output signal as long as the input signals stays within the allowed frequency range, wherein the output signal is updated by the comparator output signal, in particular at predefined points of time, and ∘ a trigger input connected for triggering the memory element, wherein the trigger input is connected to the AC-input. • an AC signal input for providing the input signal to be monitored; • a resettable circuit (2, 2') comprising: ∘ a capacitor connected to ground, ∘ a reference power supply (4, 4') for charging the capacitor, ∘ a discharge switch connected in parallel with the capacitor to control charging and discharging of the capacitor, ∘ a capacitor connected to ground, ∘ a reference power supply (4, 4') for charging the capacitor, ∘ a discharge switch connected in parallel with the capacitor to control charging and discharging of the capacitor, • a switch control for controlling the discharge switch, the switch control being electrically connected between the AC signal input and the discharge switch, wherein the switch control is adapted to detect a predetermined edge of the AC-input signal and to control opening and closing of the discharge switch in dependence of the detected predetermined edge; • a comparator with at least one comparator output (14a, 14b), the comparator being electrically connected after the resettable circuit (2, 2'), wherein the comparator is adapted for comparing the capacitor voltage with at least one threshold limiting a predetermined allowed frequency range, and for providing a comparator output signal at its comparator output (14a, 14b) indicating whether the capacitor voltage complies with the threshold; and • a memory element comprising: ∘ at least one signal input (16, 17) connected to the comparator output for receiving the comparator output signal (14a, 14b), ∘ a signal output for outputting a constant frequency-ok-output signal as long as the input signals stays within the allowed frequency range, wherein the output signal is updated by the comparator output signal, in particular at predefined points of time, and ∘ a trigger input connected for triggering the memory element, wherein the trigger input is connected to the AC-input. ∘ at least one signal input (16, 17) connected to the comparator output for receiving the comparator output signal (14a, 14b), ∘ a signal output for outputting a constant frequency-ok-output signal as long as the input signals stays within the allowed frequency range, wherein the output signal is updated by the comparator output signal, in particular at predefined points of time, and ∘ a trigger input connected for triggering the memory element, wherein the trigger input is connected to the AC-input.