A large-range input circuit for processing and forwarding digital signals which may be applied at input terminals with values both in the range of extra-low voltage signals of ≤ 120 V and in the range of low voltage signals of < 1500 V, said input circuit comprising a microprocessor controller for delivering control signals in dependence on whether input signals are to be expected in the range of extra-low voltage signals or in the range of low voltage signals, and a series circuit including the following elements: - an input-side rectifier or an input-side rectifier circuit for converting incoming digital alternating current signals into direct current signals; - a current regulator designed for managing both extra-low voltage signals of ≤ 120 V and low voltage signals of < 1500 V for adjusting the current flowing through the series circuit to a predetermined value; - a first series-connected reverse-biased Zener diode which is designed for a switching threshold of ≤ 120 V within the range of extra-low voltage signals; - a second series-connected reverse-biased Zener diode which is designed along with the first Zener diode for a switching threshold of < 1500 V within the range of low voltage signals; - a bypass circuit for bridging the second Zener diode, which includes an auxiliary optocoupler; and - a readout optocoupler for forwarding the processed digital signals to a user circuit via output connection lines (13, 14); - wherein the microprocessor controller controls the second Zener diode in terms of being bridged or not bridged for controlling the auxiliary optocoupler in dependence on whether input signals are applied in the range of extra-low voltage signals or in the range of low voltage signals; - wherein furthermore, if a digital extra-low voltage signal is applied, the second Zener diode is bridged and the first Zener diode determines, by a "conductive" or "blocking" state thereof, whether a digital signal "1" or "0" is output; and - if a digital low voltage signal is applied, at least the second Zener diode is not bridged, or the first and second Zener diodes (3, 4) remain connected in series and the non-bridged second Zener diode or the series-connected Zener diodes including the first and second Zener diodes (3, 4) determine, by a "conductive" or "blocking" state thereof, whether a digital signal "1" or "0" is output. - an input-side rectifier or an input-side rectifier circuit for converting incoming digital alternating current signals into direct current signals; - a current regulator designed for managing both extra-low voltage signals of ≤ 120 V and low voltage signals of < 1500 V for adjusting the current flowing through the series circuit to a predetermined value; - a first series-connected reverse-biased Zener diode which is designed for a switching threshold of ≤ 120 V within the range of extra-low voltage signals; - a second series-connected reverse-biased Zener diode which is designed along with the first Zener diode for a switching threshold of < 1500 V within the range of low voltage signals; - a bypass circuit for bridging the second Zener diode, which includes an auxiliary optocoupler; and - a readout optocoupler for forwarding the processed digital signals to a user circuit via output connection lines (13, 14); - wherein the microprocessor controller controls the second Zener diode in terms of being bridged or not bridged for controlling the auxiliary optocoupler in dependence on whether input signals are applied in the range of extra-low voltage signals or in the range of low voltage signals; - wherein furthermore, if a digital extra-low voltage signal is applied, the second Zener diode is bridged and the first Zener diode determines, by a "conductive" or "blocking" state thereof, whether a digital signal "1" or "0" is output; and - if a digital low voltage signal is applied, at least the second Zener diode is not bridged, or the first and second Zener diodes (3, 4) remain connected in series and the non-bridged second Zener diode or the series-connected Zener diodes including the first and second Zener diodes (3, 4) determine, by a "conductive" or "blocking" state thereof, whether a digital signal "1" or "0" is output.