Master/slave system • having a first movement-sensor unit having a master and a movement sensor, wherein the master is connected to the mains phase of an AC voltage mains system and has a load switching unit for switching and/or dimming an external electrical load configured as a lighting unit or a blinds motor, • having at least one second movement-sensor unit having a slave as an active subsidiary, wherein the slave is connected to the mains phase and to the neutral conductor of the AC voltage mains system, and is connected via a subsidiary line to the master, • wherein the master has a master transmitting stage, which is in the form of a voltage signal transmitter, for transmission of a signal which corresponds to the instantaneous switching state of the lighting device, which stage is configured as a low-impedance push-pull stage (Q10/Q12), • wherein the master has a master receiving stage, which is in the form of a current signal receiver, for reception of signals from the at least one subsidiary, which stage is configured as a shunt resistor connected to two transistors (Q16, Q19), • wherein the slave has a slave receiving stage (28, 30), which is in the form of a voltage signal receiver, for reception of a signal which corresponds to the instantaneous switching state of the lighting unit or of the blinds motor, • wherein the slave has a slave transmitting stage (27, 29), which is in the form of a current signal transmitter, for transmission of signals for operating the master, which stage operates as a "pull-down output stage" and has a transistor in conjunction with a total working resistance (R39, R36), and • wherein a respective PTC thermistor (R42, R43) acts in conjunction with a respective zener diode (D15, D16) as protection elements against incorrect connection and against transient, high-energy surges in the master and slave (8, 13). • having a first movement-sensor unit having a master and a movement sensor, wherein the master is connected to the mains phase of an AC voltage mains system and has a load switching unit for switching and/or dimming an external electrical load configured as a lighting unit or a blinds motor, • having at least one second movement-sensor unit having a slave as an active subsidiary, wherein the slave is connected to the mains phase and to the neutral conductor of the AC voltage mains system, and is connected via a subsidiary line to the master, • wherein the master has a master transmitting stage, which is in the form of a voltage signal transmitter, for transmission of a signal which corresponds to the instantaneous switching state of the lighting device, which stage is configured as a low-impedance push-pull stage (Q10/Q12), • wherein the master has a master receiving stage, which is in the form of a current signal receiver, for reception of signals from the at least one subsidiary, which stage is configured as a shunt resistor connected to two transistors (Q16, Q19), • wherein the slave has a slave receiving stage (28, 30), which is in the form of a voltage signal receiver, for reception of a signal which corresponds to the instantaneous switching state of the lighting unit or of the blinds motor, • wherein the slave has a slave transmitting stage (27, 29), which is in the form of a current signal transmitter, for transmission of signals for operating the master, which stage operates as a "pull-down output stage" and has a transistor in conjunction with a total working resistance (R39, R36), and • wherein a respective PTC thermistor (R42, R43) acts in conjunction with a respective zener diode (D15, D16) as protection elements against incorrect connection and against transient, high-energy surges in the master and slave (8, 13).