Abstract: A master controller is connected via a pair of wires to a plurality of loads and is capable of generating pulses having either positive or negative polarity and a voltage level either above a first threshold or below a second threshold or above the second threshold. Some of the slave units connected via these two wires to the master controller are singly handicapped and have a single zener diode whose avalanche voltage corresponds to the first threshold so that the load is energized whenever this first threshold is exceeded. Other units have a second zener diode in series of the first diode and itself having an avalanche voltage equal to the second threshold so that only when this second threshold is exceeded is the load energized. In such a system it is possible to provide a memory circuit to short circuit this second zener diode if it is desired to remove the double handicap and partially emancipate the slave unit.