Abstract: A switch is closed to generate a short duration pulse. The pulse activates a semiconductor switch which powers an indicator lamp and generates a D.C. control signal on a single conductor. A latching unit responds to the D.C. signal on the conductor to latch the switch in the activated state. A reset signal consisting of half wave pulses is transmitted over the conductor. These pulses charge a capacitor coupled to the switch input to substantially the peak voltage of the half wave pulses, which deactivates the switch. The capacitor holds a voltage of sufficient level to maintain the switch in the deactivated state until the latch unit is deactivated when transmission of the reset signal stops.