PURPOSE: To display accurately the on/off state of a semiconductor switch without being affected by the presence or absence of a main circuit power supply and the quantity of a load current by connecting a special series connection circuit in parallel with the semiconductor switch.

CONSTITUTION: When an ON-signal is given externally from a signal input terminal 16, since a photo transistor (TR) in the inside of a photocoupler 17 is conductive, a forward bias current isolated from the on-signal flows in the path of positive terminal of the 2nd power supply 12 → photocoupler 17 →base resistor 18 → base of a TR 3 → emitter of a TR 3 → negative terminal of the 2nd DC power supply 12 to conduct the TR 3. When the TR 3 is conductive, the current also flows in the path of positive terminal of the 2nd DC power supply 12 → display device 13 as power application notice means → current limiting resistor 14 → blocking diode 15 as reverse flow blocking means →collector terminal of the TR 3 → emitter of the TR 3 → negative terminal of the 2nd DC power supply 12 to operate the display device 13.