SWITCH INPUT DETECTING CIRCUIT

PURPOSE:To obtain the switch input detecting circuit which does not decide the malfunction of a switch even if signal lines at both ends of the switch are short-circuited to the ground. CONSTITUTION:With the switch 10 closed, the voltage of a DC power source 13 which is supplied through a switch 10 is inverted by an inverter buffer 5 into an interruption signal (L level), which is applied to an interruption input port 18a. A computer 18 outputs a control signal (H level) for turning ON a transistor(TR) 6 from an output port 18b according to a program, and the voltage supplied from the DC power source 13 falls to a low potential and is outputted to an input port 18c. The computer 18 decides that the switch 10 is closed from the level values (L level) inputted to the interruption input port 18a and input port 18c.