Circuit for detecting the state of a switch, in particular a car key contact in a voltage regulator of an alternator

In a circuit for regulating the voltage at which a battery is charged by an alternator, said circuit being capable of activation by a key switch wherein a first terminal of the key switch is connected to a DC source and a second terminal of the key switch is connected to said regulator circuit through a load such as a warning lamp, and a component having a voltage drop being provided for switching said load ON and OFF, a circuit for detecting the open or closed state of said key switch including:

• amplifier means receiving on a first terminal a fixed reference voltage and on a second terminal the voltage present on the terminal of the load opposite to its terminal connected to said key switch, said amplifier means being adapted for regulating said voltage drop to the value of the reference voltage as long as said key switch and said component having a voltage drop are conductive, and
• means for generating a first signal on a key switch detection output whenever said voltage present on the terminal of the load opposite to its terminal connected to said key switch is less than the reference voltage, and for generating a second signal on said detection output whenever said voltage present on the terminal of the load opposite to its terminal connected to said key switch is greater than the reference voltage, or whenever said amplifier means is in the course of regulating said voltage drop.