Noise immune power switch for automotive accessories

A noise immune electronic switch for powering an accessory device from the cigarette lighter socket of an automotive vehicle having a storage battery, a starter motor and an engine driven alternator that develops an AC ripple voltage having a frequency within a limited frequency range at said lighter socket comprising:

• an input connector for making electrical contact with said lighter socket;
• an output connector for electrically connecting said accessory device;
• filter means coupled to said input connector for passing said AC ripple voltage;
• amplifier means coupled to said filter means for amplifying and inverting said AC ripple voltage;
• normally non-conductive transistor switch means connected between said input connector and said output connector;
• integration means including an integrating capacitor for developing a voltage responsive to the negative half cycles of said AC ripple voltage from said amplifying means;
• means for developing a reference potential;
• comparator means in said integration means having one input supplied with said reference voltage and another input coupled to said integrating capacitor;
• means connecting the output of said comparator means to said transistor switch means for operating said transistor switch means;
• diode means for preventing charging of said integrating capacitor by said AC ripple voltage when said transistor switch means is non-conductive and an accessory device is connected to said output socket;
• a storage capacitor for storing energy;
• and
• a turn-on transistor coupled between said storage capacitor and said comparator means for rapidly charging said integrating capacitor and activating said comparator means to operate said transistor switch means in response to a rapid drop in voltage at said input connector upon energization of said starter motor.