Abstract: A noise immune electronic switch is connectible between a cigarette lighter socket of a vehicle and a plug-in accessory device. The electronic switch is turned on in response to the drop in B+ voltage at the lighter socket when the automobile starter is engaged for rapidly charging an integrating capacitor that is coupled to a comparator. The other comparator input is connected to a source of reference voltage. The output of the comparator, when switched, drives a transistor switch conductive to couple the B+ line between the cigarette lighter socket and an output connector. A filter passes alternator ripple voltage on the B+ line to an amplifier where the ripple is amplified and inverted. The negative half cycles of the ripple voltage charge the integrating capacitor to maintain the comparator output switched.