Abstract: An on/off circuit is provided to automatically turn "on" or "off" a radar warning receiver upon lapse of a preset period of time after an automobile engine is turned "on" or "off". The receiver is powered by a D.C. input signal from the battery of the automobile via its cigarette lighter plug. The circuit includes an on/off switch connected to the cigarette lighter plug and the receiver and being switchable between "on" and "off" states to correspondingly transmit and not transmit the D.C. input signal to the receiver for turning it "on" and "off". A noise sensor is also connected to the cigarette lighter plug for sensing the presence or absence of a varying noise signal superimposed on the D.C. input signal which depends on whether or not the automobile engine is running.