A system and method for activating vehicle sensors which are based on inductive loops. A transformer is used to couple the vehicle frame to activation circuitry. Within the activation circuitry, a voltage controlled oscillator which drives the transformer is scanned throughout the required frequency range while a balanced mixer, low-pass filter, and detector are arranged to show the presence of a signal from an inductive loop by the amplitude of the resultant signal at a difference frequency. The voltage controlled oscillator is then made to dwell at or near the frequency of the sensor loop by adjusting so as to keep the amplitude of that low-frequency difference as high as possible.