Brake light triggering device for vehicle

A circuit (111) has an adjustable capacitor and inductor (L) with armature (M) and is tuned to block the signals from an oscillator (110) when the pistons (3,6,62) of the braking master cylinder are in the rest position. When the brakes are applied and the pistons move the blocking circuit is de-tuned and the oscillator signals reach a processing unit (112) which activates the brake light control (113).