1364436 Theft alarms EURONICS Ltd 10 Nov 1971 [11 Aug 1970] 38621/70 Heading G4N [Also in Divisions H4 and H3] Unauthorized removal of articles through an exit is detected by a receiver at the exit which responds to the field produced by a batterypowered oscillator concealed in each article, and which operates an alarm. The oscillator shown produces bursts of r.f. at a pulse rate in the audio range, so that the receiver can operate an audio alarm at that frequency, and so that battery life is prolonged. The timed circuit LC1 determines the r.f. and C3R2 determines the duty ratio of the pulses of r.f. oscillation; or if R2 is shorted out and C3 omitted the feedback circuit C2R1 will control the duty ratio. The radio frequency is between 10KHz and 10MHz and is preferably 500KHz; the duty ratio is between 10 and 1000 to 1, and preferably the bursts last for 35 Ás and the gaps for 1À3 mS. By selecting different gaps, different audio tones are produced by the alarm for different articles. A portable receiver and alarm also may be provided. An alternative oscillator circuit replaces T by a dual gate FET, one gate being connected through a diode to the source, and the other gate to the upper end of LC1, the two gates being also connected across C2.