GB2105952A - Antitheft label - Google Patents

Abstract

An antitheft label incorporates an insulating substrate 1, which can be flexible or rigid plastic, cloth or paper, carrying a tuned circuit conductor pattern 2, 2a with a loop 3. The loop has a gap 4 bridged by a diode chip 5. The loop conductor is formed with a short section 6 of reduced cross section, such that the label responds in the conventional manner in a weak r.f. field, e.g in a shop doorway, but when placed in a strong field, e.g. adjacent a cash register, the section 6 acts as a fusible link and is ruptured. <IMAGE>

Description

SPECIFICATION Antitheft label This invention relates to antitheft labels such as are attached to articles for sale in shops.

The purpose of such labels is that articles which are taken out of the shop without their having been paid for cause an alarm system to be activated. It is obvious that the labels must be capable of being easily rendered ineffective by authorised shop per sonnel when a sale is completed whilst at the same time the nature of the label and the method or means of rendering it effective should not be appar ent to would-be pilferers. It is especially desirable that the label should not require to be removed from the item or visibly altered in any way when it is rendered ineffective. Preferably the actions of the shop assistant should not be indicative to the cus tomer of the particular manner in which the label is rendered ineffective.Ideally the shop assistant should not be required even to handle or otherwise manipulate the label, even to the extent of man oeuvring the article into a particular orientation or exposing to view a previously hidden label.

Antitheft systems are known in which labels incorporating radio frequency responsive circuits are attached to, for example items of clothing. In one widely used such system the label needs to be phys ically removed by the shop assistant, a process which may require destruction of the label and/or cause damage to the garment.

According to the present invention there is provided an antitheft label including a radio frequency responder circuit which comprises a tuned circuit incorporating a circuit element the characteristics of which are such that when the circuit is placed in a radio frequency field exceeding a predetermined strength of the frequency to which the circuit is tuned the circuit element is destroyed and the responder is thereby rendered ineffective.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a label with a dipole pattern, Figs. 2a and 2b illustrate a label with a 2-plate capacitor pattern, and Fig. 3 illustrates a label with a quadruple 3-plate capacitor pattern.

The label illustrated in Fig. 1 is fabricated with a circuit which is printed in conductive ink, stamped out of foil, etched or otherwise deposited on a flexible or rigid plastic, cloth or paper substrate 1. The tuned circuit consists of one or more antenna patterns 2, 2a with a loop 3. The loop incorporates a gap 4which is bridged by a diode in an integrated circuit ship 5. The loop also incorporates a short region 6 where the width of the conductor is considerably reduced compared to the remainder of the circuit.

When the label is placed within a weak radio frequency field, such as is commonly established in a shop door way, then it functions in the conventional manner and radiates a response signal which can be detected, usually indicating that merchandise is being stolen. If however the item to which the label is attached is placed in a relatively strong radio frequency field adjacent, say, a cash register, then the current induced in the circuit is sufficient to cause the narrow region 6 to fuse, thereby breaking the loop circuit. As the label is of small physical dimension and the circuit can be completely enveloped within the label it is not apparent to the shopper in what way, if any, the label has been nullified. Furthermore, nullification of the label does not require that the label be presented in any special orientation or even in view on the exterior of the article.

As an alternative to making the narrow conductor region 6the diode 5 itself can be made with a small diameter junction region, e.g. less than 12.5 microns diameter. Provided the diode junction diameter is small enough the diode itself can be destroyed in a suitably strong radio frequency field.

The size and shape of the circuit depends, of course, on the radio frequency at which the label must operate.

The fusible link 6 can be formed either as an integrable part ofthe basic circuit pattern, e.g. by etching of the conductor pattern, or by bonding a link across a gap in the basic pattern. The link can be made of any suitable metal or alloy, e.g. aluminium or tin and should be located as close to the diode chip as possible, thus ensuring a minimum r.f. pickup after nullification. Alternatively the required fusible link pattern can be etched on the diode chip itself as part of the diode metallisation. This approach has the advantage of preheating# the chip, and therefore the fuse, due to reverse breakdown avalanching and forward conduction of the diode. Thus for a given melting temperature of the fuse less "destructive" r.f. power will be required.

In all cases the resulting additional inductance is tuned out by adjusting the d.c. return loop within the overall antenna pattern.

The label shown in Figs 2a and 2b comprises a tuned circuit consisting of a planar inductance pattern 11, the two ends of which are connected to the two plates 12 of a capacitor. The whole circuit, including a fusible link 13, is formed as a printed circuit on a thin flexible insulating substrate 14 which is subsequently folded in half along the dotted line 15 to bring the two plates 12 into correspondence to form a capacitor.

A suitable method of creating the strong radio frequency field is to arrange for the labelled articles to pass through a region in which radio frequency energy from three mutually orthogonal antennae is present. Alternatively at a very high operating frequency a 'resonant chamber' approach could be adapted for this purpose. Whatever approach is adopted the prime requirement is that a strong enough signal be created in the label circuit to cause fusing of the link. Thus the orientation of the label is immaterial.

Fig. 3 shows a similar approach for constructing a quadruple tuned circuit with four 3-plate capacitor configurations. Each of four planar inductance patterns 21 is formed on a central substrate portion 22 with each pattern enclosing an individual capacitor plate 23. The rectangular substrate has four leaves 24 extending one from each side of the rectangle.

Each leaf has two capacitor plates 25 formed thereon and connected to the inductance pattern by fusible links 26 such that when the leaves are folded over four 3-plate capacitors are formed, one for each inductance.

Other fabrication techniques are possible. For example, the inductance pattern and capacitor plates can be fabricated as a double sided printed circuit with a plated-through or edge-plated connection between the capacitor plates. Mention has already been made of the incorporation of discrete components in the description of Fig. 1.

Claims (16)

1. An antitheft label including a radio frequency responder circuit which comprises a tuned circuit incorporating a circuit element the characteristics of which are such that when the circuit is placed in a radio frequency field exceeding a predetermined strength of the frequency to which the circuit is tuned the circuit element is destroyed and the responder is thereby rendered ineffective.

2. A label according to claim 1 wherein the circuit element comprises a region of the tuned circuit having reduced cross section to form a fusible link.

3. A label according to claim 1 wherein the circuit element comprises a fusible link bridging a gap in the tuned circuit conductor.

4. A label according to claim 1 wherein the circuit element is a diode having a junction region diameter small enough to cause the diode to be destroyed when the label is placed within the radio frequency field.

5. A label according to claim 1 wherein the circuit element is a portion of the diode metallisation of reduced cross section on an integrated circuit diode chip incorporated in the tuned circuit.

6. A label according to claim 1, 2 or 3 wherein the tuned circuit includes a planar inductance winding the two ends of which are connected to separate plates respectively of a capacitance one plate of which is formed in the centre of the inductance winding.

7. A label according to any preceding claim wherein the tuned circuit is printed in a conductive ink upon an insulating substrate.

8. A label according to any one of claims 1 to 6 wherein the tuned circuit is stamped out of metal and bonded to an insulating substrate.

9. A label according to any one of claim 1 to 6 wherein the tuned circuit is formed by etching of a metal clad insulating substrate.

10. A label according to any one of claims 1 to 6 wherein the tuned circuit is formed by deposition of metal upon an insulating substrate.

11. A label according to any one of claims 7 to 10 wherein the substrate is of plastics material.

12. A label according to claim 11 wherein the substrate is flexible.

13. A label according to any one of claims 7 to 10 wherein the substrate is cloth.

14. A label according to any one of claims 7 to 10 wherein the substrate is paper.

15. A label according to any one of claims 7 to 14 wherein the tuned circuit is embedded within the substrate.

16. An antitheft label substantially as described with reference to Fig. 1 or Fig. 2 or Fig. 3 of the accompanying drawings.