JPH08161655A - Device for preventing theft - Google Patents

Abstract

PURPOSE: To enhance a function of a self-ringing tag by 100% by providing a changeover means of a capacitance to capacitors in a resonance circuit so as to correct a characteristic of an antenna thereby reducing the reduction in an output level. CONSTITUTION: A capacitor addition switch 15 is provided to a capacitor 8 of a resonance circuit so as to connect either of capacitors 8a, 8b being correction purpose changeover means whose capacitance differs from each other in parallel with the capacitor 8. In the antenna characteristic at factory shipping, a frequency at a resonance point when a maximum sensitivity is obtained is set to an output frequency of a DDS 2. When a transmitter provided with the antenna is placed at an entrance/exit of a shop, the inductance of a coil is reduced by the effect of surrounding metal and the frequency characteristic is shifted to high frequencies and the reduction in the sensitivity is caused. Then the capacitor addition switch 15 is used to adjust the combined capacitance of the capacitors to shift the usual antenna characteristic toward lower frequencies, and the frequency at the resonance point is made closer to an output frequency of the DDS 2 while confirming a maximum output by a LED lamp 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention radiates radio waves from a transmitter located at the entrance of a store to a passage, which is a monitoring area, and takes out a product with a self-tapping tag attached,
The present invention relates to an antitheft device that activates alarm means of a self-tapping tag when passing through a surveillance area.

[0002]

2. Description of the Related Art An anti-theft device using a self-tapping tag adopts a resonance circuit using a coil and a capacitor,
For example, a transmitter is installed near a wall or close to a door so that an antenna that emits a radio wave of 22 kHz does not obstruct the passage of a store entrance. Therefore, the inductance of the coil decreases due to the influence of the reinforcing bars in the wall and the iron material of the door, and the antenna characteristics shift to the high frequency side, so the output level of the radio waves radiated from the antenna decreases and the total monitoring area is reduced. Cannot be covered and the original performance cannot be exhibited. Therefore, conventionally, the level of the radiated radio wave is compensated by increasing the output level of the transmitter or increasing the reception sensitivity of the self-tapping tag according to the situation of the installation location.

[0003]

In order to increase the output level of radio waves and the receiving sensitivity of the self-sounding tag, it is necessary to use high-performance parts, and the power consumption also increases. There is a strong tendency for itself and running costs to increase. However, rather than that, the problem itself is to continue to use the antenna that does not resonate at the output frequency.

[0004]

SUMMARY OF THE INVENTION The present invention is a theft prevention device for improving the ability to detect a self-sounding tag or the detection distance, and the first means has the structure of a radio wave having a resonance frequency in a monitoring area. And a self-sounding tag that emits an alarm by receiving the resonance frequency. A capacitor in the resonance circuit of the transmitter is provided with capacitance switching means to correct the antenna characteristics. It is a possible anti-theft device. As a configuration of the second means, a transmitter that radiates a radio wave of a resonance frequency to a monitoring area and a self-tapping tag that receives the resonance frequency and issues an alarm are provided in an antenna coil in a resonance circuit of the transmitter. This is an antitheft device capable of correcting the characteristics of the antenna by providing a tap and switching the coil length.

[0005]

In the structure of the first means, the antenna characteristic is shifted to the low frequency side by increasing the capacity of the capacitor so that the antenna can resonate at a predetermined frequency. Also, in the structure of the second means, the coil length is changed. As described above, the antenna characteristics can be shifted to the low frequency side by increasing the inductance to resonate at a predetermined frequency.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An anti-theft device according to the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an anti-theft device including a transmitter and a self-tapping tag. Reference numeral 1 denotes a CPU which is a central processing unit, and resonance frequency information for operating a tag is input to a storage device of the CPU 1, and the resonance frequency information is sent from a DDS (digital direct synthesizer) 2 according to a command from the CPU 1. A 22 kHz sine wave is output based on the information, and the sine wave is converted into a square wave by the pulse shaping circuit 3 and supplied to the gate voltage control unit 4. The gate voltage controller 4 receives the control signal from the CPU 1 and outputs a square wave pulse signal to the switching circuits 6 and 6 via the drivers 5 and 5. An antenna 7 is connected to the switching circuits 6 and 6, and a resonant circuit including a capacitor 8 and a transmission coil 9 is adopted for the antenna 7, and each of the circuits 6 and 6 includes a power source 10 The required power is being supplied from. As a result, a radio wave having a specific frequency of, for example, 22 kHz is radiated from the antenna 7 toward the monitoring area. If the self-tapping tag 11 enters the monitoring area, the receiving coil 12 receives the electric wave and the receiving circuit 13 gives an alarm. Means 1
4 is activated.

The capacitor 8 of the resonance circuit has a capacitor 8a, which serves as a switching means for correction having different capacities.
A capacitor addition switch 15 capable of connecting any of 8b in parallel is provided, and the resonance circuit has a capacitor capacity of only the capacitor 8 and a case of adding either of the capacitors 8a and 8b to the capacitor 8. You can switch to a total of 3 patterns. Further, the resonance circuit is connected to a voltage detection unit 16 that detects a voltage for checking the output level, and the voltage detection unit 16 is connected to the CPU 1 so that a detection signal can be output to the CPU 1.
The state of the output level is displayed by a plurality of LED lamps 17 by the program of the CPU 1.

FIG. 2 is a graph showing the antenna characteristics of the transmitter. The curve of the solid line A is the antenna characteristics at the time of factory shipment, and the resonance point at which the maximum sensitivity is exhibited is the output frequency f1 of the DDS2 of 22 kHz. Is set to. If a transmitter equipped with this antenna is set at the entrance / exit of a store, it will be affected by the surrounding metal, and the inductance of the coil will decrease, and as shown by the dotted line B, the frequency will shift to a high frequency f2 and the sensitivity will decrease. Therefore, if this is the case, DD
The output level at the output frequency f1 of S2 is from V1 to V
The number is drastically reduced to 2, and even within the surveillance area, the self-tapping tag can be activated only near the antenna.

Therefore, the capacitor addition switch 15
Is switched to increase / decrease the capacity of the capacitor, the antenna characteristic is normally shifted to a lower frequency, and the frequency at the resonance point is brought close to f1 while confirming the maximum output with the LED lamp 17. In this case, the antenna characteristic is ideally the frequency of the resonance point is the same as f1 as shown by the dotted line C,
In the embodiment, since only three patterns for switching the capacitance of the capacitor are set, it is difficult to completely match the frequency f3 at the resonance point with the output frequency f1, for example, as shown by the dotted line E. Therefore, the number of switching patterns is increased, and as shown in FIG.
By adopting No. 8, it is possible to continuously change and improve the correction accuracy. In addition, the frequency at the resonance point is the output frequency f
Even if it is exactly matched to 1, the corrected output level is lower than the original performance. Therefore, irrespective of whether the frequency at the resonance point coincides with f1 or not, for example, in the case of the antenna characteristic shown by the dotted line E, the output level of the output frequency is indicated by the dotted line D until the output level at the resonance point f3 reaches V3. As shown, the output level at the output frequency f1 can be increased to the specified level V1.

These corrections can be performed not only by manually switching the capacitor capacitance, but also by automating based on the flowchart of FIG. 4, and a modified example will be described. First, energize the installed transmitter at S1 and manually switch the mode switch of the transmitter to "install" to set the installation environment (S2), and output 22 kHz as the output frequency f1 from the DDS2. Done (S
3). A program for automatically correcting when switched to the installation mode is pre-installed in the CPU 1, the output level is checked in S4, and the output level (V
It is judged whether or not 1) has reached the specified level, and if it has reached the specified level, it is considered that the installation environment does not change from the factory set value, and the operation mode is switched to S6. If it has not reached, in S7, the capacitor capacity is added to wait for the antenna characteristics to change, the output level after the change is checked in S8, and the output level (V2) is changed to the previous output level (S9). V1) and the size are compared. Output level (V
If 2) is large, at 10 the S7 and subsequent steps are repeated for other capacitor capacities based on the output level (V2). If the output level (V1) is equal to or higher than the output level (V2), the capacitor value of the output level (V1) is fixed in S6 in S6, and the correction of the antenna characteristic ends. Next, in S12, it is determined whether the output level (V1) with the fixed capacitor value has reached the specified level,
If the specified level is not reached, the known ALC in S13
The control circuit raises the output level and returns to S11. If it reaches the specified level, it is considered that the range of the monitoring area does not change from the factory setting value, the installation mode is completed in S14, and the operation mode is switched to the anti-theft monitoring in S15. In this way, by correcting the antenna characteristics according to the installation location, it is possible to achieve matching with the frequency and prevent monitoring errors.

In the above embodiment, the antenna characteristic is corrected by changing the capacitance of the capacitor, but it is also possible to correct the antenna characteristic by changing the inductance of the transmitting coil. Next, the embodiment will be described with reference to FIG. 5. Taps 19, 19 ...
., And a tap changeover switch 20 for changing the coil length by selecting the connection with those taps.
A signal is sent from the CPU 1 by detecting the output level of the antenna, the tap changeover switch 20 is switched to change the inductance, and the antenna is corrected to the characteristics at the time of factory setting.

The switching of the inductance can be done manually, and as shown in the figure, the antenna characteristic may be corrected by using the switching of the capacitor capacity and the switching of the inductance together. Also, for manual switching, a switch is installed on the exterior panel of the transmitter so that it can be easily performed from the outside.When switching between the capacitor capacity and inductance is used together, only one of them can be used independently. It is desirable to have it. Further, the specific frequency of the radiated radio wave is not limited to 22 kHz, and is a frequency arbitrarily determined in the ultra-long wave band and the long wave band.

[0013]

As described above, according to the present invention, since the antenna characteristics which have changed depending on the environment of the installation place can be corrected, it is possible to suppress a decrease in the output level and to make the self-sounding tag function 100%. Further, high performance is not required for the circuit components that make up the transmitter, which leads to cost reduction of the product. Moreover, since high performance is not required for the parts, and the parts can be easily implemented by adding the switch circuit, it can be additionally set in the existing device.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a transmitter of an anti-theft device according to the present invention.

FIG. 2 is a graph illustrating changes in antenna characteristics.

FIG. 3 is a flowchart of an automated antenna characteristic correction.

FIG. 4 is a circuit diagram of a modified embodiment.

FIG. 5 is a circuit diagram of the modified embodiment.

[Explanation of symbols]

1 ... CPU, 2 ... DDS, 3 ... pulse shaping circuit,
4 ... Gate voltage control unit, 5 ... Driver, 6 ... Switching circuit, 7 ... Antenna, 8, 8a, 8b ... Capacitor, 9 ... Transmitting coil, 10 ... Power supply, 11 ...
Self-tapping tag, 12 ... Reception coil, 13 ... Reception circuit,
14 ... Alarm means, 15 ... Capacitor additional switch,
16 ... Voltage detector, 17 ... LED lamp, 18 ...
Variable capacitor, 19-tap, 20-tap changeover switch, A, B, C, D, E ... Curve showing antenna characteristics, S ... step.

Claims (2)

[Claims] 1. A monitoring area includes a transmitter that emits a radio wave of a specific frequency and a self-tapping tag that receives the radio wave and issues an alarm. An antenna that emits the radio wave is provided with a coil and a capacitor. An anti-theft device that employs a used resonance circuit, in which the antenna characteristics can be changed by providing capacitance switching means for a capacitor in the resonance circuit. 2. The monitoring area includes a transmitter that emits a radio wave of a specific frequency and a self-tapping tag that receives the radio wave and issues an alarm. An antenna that emits the radio wave has a coil and a capacitor. An anti-theft device that employs a used resonance circuit, in which the antenna in the resonance circuit is provided with a tap and the coil length is switched to change the characteristics of the antenna.