GB2154037A

GB2154037A - Alarm unit

Info

Publication number: GB2154037A
Application number: GB08500937A
Authority: GB
Inventors: Anthony Pearson Hall
Original assignee: Thorn Emi Ferguson Ltd
Current assignee: Thomson Multimedia Sales UK Ltd
Priority date: 1984-02-01
Filing date: 1985-01-15
Publication date: 1985-08-29
Also published as: GB8500937D0; GB2154037B

Abstract

The alarm unit 1 has an oscillator 5 powered by a dry battery 6 to generate a reference frequency signal of 10 KHz which is applied to a four-arm impedance bridge 8 including both adjustable resistances 9, 10, and coaxial cable interconnections to a television receiver and video recorder connected via terminals 11 and 12 respectively coaxial cable 2. The resistances 9 and 10 are set to values such that there is no potential difference across points A and B of bridge 8 when terminals 11 and 12 are connected to appliances 3 and 4. If the electrical connection between the alarm unit and either appliance is damaged or broken, the impedance arms become unbalanced which causes monostable circuit 13 to generate a pulse to activate a switch which diverts the oscillator output from the impedance bridge to an annunciator. A key switch must be operated to silence the annunciator and reset the alarm. <IMAGE>

Description

SPECIFICATION Alarm unit The present invention relates to an alarm unit for use with one or more electrical appliances, and particularly but not solely for use with a television receiver and video tape recorder.

The present invention provides an alarm unit for one or more electrical appliances, the unit comprising means to generate a signal of a predetermined frequency, at least one output terminal for connection to an electrical appliance, a plurality of impedance devices arranged in a configuration together with the output terminal(s) to provide a particular value of potential difference, at that predetermined frequency, between two specified points in that configuration, means to detect when a different value of potential difference exists between the two specified points whereupon means to operate an alarm signal is activated.

The present invention also provides an alarm unit for one or more electrical appliances, the unit comprising: means to generate a signal of a predetermined frequency; at least one output terminal for connection to an electrical appliance; means to note the existence of a discontinuity in a connection between the at least one output terminal and an electrical appliance, the noting means incorporating a plurality of impedance devices arranged in a configuration together with the output terminal(s) to provide a particular value of potential difference, at that predetermined frequency, between two specified points in that configuration, means to detect when a different value of potential difference exists between the two specified points whereupon means to operate an alarm signal is activated.

In one form, said particular value of potential difference is zero and said different value is any value other than zero.

Preferably the impedance devices and the output terminal(s) are arranged in a four-arm bridge configuration which is balanced when the output terminal(s) is or are connected to the respective waveguide and electrical appliance. Preferably, each of two of the arms includes connections to at least one outlet terminal.

In one form, two or more terminals are connected together such as to form one arm of the balance bridge configuration.

The alarm unit may include means to provide authorised overriding of the alarm operating means.

In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings in which: Figure 1 shows generally an alarm unit embodying the present invention in combination with a television receiver and a video tape recorder; Figure 2 shows a schematic circuit diagram for the alarm unit of Figure 1.

The illustrated alarm unit 1 is located on the coaxial cable 2 connecting a television receiver 3 to a video tape recorder 4. If the electrical connection between alarm unit 1 and either television receiver 3 or video tape recorder 4 is broken in any way, the alarm unit 1 sets off a loud continuous sound intended to frighten off anyone attempting to steal the television receiver and/or the video tape recorder.

More specifically (with particular reference to Figure 2), the alarm unit 1 has an oscillator 5 powered by a dry battery 6 to generate a reference frequency signal of 10 KHz which is applied (via a switch 7 in a first position) to a four-arm bridge 8 of impedances. The bridge 8 consists of two adjustable resistances 9 and 10, together with two terminals 11 and 1 2 which are connected to television receiver 3 and video tape recorder 4, respectively, via coaxial cable 2. For the reference frequency of 10 KHz, the input impedance at each of the terminals 11 and 1 2 will be a specific, fixed value dependent on the characteristics of the respective appliance 3 or 4 while connected to the alarm unit 1.The resistances 9 and 10 are set to values such that there is no potential difference across points A and B of bridge 8 when terminals 11 and 1 2 are connected to appliances 3 and 4 respectively; in this condition, the impedances of the four arms of bridge 8 are balanced.

However, if the electrical connection between alarm unit 1 and either appliance 3 or 4 is broken (whether for example by unplugging of the appliance from cable 2 or by cutting of cable 2), the input impedance at the corresponding terminal 11 or 1 2 is altered, resulting in the impedances of the arms of bridge becoming unbalanced thereby producing a potential drop across points A and B.

This voltage causes a monostable circuit 13, connected across points A and B, to generate a discrete pulse which passes to switch 7 and causes the latter to change to a second position whereby bridge 8 is isolated from oscillator 5 which now drives a loudspeaker 1 4 to produce an appropriate alarm sound.

In order to turn off the alarm sound, a key 1 5 must be inserted into a lock 1 6 in the housing of alarm unit 1 and rotated to one of two orientations; in the first orientation the key actuates electrical isolation of oscillator 5 from its power supply 6, whereas in the second orientation the key actuates the return of switch 7 to its first position (although of course monostable circuit 1 3 would be actuated immediately if the impedances in bridge 6 were not balanced by then). The provision of a key and lock arrangement inhibits unauthorised tampering with the alarm unit 1.

Clearly the key 1 5 and lock 1 6 could be replaced by any other suitable form of coded switching.

For example, the alarm unit may have a keyboard such that the alarm sound is only turned off when some of the keys are pressed in a predetermined sequence; the alarm unit can be turned on to its monitoring mode in the same way. The keyboard can be remote from the alarm unit; for example the keyboard may be on one of the appliances for protection, such that the signals to effect the switching are passed along the connecting co-axial cable. Alternatively the keyboard may be a separate, stand-alone unit, optionally connected to a number of alarm units to enable some or all to be operated at a time. This last feature may be particularly advantageous when a number of the alarm units are used in a shop or at an exhibition, because they could all be turned on or off together quickly and easily.

An advantage of the alarm unit 1 is that it consumes very little power when in the monitoring mode, as the oscillator 5 is then only used to send a signal down the cable 2; the alarm unit only consumes a significant amount of power when oscillator 5 drives loudspeaker 14 to produce the alarm sound.

The alarm unit 1 has an output socket 1 7 whereby oscillator 5 can drive an external loudspeaker or other alarm indicator when switch 7 is in its second position.

The oscillator 5 of the described embodiment can operate at any suitable frequency which has no harmonics causing interference with the video signal and which is in the audible range to enable driving of loudspeaker 1 4. Clearly the second condition is not essential if the alarm unit is modified to have a separate oscillator for the loudspeaker 14. In one form of the invention, the value of the operating frequency is chosen such that the impedance of at least one terminal is 75, the standard matching impedance between appliances, in order to minimise signal power loss.

The alarm unit 1 is particularly suited for use in homes, shops and at exhibitions. Once the resistances have been set to appropriate values for the appliances, the operation of the alarm unit continues regardless of whether the appliances are on or not; also, switching on or off of the appliances does not affect operation of the alarm unit. The alarm unit can be used to protect any suitable form of audio or video appliance.

The alarm unit may be modified such that it can be used for the protection of more than two appliances. For example, the television receiver and video tape recorder may be connected to terminals which are connected together to form a single arm of the bridge; then a terminal for another appliance, for example an audio centre, can be connected into the bridge to form a separate arm. Also, more than two arms of the bridge may have terminals connecting with electrical appliances. Clearly in each of the situations mentioned above, the input impedance of each terminal on the bridge must be sufficient to produce, by its absence, a detectable voltage across the points A and B.

Claims

1. An alarm unit for one or more electrical appliances, the unit comprising means to generate a signal of a predetermined frequency, at least one output terminal for connection to an electrical appliance, a plurality of impedance devices arranged in a configuration together with the output terminal(s) to provide a particular value of potential difference, at that predetermined frequency, between two specified points in that configuration, means to detect when a different value of potential difference exists between the two specified points whereupon means to operate an alarm signal is activated.

2. An alarm unit for one or more electrical appliances, the unit comprising: means to generate a signal of a predetermined frequency; at least one output terminal for connection to an electrical appliance; means to note the existence of a discontinuity in a connection between the at least one output terminal and an electrical appliance, the noting means incorporating a plurality of impedance devices arranged in a configuration together with the output terminal(s) to provide a particular value of potential difference, at that predetermined frequency, between two specified points in that configuration, means to detect when a different value of potential difference exists between the two specified points whereupon means to operate an alarm signal is activated.

3. An alarm unit according to Claim 1 or Claim 2, wherein the impedance devices and the output terminal(s) are arranged in a fourarm balanced bridge configuration.

4. An alarm unit according to Claim 3, wherein two of the arms include connections to outlet terminals.

5. An alarm unit according to Claim 3 or Claim 4, wherein two or more output terminals are connected together such as to form one arm of the bridge configuration.

6. An alarm unit according to any one of the preceding claims, having means to provide authorised overriding of the alarm operating means.

7. An alarm unit substantially as hereinbefore described with reference to and as illustrated in Figure 1 and/or Figure 2 of the accompanying drawing.