GB2119981A - Alarm Systems - Google Patents

Abstract

An alarm system includes a number of outstation each of which normally generates an unique output signal, preferably a tone of an unique frequency. The output signal is removed when an alarm condition exists. Each outstation is connected to a common line, as is a master station. The master station includes checking means operable to check cyclically for the presence of each output signal from the outstations, and indicator means operable to indicate the absence of any such signal. <IMAGE>

Description

SPECIFICATION Alarm systems This invention relates to alarm systems of the type in which a number of outstations, each with some form of detector, are connected to a central master station. Systems of this general type are well-known, and usually comprise either a type in which the outstations send an alarm signal to the master station when an alarm condition exists, or a type in which each outstation is interrogated and its response is checked. The first type is not failsafe, whilst the second requires complex circuitry and/or cabling to allow for the passage of information in both directions.

It is an object of the invention to provide an alarm system which avoids the above disadvantages.

According to the present invention there is provided an alarm system which includes a plurality of outstations each capable under normal operating conditions of generating continuously an unique output signal and responsive to an alarm condition to cease generation of said signal, a common line interconnecting all of the said outstations, and a master station connected to said common line and comprising checking means operable to check cyclically for the presence of the unique signal from each outstation, and indicator means operable to indicate the absence of any of said signals.

Preferably each outstation is arranged to generate an output signal of an unique frequency, the output signal of each outstation being harmonically unrelated to that of each other outstation.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 is a schematic block diagram of an alarm system; Figure 2 is a block diagram of an outstation, and Figure 3 is a similar diagram of a master station.

Referring now to Fig. 1, this shows, in schematic form, the general arrangement of an alarm system in accordance with the invention. The master station MS consists of a control unit CU, a display panel DP and an alarm AL. The control unit is connected by means of a common two-wire line to a number of outstations OS1, OS2. . OSn.

Fig. 2 shows one form of outstation, made up from a dual input AND-gate integrated circuit such as the RCA CD 4011. The outstation uses two of the AND gates in a simple square-wave oscillator drawing its oprating voltage from the common line and inducing in the line current pulses at a frequency determined by the values of the resistors R1 and R2 and capacitor C. The two inputs of one gate G2 are connected together, whilst one input of the other gate G1 is connected to a sensor circuit, as shown in Fig. 2. In its simplest form the actual sensor may be shown as a switch SW. The sensor circuit uses one of the other AND gates on the integrated circuit having its output connected to one input of gate G 1. The two inputs of the sensor circuit gate G3 are connected together and through a high-impedance resistor to the positive common line.The sensor switch SW is connected between the other common line in the inputs to gate G3 when the switch SW is open, gate G1 functions normally as determined by its other input, and the oscillator produces a square-wave current which is imposed on the supply line. When switch SW is closed, however, gate G1 ceases to operate and the oscillator ceases to function. The outstations of the system are arranged to generate pulses at frequencies which are not harmonically related, so as to avoid identification problems. Frequencies such 1 KHz, 1.1KHz, 1.2 KHz. .1.9KHz, are suitable, and are readily obtained from the simple oscillator circuit.

The outstation may be made to respond to a large number of alarm conditions, all of which may be arranged to cause operation of the oscillator to cease as described above.

Typical alarm conditions include changes in capacitance due to the presence of objects or persons, temperature rise or fall causing a resistive change, radiation detection, detection of light, and many others. Safety switches on equipment, or intruder-operated switches on doors, windows and the like may also be used.

Fig. 3 shows one arrangement of the master station MS. The common line is shown here as having one leg earthed, redicing the cabling requirements to a single common conductor. A constant supply voltage is supplied to the common line by an amplifier AM, having a reference voltage input VR. The line currents generated by the various oscillators may be picked off the line at the output of the amplifier. The line current variations are applied to checking means in the forms of a voltage-tunable tone decoder TD. This is a phase-locked loop the frequency of which is tuned by a voltage-variable resistance or capacitance. One example of such a device is that sold by Signetics Corp. under the type numbers NE 567 or SE 567. Such a circuit delivers an output signal when the frequency of the input signal is that to which the circuit is tuned.

The tuning of the tone decoder TD is controlled by varying the control voltage in accordance with the outputs of a decoder DC. This may be a one-out-of-ten decoder, such as the RCA CD 4017, if ten outstations are used in a system. the decoder is stepped from one state to the next by a clock CK arranged to give an output pulse at regular intervals, say every second. The outputs of the decoder DC operate switches, such as FET devices, which vary the resistance which tunes the tone decoder TD. Hence the frequency to which the tone decoder TD responds will vary at regular intervals through a sequence of ten frequencies, and the sequence will then be repeated.

The frequencies to which the tone decoder TD responds are those produced by the individual outstations. Hence, in operation, the tone decoder is tuned to each outstation frequency in turn. The presence of that frequency on the common line produces an output from the tone decoder TD. This output is used to inhibit a gate G which would otherwise apply the outputs of the decoder DC to indicating means in the form of a decoder and display drive circuit DD, driving a seven-segment display DP. Clock pulses are also applied to the display drive circuit DD.

As already stated, the presence of an output from the tone decoder TD prevents the activation of the display drive circuit, though the appropriate display character is selected.

Hence if there is no output from the tone decoder TD then the appropriate character will be displayed and an alarm indication given.

The display will identify the location of the alarm condition. The sequential checking will continue, and the display will be activated in each sequence. If more than one alarm condition exists then each location will be identified in turn.

It will be clear that the alarm indication will be given if a signal of an expected frequency is absent for any reason. Hence cutting of the common line or a power supply failure to the outstations will also produce the alarm condition.

The number of outstations may be varied as required, using tone decodered and other circuits to suit the requirements.

Claims (9)

1. An alarm system which includes a plurality of outstations each capable under normal operating conditions of generating continuously an unique output signal and responsive to an alarm condition to cease generation of said signal, a common ine interconnecting all of the outstations, and a master station connected to said common line and comprising checking means operable to check cyclically for the presence of the unique signal from each outstation and indicate means operable to indicate the absence of any of said signals.

2. An alarm system as claimed in Claim 1 in which each outstation is arranged to generate an output signal of an unique frequency, the output signals of each outstation being harmonically unrelated to that of each other outstation.

3. An alarm system as claimed in either of Claims 1 or 2 in which the supply voltage to each outstation is applied along said common line, an alarm condition being arranged to cause the interruption of the supply voltage to the appropriate outstations.

4. An alarm system as claimed in either of Claims 2 or 3 in which the checking means includes a frequency-sensitive circuit tunable to each of the oustation output frequencies and reponsive to each such frequency to deliver an output signal.

5. An alarm system as claimed in Claim 4 in which the output signal from the checking means is used to inhibit the operation of the indicator means.

6. An alarm system as claimed in any one of the preceding claims in which the indicator means comrpise a display operable to identify the outstation at which an alarm condition exists.

7. An alarm system as claimed in Claim 6 in which the indicator means includes a seven-segment character display.

8. An alarm system as claimed in any one of Claims 1 to 7 which includes an available alarm activated when an alarm condition is detected.

9. An alarm system substantially as herein described with referecne to the accompanying drawings.