Security system

Abstract

A security system for monitoring predetermined alarm conditions at multiple locations. Security units are provided at each monitored location with each unit including an emergency d.c. power source for providing power during a.c. power line failure. In the event of an alarm condition, an amplitude modulated radio frequency carrier signal having a modulation frequency indicating the location of an alarm condition is superimposed on an a.c. power line for detection at a remote monitoring location. A method is provided for checking the operability of the individual security units by the user of the unit without indicating an alarm condition at the monitoring location. A two modulation frequency method and system is provided for two alarm conditions.

Description

BACKGROUND OF THE INVENTION

This invention relates to security systems and more particularly to a security system in which the occurrence and location of a predetermined alarm condition is monitored at a point remote therefrom.

Security systems for monitoring conditions at multiple locations are well known. However, due to the danger of theft and the sophistication of the burglaries committed, especially in multiple-unit dwellings, there is a necessity for a reliable, tamper-proof alarm system. Many alarm systems operate on power delivered by a.c. power lines, in which the system may be simply inactivated by causing a power loss to occur. This may be done either by disconnecting the power line at its source or, even more simply by shorting out the power line, in which case the power is interrupted by blown fuses or circuit breakers. Moreover, power to components of the security system may be interrupted without interrupting power to the rest of a building by merely shorting out the power lines to the component thereby blowing the associated fuses without indicating, by a general blackout, that a burglary is taking place.

For the complete integrity of a security system, the operability of the components thereof must be periodically established. Heretofore, it was necessary for an attendant or security guard to make rounds and individually check each of the components while in communication with another security guard at a monitoring station. This is a cumbersome procedure involving additional personnel. There is, therefore, a need for a simple system by which each occupant can test the component of the security system protecting his premises or apartment prior to retiring for the night or leaving the premises.

Moreover, there is an overall reliability problem associated with security systems in which a number of security units are utilized to indicate the existence of a predetermined alarm condition as well as the location thereof. For instance, in multi-wire systems for connecting individual security units to a monitor unit, the frequency of system failure is a function of the number of interconnecting wires. Thus if individual wire pairs extend from monitored locations to the monitor, the integrity of each pair determines the integrity of the entire system. When a single cable system is used, the integrity of the system depends on the integrity of only one cable. In single cable systems modulated signals may be impressed on the cable to indicate a predetermined alarm condition and the location thereof. However, the more areas monitored, the more complicated and less reliable becomes the modulation system.

It is accordingly an object of the present invention to obviate many of the deficiencies of prior known systems and to provide a novel and improved method and system for reliably monitoring predetermined alarm conditions at multiple locations to indicate the fact of a security breach as well as the location of the breach.

It is another object of this invention to provide an improved security system having independent emergency powered security units whereby the operation of the system is not impaired by a general loss of power.

It is yet another object of this invention to provide an improved method and apparatus for testing security systems in which an individual component of the system may be selectively tested without indicating an alarm condition at the monitoring location.

It is a further object of the present invention to provide a novel and improved security system which utilizes an a.c. power line between monitoring and monitored locations.

It is still a further object of this invention to provide improved modulation for a security system in which a predetermined condition is sensed and a modulated carrier signal is superimposed on the signal carried by an a.c. power line to indicate the existence of the predetermined condition and in the location thereof.

It is yet another object of this invention to provide an improved demodulator for use in a security system in which a modulated carrier indicating a security breach and the location thereof is impressed on an a.c. power line.

It is important in some situations to distinguish between the fact of an alarm condition and the identity of the alarm condition. For example, it may be desirable to distinguish between the unauthorized entry of a space and the removal of an object therefrom such as a TV set from a motel. Alternatively, it may be desirable to distinguish between entry and a fire, or between a fire and the pressure of a harmful gas.

It is accordingly an object of the present invention to provide a novel two or more alarm condition method and system.

These and many other objects and advantages of the subject invention will be readily apparent to one skilled in the art to which the invention pertains from the claims and from the following detailed description when read in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of the system of the present invention;

FIG. 2 is a pictorial view of the monitor unit of FIG. 1;

FIG. 3 is a pictorial view of one of the sending units of FIG. 1;

FIG. 4 is a schematic diagram of one embodiment of the sending unit of FIG. 3;

FIG. 5 is a functional block diagram of the monitor unit of FIG. 2; and

FIG. 6 is a schematic circuit diagram of one of the demodulator units of FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1, a security system may include a plurality of security units 10 each coupled to an a.c. power line 12. Each of the security units 10 may be located at a different location to detect a predetermined alarm condition thereat. A monitor unit 14 may be coupled to the a.c. power line 12 at a position remote from each of the security units 10 for monitoring the detection of a predetermined alarm condition by any one of the security units 10.

Each of the security units 10 may include a sensor 16 for detecting a predetermined alarm condition and for providing an output signal indicative thereof. This alarm condition may be the opening of a portal, the overheating of a stove, a fire, the presence of flooding or the like. The output signal from the sensor may be applied to a sending unit 18 for superimposing on the a.c. power line 12 a signal indicating the detection of the alarm condition and the location thereof. In one embodiment, each of the sending units 18 may include means responsive to the detection of the predetermined alarm condition for coupling onto the a.c. power line 12 an amplitude modulated radio signal identifying by the frequency thereof the location of the security unit 10 at which the predetermined alarm condition is detected.

As will be described hereinafter, the monitor unit 14 may include means for demodulating and/or decoding the superimposed signal and for actuating a corresponding indicating element for indicating the detection of and location of the predetermined alarm condition.

In an apartment house or multiple-unit dwelling, the number of security units may be large. When a large number of security units are utilized, a number of security units may be grouped to facilitate signal processing. Grouping of security units may be accomplished by providing that the sending units 18 associated with a group of security units generate carrier signals of the same frequency. The individual sending units of the group may then modulate the carrier signal at different discrete frequencies to identify the individual sending unit with the group. The carrier frequency in combination with the modulation frequency may thus indicate the location of the security unit which is transmitting.

By way of example and as illustrated in FIG. 1, two groups of security units, Group A and Group B, may be connected to the a.c. power line 12. Should a predetermined alarm condition occur in the vicinity of the security unit 10A.sub.1, the sensor 16 associated with that security unit may generate a signal which may be applied to the associated sending unit 18. Upon receipt of a signal from sensor 16, the sending unit 18 of the security unit 10A.sub.1 may be activated to generate a carrier signal having a frequency C.sub.A. This carrier signal may be modulated at a frequency f.sub.1 which indicates that a predetermined alarm condition exists at the first security unit of the A group, i.e., 10A.sub.1. The signal generated by the sending unit 18 of the security unit 10A.sub.1 is coupled by the a.c. power line 12 to the monitor unit 14 where the signal is demodulated and/or decoded and an apppropriate indicating element actuated to indicate the existence and location of the detected alarm condition.

In a like manner, when the predetermined alarm condition exists at any of the other security units in Group A, a carrier signal may be generated having a carrier frequency C.sub.A and a modulation frequency f.sub.2 . . . f.sub.N corresponding to the security unit in Group A at which the predetermined alarm condition is detected.

In the same manner, a predetermined alarm condition at any one of the security units in Group B may activate the associated sending unit to generate a carrier signal having a frequency C.sub.B with a modulation frequency f.sub.1, F.sub.2 . . . f.sub.N corresponding to the security unit in Group B at which the predetermined alarm condition is detected.

In one embodiment, the monitor unit 14 may be in modular form in which a number of modules 20 as illustrated in FIG. 2 may be connected to the a.c. power line 12 at a location remote from the security units 10 of FIG. 1. Each of the modules 20 may be provided with a panel containing a number of elements 22 and indicia located adjacent each indicating element for indicating the location of the security unit at which a predetermined alarm condition is detected. The monitor unit 14 may also be provided with a control panel 24 which includes a "Power-on" indicator 26, an audible alarm 28, a reset button 30, and a key slot 32.

The monitor unit 14 may thus include a number of modules, each module corresponding to a group of security units and each module responsive to a carrier signal of a predetermined frequency. Each one of the modules may include separate frequency discrimination circuits responsive respectively to each of the discrete carrier modulation frequencies of the individual security units within the group for activating an indicating element identifying the security unit at which the predetermined alarm condition is detected.

The grouping of security units and the modular construction of the monitor unit provides an "add-on" capability when, for instance, it is desirable to increase the number of locations monitored.

In operation, and with the "Power-on" indicator 26 indicating power to the monitor unit 14, an audible alarm may be sounded upon the detection of an alarm condition at one of the security units. Simultaneously, an indicator element 22 may be actuated to indicate the detection of the alarm condition and the location thereof. The audible alarm may be designed to attract the attention of a guard who then must insert a key into the key slot 32 and turn the key to shut off the audible alarm. At the time the guard shuts off the audible alarm, he will be in a position to note the location of the predetermined alarm condition by looking at the monitor panel. The guard may then reset the monitor unit by pressing the reset button 30 and take action appropriate for the alarm.

As illustrated in FIG. 3, if the predetermined alarm condition is the opening of a portal, such as a door 33, then the sensor 16 may include a set of contacts 34 located on the inside of a door jamb 36. The door 33 may carry a shorting strip or bar 38 such that when the door is closed, electrical contact is established between the contacts 34. Thus the opening of the door 33 will open the electrical circuit through the contacts 34 and the bar 38 and thereby interrupt the power applied to a relay or other switch indicative of the alarm condition.

The sending unit 18 of FIG. 1 may include an indicator element 38, a key slot 40, a system check button 42, and an "enter" button 43, as hereinafter explained in detail in connection with FIG. 3.

THE SENDING UNIT

One embodiment of the novel sending unit of the present invention for coupling an amplitude modulated radio frequency signal on the a.c. power line 12 is illustrated in FIG. 4. In this embodiment, the sending unit 18 may be coupled across an a.c. power line at terminals 50 and 52. The terminal 52 may be connected to one side of the primary winding of a transformer T.sub.1 through normally closed switches 56 and 57. The terminal 50 may be connected to the other side of the primary winding of the transformer T.sub.1 through a capacitor 58. The switch 56 may be opened by an interrupt and delayed reconnect circuit 60 which is actuated in response to the closing of the aforementioned system check button 42 by the closing of a switch 62 to a reference potential such as a ground potential.

The interrupt and delayed reconnect circuit 60 may be any suitable and conventional time delay and relay circuit activated when the switch 62 is closed to open the switch 56. In one embodiment an internally contained relay (not shown) may be actuated for a predetermined time interval, starting with the closing of the switch 62. This time interval may be sufficiently long for the completion of system testing and the exit of the apartment by the occupant as will be subsequently explained.

The switch 57 may be opened by depressing the "enter" button 43 to permit the opening of the door 33 while the sending unit is actuated without the generation of an alarm signal on the a.c. power line. The sending unit may be in close proximity to the door 33 to permit depression of the button 43 while opening the door. Should the person at the door be an intruder, the occupant, by stepping away from the door and releasing the button 43, causes the normally biased switch 57 to close thereby to couple an alarm signal to the a.c. power line.

With continued reference to FIG. 4, the terminals 50 and 52 may be connected across a diode bridge circuit 64 for providing a direct current to a relay coil 66 from an output terminal 68 thereof. A diametrically opposite output terminal 70 of the diode bridge circuit 64 may be provided to provide the aforementioned reference potential. The output terminal 68 of the diode bridge circuit 64 may also be connected to a terminal 72 of a single pole, double throw switch 74. An independent emergency d.c. power supply 78, such as a conventional storage battery, may be connected between the other terminal 80 of the switch 74 and ground potential.

The presence of power to the unit, i.e., an a.c. signal across the terminals 50 and 52, will provide direct current through the coil 66 to actuate the switch 74 and connect the terminal 72 to the terminal 76 against the bias provided by a spring 77. Loss of a.c. power will deenergize the relay coil 66 and permit the spring 77 to connect the contacts 80 and 76 of the switch 74 to thereby energize the unit with the emergency power supply 78.

The common terminal 76 of the switch 74 may be connected through a key operated switch 82 to one terminal of a grounded incandescent lamp 84 or other indicating element such as a light emitting diode. The lamp 84 may serve as the indicator element 38 on the sending unit panel as will be described. The common terminal 76 of the switch 74 may also be connected through the key operated switch 82 and a switch 89 actuated by the sensor 16 of FIG. 1 to the power supply terminals of an oscillator 86 and a modulator 88.

The oscillator 86 may be a conventional radio frequency signal generator, amplitude modulated by an input signal applied from the modulator 88. The modulator 88 may also be conventional and may generate an output signal having a frequency determined by a resonating tuning fork (not shown). The output signal from the oscillator 86 may be applied across the secondary winding of the transformer T.sub.1.

In operation and with reference to FIGS. 2 and 3, the key closing of the switch 82 energizes the unit to apply power to the switch 89. Upon the closing of the switch 89 by the detection of a predetermined alarm condition, d.c. power from the terminal 76 is applied to the oscillator 86 and the modulator 88. Thereafter, the oscillator generates a carrier signal which is amplitude modulated in accordance with the signal from the modulator 88 and coupled onto an a.c. power line through the transformer T.sub.1 and the capacitor 58, with the capacitor 58 serving to isolate the sending unit from the a.c. power line.

TESTING METHOD

When the system check button 42 of FIG. 3 is depressed, the switch 62 associated with the interrupt and delay reconnect circuit 60 is closed thereby opening the switch 56 to disconnect the sending unit from the a.c. power line. The opening of the switch 56 also deenergizes the relay coil 66 so that the d.c. power supply 78 is connected into the circuit. The continued illumination of the lamp 84 (indicator 38) indicates that the emergency d.c. power supply to supplying power and that the predetermined alarm condition has not been detected.

The occupant may then create the predetermined alarm condition to effect the closing of the switch 89 to supply power to the oscillator 86 and the modulator 88. The closing of the switch 89 also reduces the current through the lamp 84 and dimming thereof indicates that the oscillator and modulator are energized, i.e., an alarm condition has been detected. A predetermined time after the system check button is released, the circuit 60 may close the switch 56 to reconnect the sending unit to the a.c. power line and to reenergize the relay coil 66. The time delay may be long enough for the occupant to test the sending circuit and exit the premises after ascertaining that his particular unit is working properly.

Taking, for example, the opening of a door or portal as an alarm condition, the sending unit 18 of FIG. 3 may be tested by closing the door 33 to establish the absence of the alarm condition and thereafter using the key to energize the unit. If the unit is working properly, the indicator element 38 will indicate the availability of power to the sending unit 18 and the absence of the predetermined alarm condition.

After ascertaining the delivery of power to the sending unit, the occupant of the apartment may then temporarily disconnect the sending unit from the a.c. power line by means of the system check button 42. The system check button may not only disconnect the sending unit from the a.c. power line, but also may activate a separate d.c. power supply for the sending unit. If the separate d.c. power supply is not operative, the indicator element 38 will indicate an inoperative condition.

The occupant of the apartment may then cause the predetermined alarm condition to occur, e.g., by opening the door, while the sending unit is disconnected from the a.c. power line. The indicator element 38 may then indicate that the predetermined alarm condition has occurred and that a signal is being transmitted responsively thereto. In the embodiment of FIG. 4, this indication may be a visible dimming of the incandescent lamp 84. Thus, the availability of power and the operability of the security system may be indicated without superimposing a signal indicating the occurrence of the predetermined alarm condition on the a.c. power line.

The sending unit 18 may be provided with the delayed reconnect circuit 60 of FIG. 4 such that the occupant may have time to exit the premises after ascertaining that his particular unit is working properly before the sending unit is again connected to the a.c. power line. Because of the delay, the alarm condition is terminated by closing the door prior to the reconnection of the sending unit to the a.c. line and thus the alarm signal is not coupled to the a.c. line. The reopening of the door by the occupant upon his return will couple an alarm signal to the a.c. power line and it will be necessary at that time for the occupant to phone the personnel at the monitoring station to cancel the alarm indication.

MONITOR UNIT

Referring now to FIG. 5, the monitor unit 14 may include a group of demodulators 90 coupled in parallel to the a.c. power line 12. Each demodulator may be connected to an associated bank of decode units 92 in turn connected to an associated group of indicating elements 98.

In operation, each of the demodulators 90 may respond to a carrier signal of a different frequency corresponding to a different one of the aforementioned groups of sending units such that each module responds to only the carrier frequency of that group. Each demodulator may effectively remove any a.c. power line signals and may generate a signal having a frequency corresponding to the modulation frequency of a predetermined carrier signal as will hereinafter be described.

The output signal from each demodulator 90 may be applied to the plurality of decode units 92 associated therewith. Each decode unit 92 may include conventional frequency responsive switches and relay means, including a relay coil 94 and a relay actuated switch 96, for actuating an associated indicating element 98 by completing a power supply circuit thereto. Additionally, each of the decode units may include means for maintaining the actuation of the switches 96 until the switches are manually reset.

The monitor unit 14 may be provided with its own separate power supply 100 which may be applied in parallel to one side of all of the indicating elements 98 and through a normally closed switch 102 to an audio oscillator 104. The oscillator 104 may be actuated simultaneously with the actuation of any of the indicating elements 98 associated with any of the modules by a relay actuated switch 106 for generating an audio alarm signal. The audio alarm signal may be applied to a speaker 108 for indicating the existence of an alarm condition at any one of the aforementioned security units until such time as the entire monitor unit 14 is reset by interrupting the power to the relay coils 94 to operate the switches 96 and 106.

Referring now to FIG. 6, one of the demodulators 90 of FIG. 5 may include an input stage 110, a crystal control r.f. amplifying stage 112, an amplifier stage 113, and a crystal controlled detector stage 116.

The input stage 110 may include a transformer T.sub.2 having a secondary winding S across which is connected a capacitor 118. One end 119 of the secondary winding S may be connected to ground through a parallel connected RC filter network including a resistor 120 and a capacitor 122. The end 119 of the secondary winding S may also be connected through a resistor 124 to a source V of positive potential.

The other end 125 of the secondary winding S may be connected to the base electrode of an NPN transistor Q.sub.1 which is part of the crystal controlled r.f. amplifying circuit 112. The emitter electrode of the transistor Q.sub.1 may be connected through a resistor 126 to ground potential and also through a crystal controlled filter which may include a capacitor 128 and a crystal 130 tuned to the carrier frequency for the associated demodulator. The collector electrode of the transistor Q.sub.1 may be connected through a resistor 132 to the source V and may be coupled to an amplifier 134 through an input resistor 136. The amplifier 134 may include a feedback network including a capacitor 138 and a resistor 140 coupled and parallel therewith. An output terminal 142 of the amplifier 134 may be connected through a resistor 144 to the source V and through a capacitor 146 and a resistor 148 to the crystal controlled detector circuit 116.

The crystal controlled detector circuit 116 may include an NPN transistor Q.sub.2 having a base electrode connected to an input terminal 150, with the other end of the resistor 148 being connected thereto. The input terminal 150 may be connected to ground potential through a resistor 152 and a capacitor 154 in parallel with a crystal controlled filter including a diode 156 and a crystal 158 tuned to the carrier frequency. The collector electrode of the transistor Q.sub.2 may be connected to the source V through a resistor 160 and may be connected to an output terminal 162 through a resistor 164. Feedback for the detector is provided from the output terminal 162 through a resistor 166 and the resistor 152. A capacitor 168 may be connected between the output terminal 162 and ground potential. As mentioned hereinbefore, the output terminal 162 may be connected in parallel to the input terminals of a plurality of decode units 92.

In operation, signals from the a.c. power line appear across the secondary winding S of the transformer T.sub.2 and may include substantial noise in addition to the a.c. power signal. The secondary winding S of the transformer T.sub.2 and the capacitor 118 form an LC circuit which is tuned to a carrier frequency. In addition, the filter circuit comprising the resistor 120 and the capacitor 122 serve to filter out any a.c. power line components. The signal applied to the base of the transistor Q.sub.1 is thus an amplitude modulated carrier signal with other carrier signals and a.c. line noise substantially attenuated.

The crystal controlled r.f. amplifier 112 is provided with a filter including the crystal 130 tuned to the particular carrier frequency desired so that the amplifier including the transistor Q.sub.1 will amplify only carrier signals having a frequency corresponding to that of the crystal 130. The output signal at the collector of the transistor Q.sub.1 is thus an amplitude modulated radio frequency signal having the desired carrier frequency. This signal is amplified by the amplifier 134 and applied to the crystal controlled detector 116. The carrier signal components are shunted away from the base of the transistor Q.sub.2 by the filter circuit comprising the crystal 158 and the diode 156. The resulting signal at the output terminal 162 is a signal having a frequency related to the modulation frequency of the desired carrier signal.

A TWO ALARM SYSTEM

As explained above, the number of sending units may be infinitely expanded. It is, however, also possible to modify a single sending unit 18 for two or more alarms.

With reference again to FIG. 4 where a sending unit 18 is illustrated, the sensor 16 responsive to one alarm condition may operate the switch 89 as earlier described and may in addition operate a switch 200 to connect the modulator 88 to the oscillator 86.

In the event that a second alarm condition is to be monitored, a second sensor 202 responsive thereto may be added. The sensor 202 may be utilized to operate the switch 89 which applies power to the oscillator 86 and the switch 204 which connects a second modulator 206 to the oscillator 86. As is readily apparent, each additional sensor must operate the power switch 86 and control the modulator selection so that a different modulation frequency may be imposed on the carrier frequency of the oscillator 86.

Rather than add entire modulators, a different tuning fork or other frequency controlling element may be selectively switched into the modulator circuit.

No modification of the monitor unit 14 need be made for this two alarm condition embodiment since, as earlier indicated in connection with FIG. 5, there are a plurality of decode circuits for each carrier frequency.

ADVANTAGES AND SCOPE OF THE INVENTION

It will be appreciated that the security system thus described is reliable due to the frequency discrimination afforded by the tuning fork modulation system described and is also tamper-proof in the sense that a.c. power failure will not inactivate the security system. Moreover, the security system described provides a user or occupant with a simple method for the occupant to check his component of the security system before leaving the premises without transmitting an alarm signal.

An intruder may trigger the sending unit without being advised of that fact since an audible alarm is not used at the premises protected, thereby increasing the opportunity for apprehension of the intruder. The system is moveover fail-safe in that it cannot be deactivated externally but requires communication by the occupant with the monitoring personnel.

The occupant may depress the temporary entry button while opening the door secure in the knowledge that a forceable entry will force the occupant back from the door necessitating the release of the entry button and instantly coupling the alarm signal to the a.c. power line.

When equipped with two sensors, the present system may be utilized to distinguish between alarm conditions and to convey other intelligence as well. For example, the second alarm may be a button on the sending unit which may be depressed by a maid to indicate to the office of a hotel or motel that a room is ready for occupancy.

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

What is claimed is:

1. A security system for use with an a.c. power line connected between a plurality of locations comprising:

a plurality of security units, each including a sensor for detecting the opening of a portal and a sending unit responsive to said sensor for coupling to the a.c. power line an amplitude modulated radio frequency signal identifying the location of the security unit at which said predetermined condition exists, each of said sending units including:

a radio frequency oscillator for generating a carrier signal having a predetermined frequency common to a plurality of said sending units, and

means for amplitude modulating said carrier signal, the combination of the predetermined frequency and the amplitude modulation being unique to each of said security units,

one of said sending units includes switch means biased to couple said amplitude modulated radio frequency signal to said a.c. power line and manually operable against said bias to uncouple said amplitude modulated signal to said a.c. power line,

said one sending unit being disposed in sufficient proximity to the portal to permit the manual operation of said switch means simultaneously with the manual opening of the portal to permit the opening of the portal without the coupling of said amplitude modulated signal to said a.c. power line,

the bias of said switch means being sufficient to couple said amplitude modulated signal to said a.c. power line in the event that the open condition of the portal persists beyond the manual operation of said switch means so that the natural act of stepping away from the portal upon the forceable entrance of an intruder through the portal couples said condition responsive signal to said a.c. power line; and,

a monitor unit coupled to said a.c. power line at a location remote from said plurality of security units and responsive to the coupling of amplitude modulated radio frequency signals on the a.c. power line for indicating at which of said security units said sensed predetermined condition exists; said monitor unit including a plurality of modules,

each of said modules associated with a predetermined group of said security units having a common carrier frequency different from the carrier frequency of all of the remaining ones of said security units,

each of said modules having means for indicating at which of the security units in an associated group said predetermined condition exists, and

each of said modules including a plurality of switch means responsive to a different frequency of modulation to actuate said indicating means.

2. The security system of claim 1 wherein power from the a.c. power line is utilized for the normal operation of the system, and including:

an independent source of power for each of said security units and for said monitor unit; and,

means responsive to a loss of power on said a.c. power line for connecting said independent sources of power to said security units and to said monitor unit.

3. The security system of claim 1 wherein said sending unit further includes tuning fork means for establishing the modulation frequency for each of said security units.

4. The security system of claim 1 wherein each of said modules includes means for demodulating an amplitude modulated radio frequency carrier signal coupled to the a.c. power line comprising:

crystal controlled radio frequency amplifying means including crystal controlled filter means tuned to the frequency of said carrier signal;

transformer means coupled to said a.c. power line for applying signals to said crystal controlled radio frequency amplifying means, said transformer means being tuned to the frequency of said carrier signal for removing a.c. power line signals from the signal applied to said crystal controlled radio frequency amplifying means;

means for amplifying the output signal from said radio frequency amplifying means; and,

crystal controlled detector means for generating an output signal having a frequency corresponding to that of said carrier modulation, said detector means including crystal controlled filter means tuned to said carrier frequency for removing carrier frequency signal components from signals applied to said detector means.

5. The security system of claim 1 wherein each of said security units includes a plurality of sensors; and,

wherein said sending unit is responsive to each of said plurality of sensors to couple to the a.c. power line an amplitude modulated signal having a predetermined frequency identifiable with one of said plurality of sensors.