US3866202A

US3866202A - Alarm circuitry

Info

Publication number: US3866202A
Application number: US410432A
Authority: US
Inventors: Martin H Reiss; James R Adams
Original assignee: Gulf and Western Manufacturing Co
Current assignee: ELEKTROWATT AG BELLERIVESTRASSE A SWITZERLAND CORP; Gulf and Western Manufacturing Co
Priority date: 1971-06-15
Filing date: 1973-10-29
Publication date: 1975-02-11
Anticipated expiration: 1992-02-11

Abstract

The alarm circuitry employs a plurality of pressure responsive sensors interconnected by conductor means to detection circuitry including an alarm relay, display panel, or other alarm indicating means. One sensor may be associated with each door, window or other entrance means of the protected premises and all sensors are preferably closed when the premises are secured. In one embodiment the sensors are coupled in series and the detection circuitry is voltage responsive, while in another embodiment the sensors are coupled in parallel and the detection circuitry is current responsive. In both embodiments the detection circuitry senses an alarm condition when a sensor is activated or when the conductor means becomes open or short circuited. In addition, the detection circuitry includes one or more test switches actuable by an authorized person prior to leaving the premises to determine if all entrance means are in a safe closed position, and to check the operability of the alarm circuitry itself prior to enabling it for possible energization by an unauthorized person.

Description

United States Patent 1191 Reiss et al.

[ 111 3,866,202 Feb. 11, 1975 ALARM CIRCUITRY [75] Inventors: Martin H. Reis s, Newton; James R. Adams, Framingham, both of Mass.

[73] Assignee: Gulf & Western Manufacturing Company Systems, New York, NY.

22 Filed: Oct. 29, 1973 211 Appl. No.: 410,432

Related U.S. Application Data [62] Division of Ser. No. 153,297, June 15, l97l, Pat. No.

[58] Field of Search 340/274, 280, 52 F, 253 H, 340/253 R, 256, 409'; 307/39 [56] References Cited UNITED STATES PATENTS 2,556,363 6/1951 Lord et al. 340/409 Primary Examiner-Glen R. Swann, llI Attorney, Agent, or Firm-Wolf, Greenfield & Sacks as Fla F2 2 F3 63 F4 64 F5, G5

- r1 T T3 T4 T5 k 38 5;, m1 6k 1.122 151, LR3 lOk LR4 |6k LR5 Q5 [5 7] ABSTRACT The alarm circuitry employs a plurality of pressure responsive sensors interconnected by conductor means to detection circuitry including an alarm relay, display panel, or other alarm indicating means. One sensor may be associated with each door, window or other entrance means of the protected premises and all sensors are preferably closed when the premises are secured. In one embodiment the sensors are coupled in series and the detection circuitry is voltage responsive, while in another embodiment the sensors are coupled in parallel and the detection circuitry is current responsive. In both embodiments the detection circuitry senses an alarm condition when a sensor isactivated or when the conductor means becomes open or short circuited. In addition, the detection circuitry includes one or more test switches actuable by an authorized person prior to leaving the premises to determine if all entrance means are in a safe closed position, and to 7 check the operability of the alarm circuitry itself prior to enabling it for possible energization by an unauthorized person.

12 Claims, 6 Drawing Figures *SENSOR CURRENT 2 *SENSOR 3 "SENSOR 4 "SENSOR 5 SHORT Cancun OPEN CIRCUIT DETECTOR ALARM CIRCUITRY CROSS REFERENCE TO RELATED APPLICATIONS This is a (division) of application Ser. No. 153,297, filed June 15, 1971, now U.S. Pat. No. 821,733.

FIELD OF THE INVENTION The present invention relates generally to security alarm circuitry for use in buildings such as dwelling houses, museums, commercial buildings and other premises. More particularly, the present invention is concerned with alarm circuitry that includes a plurality of pressure responsive sensors interconnected by conductor means to detection circuitry for detecting activation of the sensors or a shorting or opening of the conductor means.

BACKGROUND OF THE INVENTION In the prior art alarm circuitry is effective under most operating conditions. However, there are certain drawbacks associated with many of these known alarm circuits. For example, these circuits do not provide good noise immunity and therefore are sometimes falsely actuated into an alarm condition when in fact no alarm conditions exists. Also, in order to monitor many different alarm conditions, these circuits have become quite complex. Usually additional circuitry is needed to indicate the occurrence of an open or short in the conductor means that interconnect the sensors.

Another drawback associated with some of the prior art alarm circuitry is that when an alarm condition is sensed, there is no indication of which sensor created the alarm condition. This is especially important when many sensors are used. For instance, in a museum where a sensor ,may be used for each painting and where the museum contains many rooms, it is desirable to determine quickly which sensor has been activated.

Another problem that has been found is that the authorized operator, prior to enabling the alarm circuitry for operation, does not check to see that all entrance means are secured (closed). Upon leaving the premises a false alarm condition is then indicated at a remote location such as at a police station, for example, because the authorized operator had no way of previously checking the condition of the premises to determine if one or more of the entrance means are not secured. Also, some alarm circuits do not provide means for testing the operability of the circuit prior to enabling it for use.

OBJECT OF THE INVENTION Accordingly, it is a primary object of the present invention to provide alarm circuitry including a plurality of'sensors interconnected by conductor means and that may be switched to an alarm condition upon activation of one or more sensors ,or by opening or shorting the conductors.

A further object of the present invention is to provide alarm circuitry that is characterized by improved noise immunity.

A further object of the present invention is to provide alarm circuitry in accordance with the primary object ,Still a further object of the present invention is to provide alarm circuitry which displays an indication of the occurrence of an alarm condition and an indication of which sensor of a plurality of sensors has been activated.

Still another object of the present invention is to provide alarm circuitry that includes one or more test switches that are actuable prior to an authorized person leaving the premises to determine if the alarm system is closed-and to determine if the alarm circuitry is correctly operating.

A further object of the present invention is to provide alarm circuitry including an observation panel for displaying a plurality of discrete sensor zones with each zone including a plurality of sensors. The observation panel indicates the alarmed zone and the sensor of that zone that has been activated.

SUMMARY" or THE INVENTION The alarm circuitry of the present invention is adapted for use in a building or the like that has one or more entrance means to be protected against opening by an unauthorized person. The circuitry preferably includes a plurality of pressure responsive sensors, one or more being associated with each entrance means, and being responsive to an opening thereof to switch the sensorfrom a first preferably closed position to a second preferably open position. One or more conductor means are provided interconnecting the sensors to detection circuitry. The detection circuitry includes means for initially setting a first circuit condition when a sensor is deactivated and means responsive to either activation of a sensor, or shorting or opening of the conductor means, for establishing a second circuit condition. An alarm relay or other means coupled from the detection circuitry is used to indicate an alarm condition in response to the establishing of the second circuit condition.

In accordance with one embodiment of the invention the plurality of sensors are connected in series to the input of the detection circuit and the detection circuit includes a bistable latch means which is normally reset to the first circuit condition when no sensor has been activated and is set to the second circuit condition when one or more of the sensors is activated or the conductor means is shorted or opened. The sensors may include, in addition to a normally closed pressure responsive switch, either a resistor for causing a predetermined voltage drop thereacross or a properly poled diode also having a predetermined drop across it when conducting in the forward direction. The detection circuitry also includes one or more test switches for testing to determine that all sensors are in their closed position and secondly to determine if the circuitry is properly'operating.

In accordance with another embodiment of the present invention the plurality of sensors are interconnected in parallel to a current detector and the current through each sensor, when closed, is controlled so that each sensor has a predetermined current level. In one embodiment each sensor includes a switch that is normally closed in the non-alarm condition, a field effect transistor (PET) and a current limiting resistor. The current detector is adapted to sense a predetermined current from all of the sensors when there is no alarm condition. Upon opening of any one sensor the current changes by an amount associated with that sensor and 3 the current detector can then determine (1) an alarm condition and (2) which sensor of the plurality of sensors has been activated. The current detector may com- ,prise a limiting resistor and meter that is nulled to zero for the non-alarm condition and has its dial sweep ei- BRIEF DESCRIPTION 'OF THE DRAWINGS Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description in conjunction with the accompanying drawings in which:

FIG. 1 is a circuit diagram partiallyin block form of one embodiment of alarm circuitry constructed in accordance with the principles ofthe present invention;

FIGS. 2A and-2B show two different embodiments for the sensors depicted in FIG. 1;

FIG. 3 is a circuit diagram of a portion of the alarm circuitry of FIG. 1 showing an arrangement for a multizone system;

FIG. 4 shows another embodiment of an alarm circuit in accordance'with the present invention including a current detector; g Y

FIG. 5 shows one embodiment of the current detector of FIG. 4; and

FIG. 6 is a block diagram of a current detector, display panel and-plurality of sensor arrays constructed in accordance with-the present invention. I

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown an alarm circuit l0 constructed in accordance with this invention. The circuit 10 generaly includes a sensor array 12 interconnected by conductor means 14 which connect the sensor array to detection circuit 16. Detection circuit 16 includes an input circuit 18, a bistable latch 20, output circuitry 24, and an audible alarm 30.

The sensor array 12 is shown in FIG. 1 as including six sensors SI-S6 connected in series with resistor R1 and switch SW1 between the +6 volt supply and'the cathode of diode D1. Each of the sensors 81-86 in FIG. 1 may comprise a resistor R and pressure response switch SW4 as depicted in FIG. 2A, or a diode D and pressure responsive switch SW4 as depicted in FIG. 2B. The pressure responsive switch SW4 is normally in a closed position completing the circuit through all of the sensors when all of the system is in a closed condition meaning that all the windowsand doors in a particular building are closed, for example. The operation of switch SW1 is discussed in'more detail hereinafter.

The purpose of the input circuit 18 is to provide an alarm circuit having good noise immunity. The circuit 18 includes a tank circuit comprised ofinductor L1 and capacitors C1 and C2 which filter out both high frequency and low frequency noise components. The input to the tank circuit couples to the cathode of diode DI, the anode of which is grounded. Diode D1 clamps any input noise from the sensor array 12 at a negative level of, for example, 0.6 volts. The diode D2 connected at the output of the tank circuit couples to capacitor C3 and potentiometer P1 and is adapted to clamp any positive noise. Capacitor C3 prevents false triggering of bistable latch 20 by noise generated from array 12. I Y

The bistable latch 20 may be considered as comprising transistors Q1 and Q2 and potentiometer P1. One of the terminals of potentiometer P1 is grounded and the other terminal couples to the cathode of diode D4. The anode of diode D4 then couples to the base of transistor Q1 and by way-of resistor R2 and capacitor C5 to the +2 volt supply. The moveable arm of potentiometer P1 connects to the anode of diode D3 while the cathode of diode D3 couples to the base of transistor Q2 and by way of capacitor C4 to ground. The emitter of transistor Q1 couples to the +2 volt supply and-the collector of transistor Q1 couples to output circuit 24 and, by way of forward poled diode D5, to the base of transistor Q2. Transistor Q2 has its collector coupled to the base of transistor Q1 and its emitter coupled by way of resistor R3 to ground and also to the cathode of diode D6. Diode D6is included in the feedback reset line 17 from the contact 27C of relay K2. The operation of the circuit including line 17 is discussed hereinafter.

' Capacitors C4'and C6 are coupling capacitors that tend to block any noise that may still be present in bistable latch 20. The feedback path including diode D5 from the collector of transistor O1 to the base of transistor Q2 provides the latching feature for the bistable latch 20. If either transistor Q1 or Q2 conducts, the other conducts and both transistors are held on via diode D5 until a reset signal is received via diode D6. The operation of thev circuitry discussed to this point is as follows. Assuming that all of the sensors 51-86 are in their closed or deactivated positions, the potentiometer P1 is adjusted so that the voltage coupled by way of diode D3 to the base of transistor Q2 is just not sufficient to allow transistor O2 to. conduct. The voltage at the cathode of diode D4 is not sufficiently negative under these conditions to cause conduction of transistor Q1 either. The +2 volt supply for the emitter of transistor O1 is provided instead of the +6 volt supply in order to keep transistor Q1 out of conduction when the array 12 is not activated. With transistor Ql off there is not sufficient positive voltage at the base of transistor Q3 to turn it on.

The outputcircuitry 24 shown in FIG. 1 includes transistors Q3 and Q4, indicator Ll-L3, relays K1 and K2, test switches SW2 and SW3, and audible alarm 30.

r In the position shown in FIG. 1' switches SW1 and SW2 are in a non-test position and switch SW3 in an alarm not enabled position wherein activation of a sensor would not cause activation of audible alarm 30. Alarm 30 can not be sounded because relay K1 is kept energized, as indicated in FIG. 1 by the ground applied via contacts 25a and 250.

The output of bistable latch 20 taken at the collector of transistor Ql couples by way of capacitor C6 to ground and by way of resistor R4 to the base of transistor Q3. The emitter of transistor Q3 is grounded. An indicator light L1 couples from the collector of transistor Q3 by way of resistor R5 to the +6 volt supply. The

collector of transistor Q3 also couples by way of resistor R6 to the base of transistor Q4. The emitter of transistor Q4 is grounded and the collector couples by way of switch SW2, indicator lamp L2, and resistor R7, all in series to the +6 volt supply, and also to the cathode of diode D7. The anode of diode D7 couples to contact 25a of relay K2 and also by way of relay coil Kl to the +6 volt supply. The common contact 230 of relay Kl couples to ground and one of the other contacts 23b couples to audible alarm 30. Contact 23a is not used in the circuit of FIG. 1.

FIG. 1 also shows a circuit enabling relay K2 which is shown in its actuated position by means of switch SW3. The common contact 25c of relay K2 is grounded while the contact 25b couples by way of indicator lamp L3 and resistor R8 to the +6 volt supply. The common contact 27c of relay K2 coupled by way of switch SW2 to the anode of diode D6. The contact 27a of relay K2 couples to the +6 volt supply and the contact 27b remains open. I

As previously discussed, when the alarm circuit is in the condition shown in FIG; 1 and none of the-sensors 51-86 have been activated, transistor O3 is off and indicator light L1 is not illuminated. Sufficient current does pass by way of resistors R5 and R6 and indicator lamp Ll, however, to cause conduction of transistor Q4. Thus, a path is provided by way of relay K1 and diode D7 to the collector oftransistor Q4 keeping relay K1 energized. Similarly, with relay K2 energized by virtue of switch SW3 being in its off or closed (alarm not enabled) position a ground is continuously provided via contacts25a and 25c for relay Kl to prevent it from falling out and sounding audible alarm 30. Thus, even if one of these sensors 81-86 is activated, unless switch SW3 is opened the K1 relay remains energized and the alarm circuit is not enabled for detection.

The previously referred-to adjustment of potentiometer Pl sets a first circuit condition wherein transistor 01 and 02 are not conducting. If one of the sensors 51-86 is activated thereby opening its switch SW4, the voltage across potentiometer Pl decreases, current is drawn in a forward direction through diode D4 and transistor Q1 conducts. This action causes the collector voltage of transistor O1 to increase positively thereby turning on transistor 02 by way of latching diode D5. It theactivated sensor is subsequently deactivated by a burglar closing the door, for example, diode D4 blocks any positive voltage from causing a resetting (transistors Q1 and Q2 off) of bistable latch 20. Diode D5 also keeps transistors Q1 and Q2 on. Similarly, if-the conductor 14 is broken at any point bistable latch also becomes set. Y

Alternatively, if an unauthorized person attempts to' gain entry by shorting out one or more of the sensors 81-86 the current through inductor L1 and diode D2 increases and the voltage across potentiometer P1 also increases. This increased voltage is coupled by way of diode D3 to the base of transistor Q2 causing transistor Q2 to conduct. When. this action occurs the collector of transistor Q2 and base of transistor Q1 go negative thereby causing transistor O1 to conduct. Again, the latch is provided by way of diode D5 for transistors Q1 and Q2 and diode D3 blocks any subsequent negative voltage from turning off transistor Q2 if the short is later removed.

When either condition occurs, that is when an open or a short occurs the bistable latch 20 is set, transistor 01 is conducting and the transistor Q3 also conducts. This causes illumination of an indictor light L1 thereby indicating that an alarm condition exists. However with switch SW3 closed and the alarm circuit not enabled contacts 27a and 270 of relay K2 cause a positive voltage to be applied by way of line 17 to bistable latch 20 causing it to reset and keeping transistors Q1 and Q2 'off. This operation is provided so that transistors Q1 and Q2 are held in a non-set position until the alarm circuit is enabled for operation.

The'enabling of the alarm is provided by switch SW3 which is opened in order to enable monitoring by the alarm circuitry. When switch SW3 is opened relay K2 deenergizes and relay contact 250 couples to contact 25b causing indicator lamp L3 to lightthereby indicating that the alarm circuit is in an enabled condition. Similarly, contact 27c switches to contact 27b and the reset is no longer provided, thus enabling the bistable latch 20 to be operated from sensor array 12. It is also noted that the breaking of the contact from 25c to 25a now removes the ground from relay K1 and enables relay K1 to fall out should one of the sensors be activated or should an open or short condition exist in array 12.

Before the circuit is enabled, however, by opening switch SW3, the test switches SW] and SW2 are operated to check to see that the array is closed and that the circuitry of FIG. 1 is properly operating. Thus, the circuitry of FIG. 1 is first checked by depressing test switch SW2 just before the operator is to leave the premises. Switch SW2 includes two poles, one of which is open when the other is closed, as shown in the nontest position of FIG. 1. If all of the sensors are closed the bistable latch 20 should be reset, transistor Q4 should be conducting and when switch SW2 is thrown to the test position, indicator light L2 illuminates via one of the poles of switch SW2 thereby indicating that the entire system is secured (closed). The change in position of switch SW2 from the position shown in FIG. I, to the test position opens reset line 17 assuring that transistors Q1 and Q2 do not get reset if one of the sensors is open and relay K2 is still energized. The closing of one of the poles. of switch SW2 also connects light L2 to transistor Q4. Thus, when switch SW2 is depressed if latch 20 is reset, transistor Q3 is off and light L1 is also off indicating that the array 12 is closed. Transistor O4 is on, however, and light L2 is on indicating that transistor Q4 is also properly functioning.

With switch SW2 still depressed switch SW1 may then be moved to an open position thereby causing bistable latch 20 to set. Transistor Q3 conducts and indicator lamp Ll illuminates thereby indicating that the circuitry prior thereto is operating properly and that the transistors of the bistable latch are conducting. Transistor Q4 ceases conduction and thus indicator lamp L2 extinguish if the circuitry is operating properly.

Thereafter test switches SW1 and SW2 are returned automatically to the position of FIG. 1 and the circuit is enabled by opening switch SW3. If one of the sensors is then activated bistable latch 20 is set, transistor Q3 conducts, transistor Q4 turns off and alarm relay Kl falls out. This causes the contact 230 to apply ground to contact 23b'which in turn causes the audible alarm D7 transistor Q4, resistor R6 and lamp L1. With this arrangement instead of grounding the emitter of transistor 04 it couples to the collector of a second transistor Q4 whose emitter in turn couples to the collector of transistor Q4 whose emitter in turn connects to the collector of transistor Q4 whose emitter in then grounded. The bases of transistor Q4", Q4, Q4 couple by way of resistors R6a, R6b, R6c respectively, to lamps Lla, Llb and Llc, respectively, and additonal circuitry that may be identical to the input circuitry and bistable latch circuit of FIG. 1. Each of the lamps could couple to identical input and latch circuitry. This arrangement is used for a multi-zone building wherein a plurality of circuits identical or similar to those shown in FIG. 1 are used to actuate a single relay such as relay K1 in FIG. 1. Thus, resistor R6 may couple byway of input and latch circuitry from one sensor array and the other resistors couple to separate sensor arrays. If a sensor of an array is activated, its associated bistable latch 20 is set and one of the transistors shown in FIG. 3 is turned off. Because these transistors are in series, the turning off of any one of the transistors causes a break in the conductive path to relay K1 and thus causes a falling out of the relay K1 and an attendant ac-' tuation of audible alarm 30.

A display panel may be provided with the circuit arrangement of FIG. 3 having a plurality of lamps Ll. When a sensor is activted one of the lamps illuminates indicting an alarmcondition and the particular illuminated lamp tells which zone of the building has been entered.

Referringnow to FIG. 4 there is shown another embodiment of the invention including a plurality of pressure responsive sensors Fl-FSconnected in parallel to a current detector 34. Each of these sensors FlF5 includes a normally closed pressure responsive switch Gl-GS, a field effect transistor Tl-TS, and a current limiting resistor LRl-LRS, respectively. By connecting the gate of each of the transistors Tl-TS to ground, each of the sensors is in effect a constant current source when its associated switch is closed, the value of the current for each source being determined by its associated resistor.

One additional current source is also provided and is indicated in FIG. 4 as current source 36 which includes a field effect transistor 37 and resistor 38 connected in series between the +6 volt supply and ground. The gate of transistor 36 couples to ground to provide the constant currentoperation. The current source 36 may comprise circuitry similar to that depicted in FIG. 1 and is adapted to detect a change in current rather than a change in voltage. For example, with the values of resistance shown in FIG. 4, sensor Fl draws a current when closed of 0.6 milliamps. Similarly, sensor F2 draws 0.5 milliamps, sensor F3 draws 0.4 milliamps, sensor F4 draws 0.3 milliamps, and sensor F5 draws 0.2 milliamps. The constant current source 36 has a resistor value that causes a 0.1 milliamp current to be drawn to currentdetector 34. Thus a total of 2.1 milliamps is detected by current detector 34 when all of the sensors F l-F5 are in a closed position and the current source 36 is connected as shown.

As indicated in FIG. 4 the current detector 34 has seven output lines that detect which of the five sensors has been activted, or whether there has been a short condition or an open condition. The current detector 34, for example, could comprise a balancing circuit that is nulled at an input current of 2.1 milliamps and is indicated by a 1.4 milliamp reading.

8 is responsive to the different currents either greater or lessthan 2.1 milliamps to illuminate a predetermined indicator light correspondingto one of the sensors or a short or open condition.

The table below indicates the different conditions that can occur and the currents that are associated therewith.

CURRENT LEVEL ALARM CONDITION 2.1 milliamps No alarm 1.5 do. Fl activated 1.6 do. F2 activated 1.7 do. F3 activated 1.8 do. F4 activated 1.9 do. F5 activated 1.4 do. or less Open circuit condition. 3.0 do Short circuit condition.

In the above table it is seen that the constant current source 36 is provided in order to distinguish between an open circuit condition between sensors F1 and F2 and activation of sensor Fl. Without the source 36 both of these conditions would be indicated by a 1.5 milliamp reading. With the current source 36 in place one is indicated by a 1.5 milliamp reading and the other FIG. 5 shows one simplified embodiment for current detector 34. Thecurrent detector comprises a conventional meter 40 and associated limiting resistor 42. The value of resistor 42 is 2K ohms in FIG. 5 thereby defining the 3.0 milliamp (6 volts 2 K ohms) reading under short circuit conditions. Themeter 40 may be read by a person-monitoring the alarm system and the meter 40 may also be labeled so that when the reading is 2:1 milliamps no alarm condition exists. The other alarm conditions as depicted in the above table, are labeled on the meter to identify the different conditions that can occur.

In another embodiment of the invention the meter 40 may-be nulled to zero when reading 2.1 milliamps and then be caused to deflect either positively or negatively depending upon whether the sensor has been activated or an open or short circuit condition exists.

FIG. 6 shows one further embodiment of the invention using a plurality of sensor arrays similar to the one shown in FIG. 4. The block diagram of FIG. 6 shows sensor arrays Al-A4 each of which comprise a plurality of individual sensors such as sensor Fl-FS of FIG. 4. The current detector 34 would also comprise a display panel 44 including four zone lamps and five sensor lamps. With such an arrangement the current detector is adapted to determine which zone is being entered and which sensor in the zone is activated. Thus, with one of the zone lamps illuminated and one of the sensor lamps illuminated one can pin point which entrance means has been opened.

What is claimed is:

1. Alarm circuitry for use in a structure having entrance means, comprising;

current detection means,

a plurality of sensors each operatively associated with an entrance means and responsive to an activation thereof to change said sensor from a first state to ,a second state,

conductor means intercoupling said sensors in parallel to an input of said current detection means,

each said sensor including a current source for drawing a predetermined amount of current whereby each current source draws a different amount of current, g said detection means including means for registering a first condition when all said sensors are drawing current and means for registering a plurality of alarm conditions, each such alarm condition corresponding to activation of one of said sensors,

and an additional current source connected in parallel with said sensors.

2. Alarm circuitry as defined in claim 1 wherein each said current source includes a field effect transistor and resistor coupled in series between said conductor means.

3. Alarm circuitry as defined in claim 1 wherein said detection means includes a meter and a limiting resistor and means coupling the meter and limiting resistor in series, the input of said current detection means being defined at one end of the limiting resistor.

4. Alarm circuitry as defined in claim 1 wherein said additional current source draws a different amount of current than any of said sensors.

5. Alarm circuitry as defined in claim 4 wherein said additional current source draws less currentthan any one of said sensors.

6. Alarm circuitry as defined in claim 1 wherein said current detection means distinguishes a short circuit condition by a maximum current detection.

7. Alarm circuitry as defined in claim 5 wherein said additional current limiting means is added for distinguishing between two fault conditions.

8. Alarm circuitry for use in a structure having a plurality of ways through which access may be gained, comprising;

current detection means,

a plurality of sensor switch means each operatively associated with an access way and responsive to an activation thereof to change from a first state to a second state,

a plurality of constant current elements each arranged to draw a different amount of current,

means coupling each sensor switch means in series with an associated constant current element,

and means intercoupling the series connected constant current elements and sensor switch means in parallel to an input of said current detection means,

said current detection means including means for registering a first condition when all said sensor switch means are drawingcurrent, and for registering a plurality of alarm conditions, each such alarm condition corresponding to activation of one of said sensor switch means to its second state.

9. The alarm circuitry of claim 8 wherein each said constant current element comprises an active semiconductor device and a limiting resistor coupled in series.

10. The alarm circuitry of claim- 9 wherein said means intercoupling includes two different voltage lines.

11. The alarm circuitry of claim 10 wherein said semiconductor device comprises a field effect transistor having one electrode coupled to the sensor switch means, a second electrode coupled to the limiting resistor and a third electrode coupled to one of the two different voltage lines.

12. Alarm circuitry for use in a structure having a plurality of ways through which access may be gained comprising;

current detection means,

a plurality of sensors each operatively associated with an access way and each including a current source comprising a field effect transistor coupled in series with a current limiting resistor, each said current source drawing a different predetermined amount of current, and

conductor means intercoupling said sensors in parallel to an input of said current detection means, said current detection means including means for registering a first condition when all said sensors are drawing current and for registering a plurality of different alarm conditions each correspondingto activation of one of said sensors.

Claims

1. Alarm circuitry for use in a structure having entrance means, comprising; current detection means, a plurality of sensors each operatively associated with an entrance means and responsive to an activation thereof to change said sensor from a first state to a second state, conductor means intercoupling said sensors in parallel to an input of said current detection means, each said sensor including a current source for drawing a predetermined amount of current whereby each current source draws a different amount of current, said detection means including means for registering a first condition when all said sensors are drawing current and means for registering a plurality of alarm conditions, each such alarm condition corresponding to activation of one of said sensors, and an additional current source connected in parallel with said sensors.

5. Alarm circuitry as defined in claim 4 wherein said additional current source draws less current than any one of said sensors.

8. Alarm circuitry for use in a structure having a plurality of ways through which access may be gained, comprising; current detection means, a plurality of sensor switch means each operatively associated with an access way and responsive to an activation thereof to change from a first state to a second state, a plurality of constant current elements each arranged to draw a different amount of current, means coupling each sensor switch means in series with an associated constant current element, and means intercoupling the series connected constant current elements and sensor switch means in parallel to an input of said current detection means, said current detection means including means for registering a first condition when all said sensor switch means are drawing current, and for registering a plurality of alarm conditions, each such alarm condition corresponding to activation of one of said sensor switch means to its second state.

10. The alarm circuitry of claim 9 wherein said means intercoupling includes two different voltage lines.

12. Alarm circuitry for use in a structure having a plurality of ways through which access may be gained comprising; current detection means, a plurality of sensors each operatively associated with an access way and each including a current source comprising a field effect transistor coupled in series with a current limiting resistor, each said current source drawing a different predetermined amount of current, and conductor means intercoupling said sensors in parallel to an input of said current detection means, said current detection means including means for registering a first condition when all said sensors are drawing current and for registering a plurality of different alarm conditions each corresponding to activation of one of said sensors.