US3309689A

US3309689A - Intrusion detection system

Info

Publication number: US3309689A
Application number: US407713A
Authority: US
Inventors: Clare G Keeney
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1964-10-30
Filing date: 1964-10-30
Publication date: 1967-03-14
Anticipated expiration: 1984-03-14

Description

March 14, 1967 c, G. KEENEY INTRUSION DETECTION SYSTEM 2 Sheets-$heet 1 Filed 001:. 30, 1964 IE1 To LIGHT SOURCE & m

N Mw L R D 0 LM 6/ W T T D 9 R 5/ E P M A RU A C R A U m 4 O G/W U E R T M T 3 6/ R E cw L P M A INVENTOR.

CLARE G. KEENEY VOLTAGE DIV IDER 2O ATTORNEY United States fPatent ()filice Patented Mar. lid, "i967 3,309,689 HNTRUSION DETECHGN SYSTEM Iiare G. Keeney, Campbell, Calif assignor to Sylvania Electric Products Inc, a corporation of Delaware Filed Get. 3t 1964, Ser. No. $17,713 8 (Ilaims. (1. 340-4258) This invention relates to intrusion detection systems and more particularly to light responsive apparatus for detecting movement of an intruder in a prescribed area.

Although there are a number of different type intrusion detection systems such as radio frequency and sound detection systems, capacitance type perimeter detection systems, and so-called electric eye or beam interrupter perimeter detection systems, no effective economical system is available for protecting residential homes. This invention is directed toward provision of such a system.

In accordance with this invention, two radiation-sensitive elements, such as photoconductive cells, are physically arranged in a prescribed area so the normals to the radiation-sensitive surfaces intersect. A radiation source, such as a light, is located in front of the elements, preferably on the bisector of the angle of intersection. The elements form a network, such as a voltage divider. The output of the network, which is proportional to the relative change in the response of the elements to incident radiation, is connected to a signal processing circuit. When an intruder moves between one element and the effective radiation source (including the walls which may refiect radiation to the elements), there is a change in the intensity of the radiation incident on the one element whereas the intensity of the radiation incident on the other element remains relatively constant. Thus, the response of the one element and the output of the network vary. This variation is processed by the signal processing circuit. If the output of the processor exceeds a predetermined level, an alarm circuit indicates intrusion of the protected area. If the intensity of the radiation source varies, there is equal change in the radiation incident on and the response of each element. An alarm is not generated by this condition, however, since the output of the network remains relatively constant. In a modified form of this invention, a third radiation-sensitive element is located in the processing circuit. This third element maintains the sensitivity of the apparatus relatively constant by making the gain of the processor vary inversely with variations in the intensity of the ambient radiation environment.

An object of this invention is the provision of economical intrusion detection system for protecting residential homes.

Another object is the provision of intrusion detection apparatus that operates in conjunction with conventional sources of light.

Another object is the provision of intrusion detection apparatus which has relatively constant sensitivity and is self-compensating such that most changes in the intensity of ambient light will not alarm the system.

A further object is the provision of intrusion detection apparatus that is self-contained.

Another object is the provision of intrusion detection apparatus that is relatively simple to install and operate.

Still another object is the provision of intrusion detection apparatus which is highly portable for convenient relocation in areas to be protected.

These and other objects of this invention will be more fully understood from the following description of a preferred embodiment thereof, illustrated in the accompanying drawings in which:

FIGURE 1 is an isometric view of portable self-contained intrusion detection apparatus embodying this invention;

FIGURE 2 is a house, partly in section, illustrating the use of equipment embodying this invention to prevent undetected intrusion of a room thereof;

FIGURE 3 is a circuit diagram of a preferred embodient of this invention;

FIGURE 4 is a modified form of the embodiment of FIGURE 3 for making the processing circuitry thereof responsive to outputs of a plurality of pairs of photosensitive cells;

FIGURE 5 is a modified form of the embodiment of FIGURE 3 for preventing defeat thereof by an intruder; and

FIGURE 6 is a schematic plan view, partially in section, of a modified form of the invention showing photosensitive cells in a translucent prism.

Specific reference being had to the equipment illustrated in FEGURE 1, the intrusion detection apparatus 1 preferably comprises a box-like enclosure having a pair of

photoconductive cells

2 and 3 mounted in adjacent walls 4 and 5, respectively, of the enclosure. The photosensitive surfaces of

cells

2 and 3 lie in perpendicular planes, which may be coincident with or parallel to the planes of enclosure walls 4 and 5, respectively, intersecting at edge E. Although the planes including the photosensitive surfaces of cells 2. and 3 intersect to form an angle a of 90, these planes may be oriented to intersect at other angles between 0 and 180. The

cells

2 and 3 are electrically connected as illustrated in FIGURE 3 to form a voltage divider. A third photoconductive cell 6 is mounted in wall t. A potentiometer 7 provides for manual adjustment of the sensitivity of the apparatus. A speaker 8 mounted in wall 5 provides an audio indication of intrusion of the protected area.

The room in FIGURE 2 is illuminated by a light source 11, such as a conventional table lamp. The intrusion detection apparatus 1 is located opposite the light source between

doors

12 and 13. The light source 11 is preferably located on the bisector B of the 90 angle a, see FIGURE 1, formed by the intersection of the planes containing the photosensitive surfaces of

cells

2 and 3 to provide approximately equal illumination of each cell. Light rom source 11 is transmitted to intrusion detection apparatus 1 directly (path 14) and by reflection from walls (path 15) and objects in the room (path 16). The cells have maximum response to incident light that is normal to the plane of the photosensitive surfaces thereof.

When objects in the room are stationary, the resistance ratio of the voltage divider

network comprising cells

2 and 3 is constant and intrusion of the room is not indicated. When an intruder moves about the room, however, he shadows the cells by cutting light paths such as the one indicated at 15. As the intruder moves across the light path 15, the variation of the intensity of light incident on cell 2 is much greater than the corresponding variation in light incident on cell 3. Thus, the resistance of cell 2 changes with respect to the resistance of cell 3 and the resistance ratio of the voltage divider varies. The output of the voltage divider is processed and intrusion of the room indicated if it exceeds a predetermined level. The intrusion detection apparatus 1 illustrated in schematic form in FIGURE 3 comprises a voltage divider 20 having

photoconductive cells

2 and 3 electrically connected in series between the negative supply otential V (such as a battery) and a ground reference potential. The voltage divider may be considered to be a signal generator or voltage source and resistance connected across the input of buffer amplifier 23. The output of the voltage divider on line 21 is applied to an AC. amplifier 22 comprising amplifier stages 23 through 27, inclusive.

Amplifier stages

23 and 27 are preferably buffer stages which isolate the associated

amplifier stages

24 and 26 from the voltage divider and output circuitry, respectively.

49 Amplifier stage is preferably a buffer amplifier which isolates the

amplifier stages

24 and 26 from each other and provides means for adjusting the sensitivity of the apparatus. The buffer amplifiers may, by way of example, be emitter followers.

The sensitivity of the apparatus is controlled by a resistive biasing network in the emitter circuit of buffer amplifier 25. The biasing network is a voltage divider comprising potentiometer 7 and photoconductive cell 6. Potentiometer '7 is electrically connected in series between the emitter of transistor 25' and ground. The control arm of the potentiometer is connected through cell 6 to ground. The sensitivity is adjusted automatically by cell 6 to compensate for variations in the ambient light intensity in the protected area. The sensitivity is adjusted manually by potentiometer 7.

The output of A.C. amplifier 22 is applied on line 28 to a clamper 31 comprising capacitor 32, resistor 33 and diode 34. The clamped signal is detected by an amplitude detector 35 comprising diode 36, capacitor 37 and resistor 33.

The detected signal is applied on line 40 to a threshold circuit comprising a monostable multivibrator 41. The output of multivibrator 41 is applied on line 44 to the base of a transistor 45 of switch 45. Transistor 45 is electrically connected across the input circuit of transistor 46 of driver 46. An astable multivibrator 47, which oscillates at an audio rate, is also connected across the input of driver 46.

The output of driver 46 is electrically connected through a coupling transformer 48 to the negative supply potential V. The output of the driver is also coupled through the transformer to a speaker 49 for providing an audio indication of intrusion of the protected area.

When objects in the protected area are stationary, the intensity of light incident on the photosensitive surfaces of

cells

2 and 3 is relatively constant. The resistance ratio determining the output of the voltage divider is therefore constant and is represented as s where R is the resistance of cell 2 and R is the resistance of cell 3.

The magnitude of the output of A.C. amplifier 22 and the signal on line 49 are relatively low (the latter being a small negative D.C. signal). This signal on line 4d reverse biases and cuts off transistor 42 of multivibrator 41. The signal on line 44, therefore, is a negative voltage that forward biases transistor 45 and drives it into saturation. Conduction of transistor 45' short circuits the input of driver 4-6, cutting off the latter. Thus, the audio output of multivibrator 47 is blocked from speaker 49 and intrusion of the protected area is not indicated.

If an intruder moves about in the protected area, he will shadow one of the photoconductive cells and different amounts of light will be incident on

cells

2 and 3. Consider that movement of an intruder causes the light incident on cell 2 to decrease. This causes the resistance R of the cell to increase. The resultant resistance ratio of the voltage divider is represented as 2+ zl 2+ 2) s where AR represents the change in the resistance of cell 2. This change causes the voltage divider output on line 21 to become more negative, i.e., a negative going signal, that is amplified.

If the intruder generates a repetitive varying signal, the amplified signal on line 28 is effectively clamped to ground and is doubled by operation of clamper 31. The clamped signal is detected to provide a D.C. signal proportional to the peak magnitude thereof. If this DC. signal is more negative than a predetermined level, transistor Cal 42 is forward-biased and conducts. The output of transistor 42 is a very small negative potential (approximately 0.3 volt).

Transistor 55, reverse-biased and cut off by the output of transistor 42, is effectively an open circuit across the input of driver 46 and the latter conducts. The audio output of multivibrator 47 is amplified by the driver and is coupled to speaker 49 to provide an audio indication of intrusion of the protected area.

It is desirable that changes in intensity of the light source do not cause the detection apparatus to indicate intrusion of the protected area. This is accomplished by employing

photoconductive cells

2 and 3 as elements of voltage divider 29.

A change in the intensity of the light source 11 (see FIGURE '2) is reflected equally on

cells

2 and 3, since the light source is positioned to provide approximately equal illumination of each cell. This change in light intensity causes an equal change in the resistance of each cell. The resultant resistance ratio of voltage divider 29 is represented as R2+ z 2+ 2)+( 3+ s) where AR; and AR;, represent the change in resistance of

cells

2 and 3, respectively. This resistance ratio and the output of the voltage divider are relatively constant for such changes in light intensity since the cell resistance R is equal to the cell resistance R and the resistance change AR is equal to the resistance change AR (assuming the response curves of the cells are identical). Thus, a change in the intensity of light source 11 does not cause detection apparatus 1 to indicate intrusion of the protected area, unless the light source is abruptly extinguished.

Intrusion detection apparatus 1 may be employed in a home to protect a detached building, such as a garage, over a time period from sunset until after sunrise. During such an operating period, the apparatus is subject to a wide variation of ambient light intensity. It is desirable in such applications that system sensitivity to intruder movements remain relatively constant as the intensity of the light source varies.

When an intruder moves in the protected area and shadows only one cell, there is a change in the resistance of the shadowed cell, the resistance ratio (see Equation 1) and the output of the voltage divider. The resistance of the cell is represented as ln(R):Kln(I)+ C (4) in which ln is the natural logarithm, R is the cell resistance, K and K are constants and I is light intensity. The rate of change of the resistance of the cell, with respect to the rate of change of light intensity, is proportional to the ratio of the cell resistance and light intensity and is represented as where d is the derivative.

The absolute value of the output V of the voltage The rate of change of the output V of the voltage divider, with respect to the resistance of cell 2, is obtained by differentiating Equation 6, and is represented as law) l iRi) l 1) The rate of change of the output V of the voltage divider, as a function of the rate of change of the light intensity is obtained from Equations 5 and 7 (where R:R =R and is represented as 1 (8) Thus, the rate of change of the output V of the voltage divi er (which is proportional to the system sensitivity) is proportional to the ratio of the rate of change of the light intensity to the absolute light intensity. This ratio, d(I)/1, represents the percentage change in the tensity of light incident on the shadowed cell. This percentage change in light intensity, for a given movement within the protected area, is the same whether the effective light intensity is bright or dim, since the shadowing of the cell is the same in each case. Thus, the sensitivity of the intrusion detection apparatus is constant for different intensities of the light source.

In an actual embodiment and application of intrusion detection apparatus 1, the system sensitivity is not constant; since ditferent photoconductive cells do not have identical responses, the cells are loaded by external circuitry and the apparatus may not be adjusted so equal amounts of light are incident on each cell. The third photoconductive cell 6 is employed in this invention to automatically adjust the sensitivity of apparatus 1 to make the system sensitivity more nearly constant for changes in intensity of the light source.

Cell 6 is preferably located so its photosensitive surface faces the same direction as one of the cells 2 and 3 (see FIGURE 1). The cell 6 is electrically connected to change the gain of A.C. amplifier 22 (see FIG- URE 2).

The resistance of

cells

2 and 3 decrease as the intensity of the light source increases. This decrease in cell resistance decreases the internal impedance of the voltage divider (signal generator) and the system sensitivity therefore increases.

The resistance of cell 6 also decreases as the intensity of the light source increases. This resistance change decreases the gain of buffer amplifier 25 and less of the AC. signal is applied to transistor 26'. This decrease in amplifier gain caused by cell 6 tends to cancel the increase in amplifier gain and system sensitivity caused by the impedance change of the voltage divider and the system sensitivity therefore remains relatively constant.

Conversely, the resistance of

cells

2, 3 and 6 increase as the intensity of the light source decreases. This change in resistance of

cells

2 and 3 increases the internal impedance of the voltage divider (signal generator) and the system sensitivity therefore decreases. The increase in the resistance of cell'6, however, increases the gain of buffer amplifier 25 and more of the AC. signal is applied to transistor 26. Thus, the increase in gain caused by cell 6 cancels the decrease in gain and system sensitivity caused by the voltage divider and the system sensitivity remains relatively constant. The system sensitivity is manually adjusted by varying potentiometer 7.

In certain applications, it may be necessary to employ a plurality of pairs of

photoconductive cells

2 and 3 to protect a large or irregularly shaped area or to simultaneously protect a number of physically separate areas. In such applications, it may not be desir able or economically feasible to employ more than one of the self-contained intrusion detection apparatus illustrated in FIGURE l. A modified form of this invention in which a plurality of cell pairs are connected to common processing circuitry in one detection apparatus is illustrated in FIGURE 4.

Voltage dividers 20a, 29b, and 28c comprise cell pairs 2a3a, 212-312 and 2c3c, each electrically connected between the negative supply potential V and the ground reference potential. The cell pairs may be located in physically separate areas such as kitchen, bedroom and detached garage. The outputs of the voltage dividers are applied on associated

lines

51a, 51b and 51c to a summing

network comprising resistors

52a, 52b, 52c and 53. The sum of the respective outputs of the cell pairs is applied on line 2 to the base of transistor 23' of AC. amplifier 22 (see FIGURE 3).

In certain instances, intruders may deliberately attempt to defeat the intrusion detection apparatus. The embodiment shown in FIGURE 5 comprises means for preventing an intruder who has gained access to the light source from defeating the system by extinguishing the ight or increasing the intensity of the light source to such a degree as to saturate the cells and thus minimize or eliminate their response to changes in ambient light. The voltage divider 2%? comprises cell 3' connected through res tor 55 to the negative supply potential and cell 2 connected through resistor 57 to the ground ref rence potential. The voltage divider output on line 58 is applied to processing circuitry comprising a 11C. amplifier 59', a threshold detector 60 and an alarm circuit 61. The voltage divider output on line 62 is applied to similar processing circuitry comprising amplifier 63, detector 64 and alarm circuit 65. Detectors 60 and as may, by way of example, comprise Schmitt trigger circuits. The voltage divider output on line 21 is applied to the base of transistor 23' of AC. amplifier 22 (see FIGURE 3).

The resistance of cells 2. and 3 increase as the intensity of incident light decreases, such as when the light source is gradually turned off. This resistance change causes the potential on line 58 to decrease (become more negative). When the potential on line 58 exceeds (is more negative than) a predetermined level, threshold detector 60 conducts to indicate tampering or failure of the light source.

Similarly, the resistance of cells 2 and 3' decrease as the intensity of incident light increases, such as when the intensity of the light source is increased. This decrease in resistance causes the potential on line 52 to decrease (become more negative). When the potential on line 62, is more negative than a predetermined level, threshold detector 64 conducts and tampering with or failure of the light source is indicated.

Another embodiment of the invention shown in FIG- URE 6 features means for protectively covering the active surfaces of the photocells and for preventing false alarms due to insects such as flies moving on those surfaces. A translucent prism 79, such as a cubical body of glass, has

photoconductive cells

71 and 72 mounted adjacent to prism surfaces 73 and 74, respectively. The photosensitive surfaces '71:: and 72a of the cells face into the prism body 7% so that rays r and 7- from the li ht source, not shown, pass through the body before reaching the active cell surfaces. The prism dimensions a between cell surfaces 71a and 72a and the opposite prism surfaces and 7011, respectively, preferably are sulficient to adequately space insects from surfaces 71:1 and 72a and thus minimize deleterious shadowing effects on the cells from these sources. The maximum responses of

cells

71 and 72 occur for light rays that are normal to surfaces 71a and 720, as for the previously described embodiments.

Cells

71 and 72 are electrically connected to processing circuits described in conjunction with FIG- URE 3 to indicate intrusion.

Although this invention has been shown and described in relation to preferred embodiments thereof, variations and modifications will be apparent to those skilled in the art. The scope and breadth of this invention is therefore to be determined from the following claims rather than from the above detailed description.

What is claimed is:

1. A system for detecting intrusion of a room comprising an electric lamp at a first location in said room,

detection apparatus at a second location in said room,

said detection apparatus comprising a body having a plurality of plane sides with the planes of the sides intersecting each other,

said body being oriented with the intersection of the planes of two of said sides closer to said lamp than the remainder of the body,

a light responsive element mounted on the exterior of each of said two sides and substantially equally spaced from said lamp and at a greater distance therefrom than said intersection of the planes of said two sides,

each of said elements being characterized by an electrical resistance that is a function of the intensity of light incident thereupon,

a source of electrical energy connected to said elements,

said elements being electrically connected in series to form a voltage divider network,

a signal processing circuit connected to said network and being responsive to a relative change in voltages across said elements to produce an output, and

an alarm circuit responsive to said output of the signal processing circuit to produce an alarm.

2. A system according to claim 1 with a plurality of pairs of said elements at dilferent locations,

the elements of each pair being oriented relative to each other so that normals from their surfaces diverge,

said signal processing circuit being responsive to each of the outputs of said pairs of elements whereby an alarm is produced by an intrusion at any of said locations.

3. A system for detecting intrusion of a room comprising an electric lamp at a first location in said room,

detection apparatus at a second location in said room.

said body being oriented relative to the lamp so that the planes of two of said sides converge in a direction toward said lamp,

a light-responsive element mounted on the exterior of each of said two sides and substantially equally spaced from said lamp and at a great-er distance therefrom than the inter section of the planes of said two sides,

"a source of electrical energy connected to said elements,

said elements being electrically connected in series form a voltage divider network,

a signal processing circuit connected to said network and responsive to a relative change in voltages across said elements to produce an output,

an alarm circuit responsive to said output of the signal processing circuit to produce an alarm, and

a third light-responsive element mounted on the exterior of said body and connected to said signal processing circuit for changing the sensitivity of the signal processing circuit in inverse proportion to the change in absolute level of light intensity in the room.

4. A system for detecting intrusion of a room comprising :an electric lamp at a first location in said room, and

detection apparatus comprising 7 a body at a second location in said room having a plurality of plane sides with the planes of the sides intersecting each other,

said body being oriented relative to the lamp so that the planes of two said sides converge in a direction toward said lamp,

a light-responsive element mounted on the exterior of each of said two sides and substantially equally spaced from said lamp and at a greater distance therefrom than the intersection of the planes of said two sides,

each of said elements being characterized by an electrical resistance that is a function f the intensity of light incident thereupon,

a source of electrical energy connected to said elements,

an alarm circuit responsive to said output of the signal processing circuit to produce an alarm,

circuit means responsive to the total change in voltage across said elements producing an output when said total voltage reaches a limit, and

a second alarm circuit connected to said circuit means and responsive 0t an output therefrom for producing an alarm.

5. A system for detecting intrusion of a room comprising in combination a source of light at a first location in said room,

and detection apparatus at a second location in said room,

comprising a body having at least two sides lying in intersecting planes,

a light-responsive element having a photosensitive surface and mounted on each of said sides whereby to change a condition in response to a change in the intensity of light incident on said photosensitive surface,

said light source being spaced farther from said elements than from the intersection of said planes whereby said photosensitive surfaces are divergent relative to each other,

means for comparing said conditions controlled by said elements for deriving an output when one condition changes relative to the other,

signal processing means responsive to said relative change of conditions for indicating an alarm and a third light-responsive element on said body elec trically connected to said signal processing means for changing the sensitivity thereof inversely with the change in absolute level of intensity of light in said room.

6. A system for detecting intrusion of a room comprising in combination a source of light at a first location in said room,

and detection apparatus at a second location in said room comprising a body having at least two sides lying in intersecting planes,

a light-responsive element having a photosensitive surface and mounted on each of said sides whereby to change a condition in response to a change in the intensity of light incident on said photo-sensitive surface,

said light source being spaced farther from said elements than from the intersection of said planes whereby said photosensitive surfaces are divergent relative to each other, means for comparing said conditions controlled by said elements for deriving an output when one condition changes relative to the other, signal processing means responsive to said rela tive change of conditions for indicating an alarm, circuit means responsive to the total change of intensity of light in said room and producing an output when said total change of light intensity reaches a limit, and a second alarm circuit connected to said circuit means and responsive to said output therefrom to produce an alarm. 7. A system for detecting intrusion of a room comprising in combination a source of light at a first location in said room,

and portable detection apparatus comprising a body at a second location in said room and having at least two external sides lying in intersecting planes so that normals from said sides diverge therefrom, light-responsive elements having maximum response to light received along the said normals being mounted on said body sides, respectively, each of said elements being adapted to change a condition in response to a change in the intensity of the light received by the element,

of con- References Cited by the Examiner UNITED STATES PATENTS 1,877,279 9/1932 Dawson 340-258 2,016,036 10/1935 Fitzgerald 340228 2,227,147 12/1940 Lindsay. 2,656,527 10/1953 Tillman. 3,089,065 5/1963 Worden. 3,191,048 6/1965 Cowen 340228 X 3,221,317 11/1965 Ferrigno 3403S4 X NEIL C. READ, Primary Examiner.

THOMAS E. HABECKER, Examiner.

R. GOLDMAN, D. L. TRAFTON, Assistant Examiners.

Claims

1. A SYSTEM FOR DETECTING INTRUSION OF A ROOM COMPRISING AN ELECTRIC LAMP AT A FIRST LOCATION IN SAID ROOM, DETECTION APPARATUS AT A SECOND LOCATION IN SAID ROOM, SAID DETECTION APPARATUS COMPRISING A BODY HAVING A PLURALITY OF PLANE SIDES WITH THE PLANES OF THE SIDES INTERSECTING EACH OTHER, SAID BODY BEING ORIENTED WITH THE INTERSECTION OF THE PLANES OF TWO OF SAID SIDES CLOSER TO SAID LAMP THAN THE REMAINDER OF THE BODY, A LIGHT RESPONSIVE ELEMENT MOUNTED ON THE EXTERIOR OF EACH OF SAID TWO SIDES AND SUBSTANTIALLY EQUALLY SPACED FROM SAID LAMP AND AT A GREATER DISTANCE THEREFROM THAN SAID INTERSECTION OF THE PLANES OF SAID TWO SIDES, EACH OF SAID ELEMENTS BEING CHARACTERIZED BY AN ELECTRICAL RESISTANCE THAT IS A FUNCTION OF THE INTENSITY OF LIGHT INCIDENT THEREUPON, A SOURCE OF ELECTRICAL ENERGY CONNECTED TO SAID ELEMENTS, SAID ELEMENTS BEING ELECTRICALLY CONNECTED IN SERIES TO FORM A VOLTAGE DIVIDER NETWORK, A SIGNAL PROCESSING CIRCUIT CONNECTED TO SAID NETWORK AND BEING RESPONSIVE TO A RELATIVE CHANGE IN VOLTAGES ACROSS SAID ELEMENTS TO PRODUCE AN OUTPUT, AND AN ALARM CIRCUIT RESPONSIVE TO SAID OUTPUT OF THE SIGNAL PROCESSING CIRCUIT TO PRODUCE AN ALARM.