Alarm System Including Remote Signalling Means

The present invention generally relates to alarm systems, and more specifically to an alarm system which includes one or more remote stations connected to a control station by only three leads but which nevertheless permits remote control of the alarm system and which provides numerous visual indications of the status of the alarm system at the central as well as remote locations.

There are numerous alarm systems, including intrusion as well as fire, already known. While the basic circuitry of such alarms is relatively simple, many of the known alarms incorporate means for increasing the effectiveness of the alarm system, decreasing the number of false alarms, minimizing power consumption or preventing tampering with the system. Typical alarms of the aforementioned types are disclosed in U.S. Pat. Nos. 3,351,934; 3,641,552 and 3,828,340.

The use of remote stations which permit supervision of the alarm system are also known. With such an arrangement, there is typically provided central control box which includes the bulk of the circuitry of the system as well as the power supplies and stand-by batteries for energizing the system. The control boxes are normally concealed and locked so that tampering therewith is not possible without actuating the alarm. When remote stations are provided, these are connected to the control box and permit limited control of the alarm system. Sometimes, the remote stations provide an indication of the status of the alarm system. U.S. patents which disclose alarm systems having remote stations are U.S. Pat. Nos. 3,174,143 and 3,747,093. The use of LEDs in alarm systems is also known and disclosed in U.S. Pat. No. 3,706,987.

As is well known to those skilled in the art, a major expense involved in the installation of alarm systems is the labor required to wire the electrically conductive leads or wires throughout the premises to be protected. When remote locations are utilized, these must clearly also be wired to the control box, and this involves additional time and expense. In this connection, an objective of numerous prior art alarm systems has been to connect the remote stations which cooperate with the central control box by the minimum number of leads while providing the maximum number of indications of the status of the alarm system. The invention to be described permits the use of only three leads to connect each remote station to the control station while providing four or five distinct visual indications of the status of the alarm system, as well as providing supervisory control over the system to permit the same to be "enabled" or "disabled" from any of the remote locations. The known alarm systems have not provided such extensive information concerning the status of an alarm system, as well as providing control of the system at remote locations when the remote stations have been connected to the control station by only three leads or conductors.

It is an object of the present invention to provide an alarm system which does not have the above-described disadvantages associated with the prior art known systems.

It is another object of the present invention to provide an alarm system which permits supervisory control at remote locations as well as numerous indications concerning the status of the alarm system at the remote locations.

It is still another object of the present invention to provide an alarm system as in the last object which is both simple in construction and economical to manufacture.

It is yet another object of the present invention to provide an alarm system as in the above objects, wherein the remote stations are connected to the central control station by only three leads while providing supervisory control as well as five distinct visual indications concerning the status of the alarm system at each of the remote stations.

It is a further object of the present invention to provide an alarm system having both instant and delay loops, and which has means for providing five distinct visual indications concerning the status of the alarm system and the conditions of the loops at one or each of a plurality of remote stations.

The alarm system of the present invention includes an alarm circuit and first and second alarm sensing means, the state of each of which is changable between a non-alarm condition and an alarm condition. According to the present invention, the alarm system comprises pulsing means connecting to said first and second alarm sensing means for generating a substantially constant voltage, in said non-alarm condition of said first alarm sensing means and the alarm condition of said second alarm sensing means, and a pulsating voltage in said alarm condition of said first alarm sensing means. Switch means is provided for enabling and disabling the alarm circuit and for providing higher and lower reference voltages as a function of the enabled and disabled conditions of the alarm circuit. Indicating means are provided connected between said pulsing and switching means and adapted to permit current flow in each of two opposite directions therethrough as the function of the relative voltages provided by both said pulsating and switching means across said indicating means. Said indicating means provides a different indication for each of the two directions of current flowing therethrough as well as a no-current indication. In this manner, a total of five indications may be obtained by selective passage of currents through said indicating means which currents may flow in one of the two opposing directions and which may be zero, constant or pulsed in each of said directions to thereby provide two different indications in both the constant or pulsed modes, as well as one indication when the current flow is reduced to zero.

Further switch means is provided for enabling and disabling the alarm system. The indicating means as well as said further switch means are connected between said pulsing and switching means by only three leads or conductors. Where remote stations are provided, each of the remote stations includes indicating means and further switch means connected in parallel to the first-mentioned indicating means and further switch means by only three leads, each remote station permitting supervisory control over the alarm system as well as providing the numerous visual indications which are provided at the control station.

The first alarm sensing means includes an instant loop while the second alarm sensing means includes a delay loop. The delay loop is connected to said pulsing means by way of still further switch means which is arranged to disconnect said delay loop from said pulsing means when the instant loop is open. In this way the alarm system gives priority to said instant loop over the delay loop so as to provide a pulsating signal through said indicating means when the instant loop is open irrespective of the condition of the delay loop. However, when the instant loop is closed, a constant indication is provided when the delay loop is open.

Other objects and advantages of the invention will become apparent from the following detailed description, reference being made to the accompanying drawings.

FIG. 1 is a schematic representation of an alarm system in accordance with the present invention, showing a remote station having an indicator light and a key switch connected to a central or control station by only three leads;

FIG. 2 is a schematic representation, partially in block diagram form, showing the details of the control station and remote station shown in FIG. 1, and further showing the manner in which a plurality of remote stations are connected parallel to each other and connected to the control station by only three leads or conductors;

FIG. 3 is similar to FIG. 2, but only showing the details of the alarm system at the control station during a non-alarm condition of the system, as opposed to the alarm condition shown in FIG. 2; and

FIG. 4 is a table summarizing the visual indications which are provided at each of the stations, control and remote, during different conditions of the alarm system.

Referring to the figures, wherein the similar or identical parts are identified by the same numerals throughout, and first referring to FIG. 1, an alarm system in accordance with the present invention is generally designated by the reference numeral 10.

The alarm system 10 includes a control station 12, the circuitry of which is generally housed within a control box 14 which is made tamper proof and concealed.

Most simple present-day alarm systems include a single closed "instant loop" and are known as "closed loop" systems. Such a closed instant loop is designated by the reference numeral 16 in FIG. 1, and such a loop is generally preferred because it is somewhat more effective and tamper proof than "open loop" systems. While the "closed loop" alarm systems require a continuous flow of current through the loop during the non-alarm condition, these currents can be made sufficiently small so that there is minimal drain on the source of electrical energy. Such simple, closed instant loops used for intrusion alarm systems typically include switches at the openings to the premises to be protected, including doors and windows, which switches are closed during the non-alarm condition when the openings are also closed. When one of the openings is violated, the respective switch associated with that opening breaks or opens the loop to interrupt the current flow therethrough. Such termination of current flow is detected by the alarm system and suitable visual or audio indications may be provided. The opened loop is designated by the reference numeral 16' in FIG. 2.

There are several disadvantages inherent with the simple, single closed loop system described above. Firstly, the use of a single continuous loop throughout the premises to be protected makes it difficult to quickly locate the precise point at which the loop was opened or which of the openings of the premises was violated. To overcome this problem, more sophisticated alarm systems have utilized the concept of multiple-zone protection, where, in essence, a different loop is utilized to cover one of a large number of zones within the premises so that an indication may not only be given of the existence of an alarm condition but also in which zone of the premises the alarm condition was created.

A second problem which has arisen in conjunction with the simple single instant loop arrangements is that an alarm is produced as soon as the instant loop is opened, irrespective of the opening of the premises which is violated. This prevents the owner of the premises from opening the door, for example, and leaving the premises without simultaneously issuing an alarm. To solve this problem, it has been proposed to utilize delay loops which are similar to the instant loops except that opening of a delay loop actuates a delay circuit which prevents an alarm from issuing until a predetermined amount of time has elapsed from the initial opening of the door or the like. Such a delay loop normally covers those openings to the premises which are contemplated to be utilized for entry and exit of the premises so as to avoid premature issuance of an alarm upon entry or exit by the owner from the premises. In effect, an alarm system utilizing an instant and a delay loop is equivalent to a two zone alarm wherein the first zone comprises all those openings which are not typically utilized for entry and exit by the owner and the second zone comprises those openings so utilized by the owner. It is in such a case desirable to provide a separate indication as to the status of each of the loops, namely the instant and the delay loops. The reason for this is that an indication to the owner that either the instant or delay loops are open aids him in locating the opening and closing the same.

The initial discussion will primarily relate to an instant loop arrangement only. With such an arrangement, as will become evident, four possible visual indications may be provided at the control box as well as each of the remote stations. When a delay loop is utilized, as will be described hereafter, a fifth indication may be provided on the same indicating means. In this manner, the indicating means of the present invention provides, in an essentially single indicator, a complete status of the alarm system.

Referring to FIG. 1, an important feature of the present invention is the provision of one or more remote stations 18, each of which includes the maximum supervisory control as well as remote indications concerning the status of the alarm system while being connected to the control station by only three electrical conductors or leads. Accordingly, the remote station 18 includes an indicator light 20 and a key switch 22, the remote station 18 being connected to the control station by means of only three electrical conductors or leads 23-25. As to be more fully described hereafter, the key switch 22 permits the alarm system 10 to be "enabled" or "disabled" at any remote station. Additionally, the indicator light 20 provides four distinct visual indications when only an instant loop 16 is used, or five indications when a delay loop is also used, concerning the status of the alarm system.

In FIG. 2, the control station 12 is shown to include an alarm monitoring means 26 which is connected to the open instant loop 16' during an alarm condition. The alarm monitoring circuitry 26 typically includes means for driving a current through the normally closed instant loop 16 as well as means for detecting when such current flow has terminated due to a violation of the loop or one of the protected entrances or openings.

The alarm monitoring circuitry 26 is connected to pulsing means generally designated by the reference numeral 28, to be more fully described below. Advantageously, the alarm monitoring circuitry 26 is also connected to timing circuitry 30 which in turn is connected to audio producing means 32. As will be described below, visual indications are provided concerning the status of the alarm system 10. However, as with most known alarm systems, there is advantageously generated a loud audio signal upon the actuation or energization of the alarm system during the alarm condition. Such a loud sound is intended to frighten a trespasser who has violated one of the openings and the closed condition of the instant loop 16, in the case of burglary for example, or to alarm the inhabitants of the dwelling in the case of fire. Such a loud sound is most effective during the first few moments and continued generation of such loud sound frequently serves no useful purpose. Accordingly, the timing circuitry 30 is advantageously adjusted so that upon the generation of an alarm condition the audio producing means 32 is energized for only a predetermined time period, such as for example 15 minutes. After such time interval, the audio producing means is de-energized so that further sound is terminated. However, the visual indications are advantageously maintained so that information may be obtained as to the present status of the alarm system, namely whether the alarm system is in an "enabled" or a "disabled" condition and whether the instant loop 16 is closed or open.

As described above, the central or control station 12 is connected to the remote stations by means of three leads or conductors 23-25. Referring specifically to FIG. 2, the pulsing means 28 is shown to include a multi-vibrator 34 connected to a transistor 36 by means of a current-limiting resistor 38. It should be clear that any suitable pulsing means may be used and that the pulsing means being described is merely illustrative of the type which has been found satisfactory in this application.

The transistor 36 has a grounded emitter electrode, with the base being connected to the resistor 38. The base of the transistor 36 is also connected to the optional delay loop circuitry, as will be fully discussed below. A collector resistor 40 is connected between the collector of the transistor 36 and a source of voltage B.sup.+. As is well known, the application of a very low voltage approaching ground potential at the base of the transistor 36 places the transistor in a non-conductive mode and the supply of voltage B.sup.+ appears at the collector of the transistor. On the other hand, when a sufficiently positive voltage is applied to the base, typically above several tenths of a volt, the transistor 36 becomes saturated and the collector voltage drops to a potential close to ground potential.

A relay generally designated by the reference numeral 42 is provided which has a relay coil 42a, one terminal of which is connected to the source of voltage B.sup.+. The relay 42 also has a pair of contacts 42b one of which contacts is grounded or connected to ground potential and the other contact of which is connected through a resistor 44 to the source of voltage B.sup.+. For reasons which will become apparent hereafter, the relay 42 is advantageously a ratchet-type relay which permits successive opening and closing of the contact 42b upon each energization of the relay coil 42a. Accordingly, subsequent to one energization of the relay coil 42a, the contacts 42b are, for example, opened or separated and remain open. Upon a subsequent energization, the contacts 42b close or come into contact and remain in contact until the relay is again energized.

Connected between the collector of the transistor 36 and a point between the contacts 42b and resistor 44 there is provided an indicator light 20 which, in the present invention comprises a pair of back-to-back LED diodes d.sub.1 and d.sub.2. The diodes d.sub.1 and d.sub.2 are advantageously packaged in a single housing or encapsulated within a single package, with the anode of d.sub.1 and the cathode of d.sub.2 forming a terminal 46 while the cathode of d.sub.1 and the anode of d.sub.2 forming a terminal 48. Accordingly, the terminal 48 of the indicator light 20 is connected to the collector of the transistor 36 while the terminal 48 is connected to the contacts 42b and resistor 44 as above described.

Each of the diodes d.sub.1 and d.sub.2 is adapted to permit current flow in one directon only. Thus, the diode d.sub.1 permits current flow from the collector 36 towards the resistor 44 while the diode d.sub.2 only permits current flow in a reverse direction or from the resistor 44 towards the transistor 36. In accordance with the presently preferred embodiment, the diode d.sub.1 generates one color when current flows therethrough, while the diode d.sub.2 generates a different and distinct color when current flows through it. Accordingly, depending upon the direction of current flow through the indicator light 20, a corresponding color will be emitted by the indicator light 20.

The key switch 22 is connected between the collector of the transistor 36 and the other terminal of the delay coil 42a. Since one terminal of the relay coil is connected to a positive source of voltage B.sup.+, it should be clear that the grounding or lowering of the potential of the other terminal of the relay coil to a sufficient degree energizes the relay 42 to alter the condition of the relay contacts 42b.

What has been described above is the circuitry within the control station 12. However, each of the remote stations 18' and 18" similarly includes an indicator light 20', 20" and a key switch 22', 22" connected in parallel to the corresponding indicator light 20 and to key switch 22 at the control station. It should be clear that while only two remote stations are shown, any desired number of remote stations may be utilized and connected in parallel across the leads or conductors 23-25 as suggested by the dashed lines in FIG. 2.

The multi-vibrator 34 is connected to the alarm monitoring circuitry 26 as above described. One condition of the alarm circuit 10 is shown in FIG. 2. When the loop 16' is open, well-known circuitry may be utilized to cause multi-vibrator 34 to begin pulsating and generate a square wave as shown in FIG. 2. The output voltage of the multi-vibrator 34 need only fluctuate from approximately ground potential to over several tenths of a volt in order to turn the transistor 36 on and off respectively. When the potential at the base of the transistor 36 drops to near ground potential, the transistor 36 is turned off and the collector voltage rises to approximately B.sup.+. On the other hand, when the voltage at the base rises above several tenths of a volt, the transistor 36 become saturated and the collector drops to approximately ground potential.

The alarm monitoring circuitry 36 and the multi-vibrator 34 are selected to produce a fluctuating or pulsating voltage as shown only when the loop 16 is open. On the other hand, when the loop is closed, the output of the multi-vibrator 34 is advantageously at the positive as opposed to the ground potential of the fluctuating voltage. Thus, when the loop 16 is closed, a contant, permanent or non-fluctuating voltage of over several tenths of a volt appears at the base of the transistor 36, except when the base is grounded by the delay loop circuitry, as to be described below. When the voltage at the base of the transistor 36 is maintained at a positive potential, the transistor remains on and the collector voltage thereby remains at the low saturation level near ground potential. On the other hand, when the base is grounded the collector voltage becomes equal to approximately B.sup.+.

Only the pulsating means 28, including the multi-vibrator 34 and the transistor 36, thereby controls the potential at the terminal 46 of the indicator light 20, when an optional delay loop is not used. The terminal 48 has the potential thereof controlled by the condition of the relay contacts 42b. When the contacts are closed as shown in FIG. 2, the terminal 48 is grounded or at zero potential while opening of the contacts 42b places a positive potential at the terminal 48 close to B.sup.+.

When the potential at the terminal 46 is higher than the potential at the terminal 48, a current will flow only through the diode d.sub.1 in the direction indicated by the arrow and marked by "I". When the potential across the indicator 20 is reversed, or when the potential at the terminal 48 is higher than the potential at the terminal 46, the current flow is reversed and flows in a direction opposite to that indicated by the arrow.

For purposes of illustration only, the diodes d.sub.1 may be selected to generate a green light when current flows therethrough while the diodes d.sub.2 generate a red light when currents flow therethrough. Clearly, since all the indicators 20, 20' and 20" are connected in parallel, application of voltage potentials as suggested above simultaneously applies the same voltages across similar terminals on all the indicators and comparable currents flow with resultant comparable colors being emitted by all the indicators at all the remote locations. Similarly, since all the key switches 22, 22' and 22" are connected in parallel, the closure of any of the key switches is effective to energize the relay coil 42a and thereby change the condition of the relay contacts 42b as above described.

The operation of the alarm system 10, which does not use a delay loop, will now be described to the extent to which it has not been described above. Referring to FIG. 2, the relay contacts 42b are shown closed to correspond to the off or disabled condition of the alarm system. Accordingly, the terminal 48 of each of the indicators is grounded. Since the instant loop 16' is open, indicating an alarm condition, the multi-vibrator 34 is caused to generate a square voltage output at the base of the transistor 36 to thereby turn the transistor on and off and correspondingly apply a fluctuating voltage at the terminal 46. When the potential of the collector drops to near ground potential, little or no voltage appears across the indicators 20, 20' and 20" and no light is emitted therefrom. However, when the transistor 36 becomes non-conductive, the potential of the terminal 46 is raised to approximately B.sup.+ and the current "I" flows through the diode d.sub.1 with resultant pulsating green light being generated at each of the indicators at the control station 12 as well as at the remote stations 18' and 18".

Prior to opening of the instant loop 16, that is with the loop 16 closed, a positive potential appears at the base of the transistors 36 and the terminal 46 is maintained at a low, near ground potential. In this condition, when the contacts 42b are closed as shown in FIG. 2, little or no potential appears across the indicator lights and no light at all is emitted therefrom. This condition indicates that the alarm system 10 is "disabled" and that the instant loop 16 has not been violated.

When any of the key switches 22, 22' or 22" are closed, the relay contacts 42b are opened and maintained open to raise the potential at the terminal 48 of the indicator lights. Now, when the loop 16 is closed as shown in FIG. 4, a positive potential appears at the base of the transistor 36, thus lowering the potential at the terminal 46 and a constant current I' flows through the diode d.sub.2 to cause a constant red light to be emitted at each of the stations. The constant red indication signifies that the alarm system 10 is "enabled" but that the instant loop 16 has not been violated and that therefore the openings of the protected area covered by the instant loop are closed. Opening of the instant loop 16 with the contacts 42b open causes a pulsating current to flow through the diode d.sub.2 due to the fluctuating voltage at the collector of the transistor 36. Such a pulsating red light indication at each of these stations signifies that the alarm system is "enabled" and additionally that the loop 16 has been violated and that therefore at least one of the openings to the premises is open.

The above-mentioned conditions of the alarm system are summarized in the first four rows of the table shown in FIG. 4. Clearly, four visual conditions are provided at each of the stations, namely no light emission, a pulsating green light, a constant red light or a pulsating red light, each of these visual conditions signifying the status of the alarm system as above described which does not use an optional delay loop. A fifth indication, namely a constant green, may be obtained when a delay loop is used by slight modification of the circuit, as to be described below and as shown in FIGS. 2 and 3.

As described above, the relay 42 becomes energized when the terminal of the relay coil 42a connected to the key switches is grounded or substantially lowered in potential. Since the key switches are connected between the relay coil 42a and the collector of the transistor 36, it is important that the collector of the transistor 36 be brought to zero potential or substantially decreased while one of the key switches is closed in order to change the condition of the relay contacts 42b and therefore the on or off condition of the alarm system. When the loop 16 is closed, a positive potential appears at the base of the transistor 36, and the potential at the collector drops substantially to ground level as described above. On the other hand, when the loop 16 is open, the base voltage fluctuates to raise and lower the collector voltage in a fluctuating manner substantially between ground level and the positive supply voltage B.sup.+. In either case then, whether the loop 16 is closed or open, there appears at least brief periods or points when the collector voltage drops sufficiently in order to energize the relay coil 42a upon the closing of one of the key switches. In this connection, it may be mentioned that the frequency of pulsation as determined by the multi-vibrator 34 is selected to be in the range of a fraction of a cycle to several cycles per second. With this relatively low frequency, the on periods of the transistor are sufficiently long to energize the relay coil 42a. It has been found that such an arrangement of the relay coil 42a provides satisfactory results.

Based on the above, it will be evident that four distinct visual indications are provided at each of the stations, including the control station 12 and each of the remote stations, to signify different conditions of the alarm system 10 when a delay loop is not used. Additionally, each of the stations includes a key switch which permits local or remote supervisory monitoring or control of the alarm system. All of these functions and indications are achieved by mere utilization of three conductors 23-25 which connect the remote stations to the central or control station 12. This relatively large number of functions has not heretofore been possible with such a low number of inconnecting leads of conductors.

As suggested above, a fifth indication may be obtained when a delay loop is used by slight modification of the circuit. Referring to FIG. 4, it will be noted that the only condition which has not been discussed up to this point is one where a constant current flows through d.sub.1, which would provide a constant green indication. The operation previously discussed describes the base of the transistor 36 as either being at a constant positive potential, when the instant loop 16 is closed, or a pulsating potential which is only at the ground or reference potential during a small portion of each cycle. For this reason, the collector voltage is either fluctuating or nearly grounded with the circuit above described. In order to maintain the collector voltage at a constant positive level, the base must be grounded on a continuing basis. Now, closing the relay contacts 42b with the transistor in the "off" or non-conducting condition causes a constant current to flow through d.sub.1 which, with the above described LED diodes, would provide a constant green indication. The manner of grounding the base is not in and of itself critical. For example, a switch can be connected to the base which grounds the same on the occurrence of any desired event, the occurrence of which is to be monitored. One condition which can advantageously be monitored at all remote locations is the condition of an additional time-delay loop which is used in some alarm systems. When the time delay loop is open and the instant loop is closed, a switching circuit may be provided which automatically grounds the base of the transistor 36. Clearly, when the base is directly grounded, the collector voltage will remain at the high positive potential irrespective of the potentials appearing at the output of the multi-vibrator 34. Alternately, it may be possible to modify the multi-vibrator 34 so that it provides a grounded output when the specified fifth condition occurs.

Referring now more specifically to FIGS. 2 and 3, it will be noted that an optional delay loop is shown which, as described above, may be used to cover those openings of the premises which are primarily utilized for exit and entry by the owner. The delay loop cooperates with the alarm monitoring circuitry 26 and the timing circuitry 30. While the first alarm sensing means of the alarm circuit comprises the instant loop 16, the second alarm sensing means includes a delay loop 50 and, in a presently preferred embodiment, a transistor 52. The base of the transistor 52 is connected to the delay loop while the collector is connected to the base of the transistor 36 and the emitter to the switch contact 42b of the switch 42. The voltage applied by the delay loop 50 to the base of the transistor 52 is supplied through the alarm monitoring circuitry 26 and the circuitry is advantageously arranged so that a voltage is applied to the delay loop only when the instant loop 16 is closed. As soon as the instant loop 16 opens, the voltage to the delay loop 50 is interrupted and the delay loop circuitry becomes inoperative. In this manner, the instant loop circuitry takes priority and provides the suitably pulsed indications when the instant loop is open.

The delay loop 50 is shown to cooperate with the timing circuitry 30 so that when the delay loop 50 is open, a predetermined time period must elapse before the audio producing means 32 generates the desired alarm. This permits exit and entry into the premises through certain predetermined openings of the premises. As suggested above, the timing circuitry 30 also serves to terminate the operation of the audio producing means after a predetermined period of issuing an alarm, as for example after a 30 second alarm period. Additionally, the timing circuitry 30 serves to recycle and reset the alarm system once the openings in the premises have been closed and, therefore, the loops are also closed.

Still referring to FIGS. 2 and 3, when the delay loop applies a positive potential to the base of the transistor 52, the base of the transistor 36 will be grounded when the switch terminal 42b is grounded. This will provide a voltage approximately equal to B.sup.+ at the junction 46 while the junction or termimal 48 is grounded. Such relative voltages at the indicating means 20 will cause a constant voltage to flow through the diode d.sub.1 to thereby cause a green light to be emitted. In order to obtain such constant green indication, as shown in the last row or line of the table in FIG. 4, the delay loop 50 is arranged so that a positive potential is applied to the base of the transistor 52 when the delay loop is open but the instant loop is closed.

Referring to the table shown in FIG. 4, it will be noted that the introduction of the delay loop 50 and the transistor 52 as shown in FIGS. 2 and 3 does not modify in any way the results previously obtained insofar as the indicating means are concerned. When the instant loop is closed and the relay contacts are closed, the first and last rows of the table indicate that the current flow is interrupted through the indicator 20 when the delay loop is closed while a steady current flows through diode d.sub.1 when the delay loop is opened.

In the other conditions listed in rows or lines 3-5 of the table in FIG. 4, the indications are independent of the condition of the delay loop. When the instant loop is closed, a high positive potential is applied to the base of the transistor 36, as described above. Opening of the relay contacts, as indicated in line 2 of the table, applies a voltage approximating B.sup.+ to the emitter of the transistor 52 so that the collector of that transistor cannot be grounded independently of the condition of the delay loop. In that case, a constant current will flow through the diode d.sub.2 independently of the condition of the delay loop 50.

As suggested above, when the instant loop is open, the alarm monitoring means 26 removes the potential from the delay loop 50 so that the delay loop cannot saturate the transistor 52 or place the collector thereof at the ground potential even when the switch contact 42b is grounded. For this reason, lines 3 and 4 of the table shown in FIG. 4 indicate that pulsed currents flow through diodes d.sub.1 and d.sub.2 when the relay contacts are respectively closed and opened independent of the condition of the delay loop.

What has been described above is a description of the presently preferred embodiments of the alarm system in accordance with the present invention which provide four indications of the status of the alarm system when only an instant loop is used, and five indications when both instant and delay loops are used. However, various changes may be made in the form, construction and arrangement of the parts herein described without departing from the spirit and scope of the invention and without sacrificing any of its advantages.