US3436554A - Radiation sensitive intrusion detection system - Google Patents

Description

April 1, 1969 R. E. JAMES ET AL RADIATION SENSITIVE INTRUSION DETECTION SYSTEM Filed Dec. 30, 1965 ALARM CIRCUIT DETECTOR CIRCUIT ASTABLE MULTIVIBRATOR INVENTOR. ROBERT E. JAMES CLARE G. KEENEY ATTORNEY United States Patent 3,436,554 RADIATION SENSITIVE INTRUSION DETECTION SYSTEM Robert E. James, Saratoga, and Clare G. Keeney, Campbell, Califi, assignors to Sylvania Electric Products Inc.,

a corporation of Delaware Filed Dec. 30, 1965, Ser. No. 517,560 Int. Cl. H01j 39/12 U.S. Cl. 250-221 4 Claims ABSTRACT OF THE DISCLOSURE Thi invention relates to intrusion detection systems and more particularly to such a system utilizing photosensitive elements for detecting introducers in a protected area.

Intrusion detection apparatus of the type described in Patent No. 3,191,048 utilizes photocells in a balanced circuit in conjunction with a light source such as a lamp, on the opposite side of the area to be protected. The circuit is in the no alarm Or quiescent state if the intensity of light incident upon the photocells is equal. A change in the amount of light on one photocell as compared to the other such as may be produced by an intruder in the room unbalances the circuit and causes an alarm.

Flickering light or deliberate or accidental extinguishing of the light from the lamp does not unbalance the circuit so as to cause an alarm provided the response characteristics of the photocells are identical over their full dynamic range. However, standard photocells rarely have matched responses beyond a 3% variation in light intensity because of the diiferences in composition and structure due to tolerances involved in their manufacture. Therefore, photocells that are matched over the full range of high variation must be specially manufactured at higher cost or must be obtained by time-consuming and expensive trial and error selection from large quantities of standard photocells. Even then, a perfect match over the full dynamic range is difiicult to achieve. The result of imperfectly matched photocells is such a system is false alarming when the change of light intensity exceeds the limits of the match.

For the reasons outlined above, a balanced light-responsive intrusion detection system preferably has a light generator or lamp which radiates light across the area to be protected at a constant level of intensity. This presupposes the availability of a constant level of power to energize the lamp. However, normal fluctuations in central power systems and even the occasional loss of power from that source introduce discontinuities in the supply of power to the lamp such as to tend to cause false alarms. Problems of this sort resulting from line voltage fluctuations may be minimized by the use of voltage regulators and similar voltage control devices which, however, increase the cost and complexity of the intrusion detection system. Furthermore, such devices are inetfective in event of a power failure.

In order to accommodate complete loss of primary 3,436,554 Patented Apr. 1, 1969 power, standby power in the form of batteries may be provided. However, unless the photocells are matched over a wide dynamic range, the perturbation in the supply of power to the lamp caused by the switching from primary to standby power often is sufficient to unbalance the photocell and produce a false alarm.

An object of this invention is the provision of a lightresponsive intrusion detection system which is insensitive to large fluctuations in primary power.

Another object i the provision of such a system with standby power connected to the light generator in such manner that changes in or loss of primary power has no significant effect on the intensity of the light generated.

A further object is the provision of a light-responsive intrusion detection system with means for energizing a lamp from two independent power sources such that failure of one power source has no effect on surveillance of the area to be protected.

Still another object is the provision of such an intrusion detection system with a low-cost, reliable circuit arrangement for providing regulation as well as standby power for the light generator of the system.

These and other objects are achieved by supplying power to the lamp of the light-responsive detection system from a storage battery which is continuously charged by the primary power source during normal operation of the system. The output of the battery is changed to alternating current by a converter which in turn is connected to the light generator, preferably a fluorescent lamp or the like. The battery is charged by rectified alternating current. The lamp is thus separated from the primary power by the battery which functions as a voltage regulator as well as a source of standby power in event of failure of the primary power. Since the battery is connected to the lamp at all times, the intensity of light from the lamp remains constant within matched range of the photocells, even when the primary power fails completely.

These and other objects of the invention will become apparent from the following description of a preferred embodiment thereof reference being had to the accompanying drawings in which:

FIGURE 1 is a perspective cutaway view of a room guarded by an intrusion detection system embodying this invention; and.

FIGURE 2 is a schematic circuit and block diagram of the intrusion detection system shown in FIGURE 1.

Referring now to the drawings, an intrusion detection system utilizing this invention is shown in an operating position in a room 10 in FIGURE 1 for protecting the area within the room from unauthorized intrusions, for example, through door 11 or through window 12. The system comprises a generator of light 14, such as a fluorescent lamp, at one corner of the room and a lightresponsive receiver and alarm unit 15 at the opposite corner of the room. Receiver 15 has a pair of photocells 16 and 17 mounted on the exterior of the receiver housing so as to respond to relative changes in intensity of light upon them for generating a signal through the receiver circuits and giving an alarm. Briefly, photocells 16 and 17 are connected in a balanced network in the receiver circuit so as to produce an output when more light strikes one cell than the other. This output results in an alarm. When the intensity of light on both cells is equal and steady, the circuit is balanced and no alarm is produced. By way of example, a receiver and alarm unit that may be used in the practice of this invention is described in the copending application of Clare G. Keeney, Ser. No. 407,713, filed Oct. 30, 1964, now Patent No. 3,309,689.

Light from lamp 14 reaches the photocells 16 and 17 as reflections from the walls of the room as well as by direct path transmission. Although receiver 15 is conditioned with the photocells substantially balanced to have no output under no-alarm (no intruder) conditions, nevertheless a substantial change in intensity of light from the lamp, such as a to increase or decrease in that light, does tend to unbalance the photocells and to produce a false alarm. This is due to dissimilar responses of these cells to wide changes in the light caused by slight differences in the composition, configuration, and dimensions of the cells resulting from manufacturing tolerance errors. The cells therefore have similar dynamic response characteristics for only a limited range of change in light intensity beyond which the cells respond differently and become unbalanced. Thus, if lamp 14 changes its light output for any reason such as a drop in supply voltage due to line loading or the switching to standby power in event of supply power failure, there is a tendency for the photocells to become unbalanced and to produce a false alarm. In accordance with this invention, lamp 14 is energized from a constant voltage source 20 connected between the lamp and the source of primary power, shown in FIGURE 1 as a household convenience outlet 22. The constant voltage source 20 compensates for any fiuctuations in the primary power supply and provides standby power in event of failure of the primary supply without introducing a disturbance in the supply of energy to the lamp.

The constant voltage source 20, see FIGURE 2, comprises a transformer 25, a rectifier 26 connected to the output of the transformer, a storage battery 27 directly connected to rectifier 26, and a DC to AC converter 28 connected to the output of storage battery 27 and having an output connected to lamp 14.

The two photocells 16 and 17, shown as variable resistors in FIGURE 2, are connected to a detector circuit 30 which energizes and alarms circuit 31 if the photocells are unbalanced due to relative change of the amount of light incident upon them.

Transformer 25 preferably produces a relatively constant output voltage for efficient charging of the battery and has a secondary winding 25a connected to diodes 26a and 26b which function as a full wave rectifier for continuously charging the battery on line 34. Thus, transformer 25 and rectifier 26 constitute a battery charging circuit.

The capacity of battery 27 is sufficient to provide standby power for lamp 14- for a period of several hours and may, by way of example, comprise the well known lead-acid type storage battery. The output line 34 from rectifier 26 is connected across the battery 27 at all times during operation of the circuit and the battery is there fore constantly being charged under normal operating conditions with the primary power on.

The function of the converter 28 is to change the output of battery 27 from a unidirectional to an alternating current and also to increase the frequency of the energy substantially above that of the primary power source. Converter 28 comprises an astable multivibrator 36 connected directly to battery 27 and has an output coupled to a stepup transformer 37, the secondary of which is directly connected to lamp 14. By way of example, multivibrator 36 has an operating frequency of 5 kilocycles or more, and at this frequency the gas in fluorescent lamp 14 is ionized so as to further insure the radiation of flickerless uniform intensity light which maintains the photocells 16 and 17 in a balanced state.

In the event of failure of the primary power source S, the charging current on line 34 disappears but energy continues to be supplied from battery 27 through the converter 28 to lamp 14 without significant change. Since battery 27 is always in the circuit, no switches are required to initiate its function as a standby power source in event of failure of the primary power source and, importantly, the transition occurs without any undesirable perturbations in the flow of energy to the lamp.

By way of example, an intrusion detection system embodying the invention was constructed, successfully operated and tested, and had the following characteristics and performance results:

(A) GRADUAL CHANGE OF PRIMARY POWER Source S (volts) Light output of lamps (toot candles) 40 39. 2 30 Alarms None (B) RAPID CHANGE OF PRIMARY POWER Source S (volts)- 95 130 Light output of lamps (foot candles) 40 38. 8

larms None (0) ON-OFF ACTION Source S (volts) 130 0 Light output of lamps (foot candles) 39 38 Alarms None (D) SYSTEM PERFORMANCE (UNDERWRITERS LABORATORIES) Room size cu. ft 2200 Duration of test (continuous) hours 1 1680 Variation in primary power:

Line fluctuations volts 115:15

Number of power failures 3 Number of false alarms 0 Number of attempted intrusions 20 Number of attempted intrusions detected 2O 1 10 weeks.

From the foregoing description, it will be seen that there is provided a light-responsive intrusion detection system which is essentially insensitive to changes in primary line voltage, even a total failure of that power source. Battery 27 translates a potentially variable power source into a stable one so that light is generated with an intensity that is constant within the balance limits of the light-responsive cells. Battery 27 is also the source of standby power and is present in the lamp energization circuit at all times. This arrangement insures continuous reliable protection of the secured area.

Changes, modifications, and improvements may be made to the above-described system without departing from the spirit and scope of the invention. Therefore the appended claims define the patentable features of the invention.

5 We claim: 1. An intrusion detection system comprising: light generating means adjacent to an area to be protected and connected to a primary source of alternating current, a light-responsive receiver spaced from said light generating means on the opposite side of said area,

said receiver having a plurality of photo-sensitive elements adapted to produce an alarm signal when one element receives more light than another, said light generating means comprising a light source,

a storage battery connected to supply electrical energy to said light source, and a battery charging circuit connected between said primary source and said battery for continuously charging the latter during energizatio nof said light source. 2. The system according to claim 1 in which said light source is an alternating current lamp, and converter means connected between said battery and the lamp for changing the output of the battery into alernating current.

3. The system according to claim 2 in which said lamp is a gas type and the frequency of the output of said converter means is greater than 1 kilocycle.

4. The system according to claim 2 in which said lamp is a fluorescent type, said battery charging circuit comprising a transformer and a full wave rectifier connected directly to said battery.

References Cited UNITED STATES PATENTS 2,557,167 6/ 1951 Armagost 250-221 X 2,813,230 11/1957 Fruengel 250-221 X 2,920,260 1/ 1960 Gotfstein 3202 X 3,191,048 6/1965 Cowen 250-221 3,317,809 5/1967 Bowers et a1. 320-30 WALTER STOLWEIN, Primary Examiner.

US. Cl. X.R.

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