United States Patent 3683352

An alarm system using a laser generated light beam which extends through a plurality of isolated detectors and ends at a control station. Each detector includes two light-to-electric transducers, one for detecting smoke by scattering and the other to detect an intruder by the temporary blocking of the beam. There is no electrical wiring connecting the detector stations. A signal denoting smoke or an intruder is sent over the laser beam in the form of an amplitude or polarization modulation at a characteristic frequency. This signal is received at the control station, its frequency of modulation is determined and an alarm is activated.

Henry W. West (Red Bank, NJ)
Frederick Gans (Jamaica, NY)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
340/630, 340/870.09, 340/870.28, 359/240, 359/246
International Classes:
G08B13/183; G08B19/00; (IPC1-7): G08B13/18
Field of Search:
340/228S,258B,237R,189 350
View Patent Images:
US Patent References:
3471845SECURITY SYSTEM1969-10-07Sokoloff
3370285Detection system1968-02-20Cruse et al.
3335285Photoelectric system for detecting objects in a zone including vibrating light source1967-08-08Gally et al.

Primary Examiner:
David, Trafton L.
Attorney, Agent or Firm:
Albert, Kronman F.
1. An alarm system for sensing the presence of a light barrier comprising: a source of light collimated into a narrow parallel beam; a first polarizing means for making the light beam plane polarized; a plurality of sensor stations positioned at places where it is desired to detect a light barrier, each of said stations including a container having an entrance and an exit opening for the passage of the light beam, each of said stations also including a photo-transducer adapted to sense the reduction in intensity of the light beam; an oscillator coupled to each sensor station for generating a distinctive frequency; coupling means between the photo-transducer and the oscillator in each station for starting the oscillator only when the transducer detects a light barrier; a crystal modulator controlled by the oscillator for modulating the light beam as it passes through the sensor container; a control station for receiving the light beam and for interpreting its modulated information, said station including a second polarizing means set at an angle to the first polarizing means for converting the modulated light beam into a variable intensity beam; a photo-transducer for converting the variable intensity beam into an alternating current; a plurality of filter circuits respectively tuned to pass a frequency corresponding to the frequency of one of the oscillators; and an alarm coupled to each of said filter

2. An alarm system as claimed in claim 1 wherein said source of light is a

3. An alarm system as claimed in claim 1 wherein the light from said source

4. An alarm system as claimed in claim 1 wherein at least one light filter is positioned in the path of the light beam for eliminating ambient light

5. An alarm system as claimed in claim 1 wherein a photo-detector in a sensor station is arranged for detecting an intruder and normally receives

6. An alarm system as claimed in claim 1 wherein a photo detector in a sensor station is arranged for detecting smoke, said photo-detector positioned to one side of the beam and receiving light only when smoke

7. An alarm system as claimed in claim 1 wherein the crystal modulator is

8. An alarm system as claimed in claim 1 wherein the crystal modulator includes two plate electrodes for creating an A.C. electric field in the

9. An alarm system as claimed in claim 1 wherein each alarm coupled to a filter is designated by a symbol to indicate the location of the sensor

10. An alarm system as claimed in claim 8 wherein the capacity created by the two crystal electrodes causes resonance with an inductance in the oscillator output circuit.

There are many smoke detectors and intruder alarms available today for the protection of property. These systems work on various principles, some using radar, some ultra-sonic beams and some using light beams. All these prior systems have some disadvantages. Radar systems are difficult to contain in the area to be protected and disturbances outside the area may trigger the alarm. Ultrasonic beam systems require a separate generator and detector for each room since the beams cannot penetrate walls. Light beams offer the best protection but focusing problems limit their range and, again separate and complete installations must be made for each room or protection area.

The present invention has none of the above listed disadvantages. A single laser generator and a single control sensor are all that are necessary for an entire building. A laser beam is sent through each room in the building at places where an intruder is sure to be found. The beam is reflected by a plurality of mirrors until it reaches a control center where the alarm annunciator is installed. A plurality of sensors are positioned along the beam and are provided with individual battery power but there is no wiring from the sensor stations to the control station. The laser light beam carries the intruder information to the control station and indicates the room in which the intruder broke the beam. Each sensor station is provided with a smoke detector in addition to the intruder detection means.

One of the features of the invention is a single light generator and a single control station, covering a large building having many rooms.

Another feature of the invention is the transmission of intruder-smoke information by means of a light beam, either amplitude or polarization modulated.

Still another feature of the invention is the absence of signal wiring between the sensor stations and the control station.

For a better understanding of the present invention, together with other details and features thereof, reference is made to the following description taken in connection with the accompanying drawings.


FIG. 1 is a schematic diagram of a building having two rooms showing, the laser generator, six sensor stations and a control station.

FIG. 2 is a perspective drawing of a sensor.

FIG. 3 is a cross sectional view of the sensor shown in FIG. 2 and is taken along line 3--3 of that figure.

FIG. 4 is a cross sectional view of a sensor station having two photoconductive cells, one for smoke and one to sense the blocking of the laser beam.

FIG. 5 is a schematic diagram of connections of the sensor station shown in FIG. 4.

FIG. 6 is a schematic diagram of the complete system showing the laser generator, four polarizing crystals, four oscillators, a control station which includes four filters and four annunciators.


Referring now to FIG. 1, two rooms are indicated, one above the other with a laser beam generator 10 positioned in the upper room. The laser beam is directed to a first sensor 11, past a window 12, to a first mirror 13. Mirror 13 sends the beam to several other mirrors 14, 15, and 16, in the same room, the beam passing through sensors 17 and 18. The last mirror 16 in the upper room directs the laser beam through a hole in the floor and to mirrors 20, 21, 22, and 23.

Three sensor stations 24, 25, and 26 are positioned in the path of the beam which passes a door 27 and several other windows. The beam is terminated at a control station 28. The detection system is not limited to the number of mirrors shown nor to the number of sensor stations. There is no theoretical limit to the number of mirrors and sensors and the laser beam may be directed across the room areas and is not confined to any particular range or angular direction. The system may be used outside a building to detect intruders and smoke in a restricted area.

Referring now to FIGS. 2, 3, and 4, the sensor detectors are positioned in a light tight box 30 having an entrance tube 31 and an exit tube 32. The preferred system includes light filters 33, but these are not always necessary. The light filters are used only in rooms where there is considerable ambient illumination that might be reflected into the boxes 30 and give a false alarm. It is well known that lasers can produce light beams in the near infra-red region of the spectrum. Such beams are preferred since there is almost no light of such wave length generated by modern systems of area illumination. The boxes 30 are formed with louvers 34 in the top and bottom sides so that smoke can pass freely through the box space.

Inside the sensor box 30 is a first photoconductive cell 35 for detection of intruders. A second photoconductive cell 36 is preferably mounted inside the sensor box 30 for detection of smoke. However, sensor units may be constructed with only one cell to detect either intruders or smoke if desired. A small piece of glass 37 is mounted in front of cell 35 and in the path of the laser beam in order to reflect a portion of the beam to the cell and keep it normally conductive. A plain piece of glass without silvering will be sufficient to reflect about 6 percent of the light (if unpolarized). By varying the angle of polarization, values ranging from two percent to 10 percent can be obtained without an additional reflective coating (u = 1.523). Since the laser beam is concentrated and of small diameter, better results are obtained if the reflecting surface is convex, as shown, thereby spreading the reflected light over a large area and reducing the probability of hot spots on the cell surface.

Photoconductive cell 36 is normally non-conductive and is designed for detecting smoke. When the air is clear, the laser beam 38 passes through the box 30 without producing any action. When smoke fills the box, the scattered light activates cell 36 and makes it conductive.

In each sensor box, a light modulating means must be mounted. Light modulators are of many types. There are slow moving shutters, sectored wheels, polarizers, both electrostatic and electromagnetic, and Kerr cells. While the invention is not limited to any particular type of modulator, the electrostatic type of polarizer is preferred since it requires less power than other types and since there is no mechanical motion. One type, made of crystal potassium dihydrogen phosphate, changes the angle of the plane of polarization when under the influence of an electrostatic field. This type of modulator together with other types is described in the Proceedings of the IEEE for October 1966. Such a crystal 40 with electrodes 41 is positioned so that the light beam makes an angle of 45° with the electric field produced by an oscillator. A steady AC wave from the oscillator applies a polarization angle shift to the light beam passing through the crystal.

FIG. 5 shows one manner of connecting the two photoconductive cells to the oscillator and the crystal. A first relay 42 has a winding 43 connected in series with a battery 44 and the cell 35. The cell 35 is for detection of intruders and is normally illuminated by the laser beam 38 reflected by the glass reflector 37 so that contacts 45 on relay 42 are normally closed. A second relay 46 has a winding 47 connected in series with the second cell 36 and the battery 44. Cell 36 is for smoke detection and normally is not conductive so that contacts 48 and 50 are open. Contacts 48 connect the battery 44 to an oscillator 51 when closed. Contacts 50 are locking contacts and hold the relay in its operated condition once smoke has been detected. The oscillator, which may be any suitable type, delivers a predetermined frequency wave to the crystal 40. The locking circuit through contacts 50 can be broken and the circuit normalized by manually operating a switch 52. The circuit is battery operated to make sure the system will continue in operation even if the main AC supply lines are disconnected. However, to keep the battery in good condition, a trickle charger circuit is generally supplied. This circuit includes a step-down transformer 53 and a rectifier 54.

The operation of the above circuit is as follows: Relay 42 is normally conducting and contacts 45 are open because cell 35 is activated. Relay winding 47 is not receiving current because cell 36 is not receiving any light and contacts 50 and 48 are open. The oscillator 51 also receives no current because contacts 48 are open and the crystal 40 produces no modulation on the light beam. Now, let it be assumed that an intruder steps in front of the laser beam. The light from reflector 37 no longer makes cell 35 conductive and current is cut off from winding 43, closing contacts. Current now flows from the positive battery terminal over conductor, through contacts 45, over conductor 56, to winding 47 of relay 46, then over conductor 57 to the negative terminal of the battery. This current activates the relay 46, closing contacts 48 and 50 and delivering battery power to the oscillator 51 and a high frequency wave to the crystal 40. Holding contacts retain the relay 46 in operation until an operator pushes switch 52 open. As long as the intruder remains in front of the sensor, blocking the laser beam, there will be no alarm indicated, but it is assumed that the intruder moves to one side in the course of his operations and, as soon as the beam is again established, a modulated light is delivered to the control station 28 where it will be detected and the alarm sounded.

Now let is be assumed that smoke enters the sensor box 30 and causes scattering of the laser beam. Cell 36 is made conductive and current from the battery 44 passes through the cell, then over conductor 56 to winding 47 on relay 46, then back to the battery by way of conductor 57. This current activates the relay, closing contacts 48 and 50 sending DC current to the oscillator and AC power to the modulating crystal. Again, holding contacts 50 retain the relay in operated condition until switch 52 is opened manually by an operator at the sensor. In this case, the laser beam transmits a modulated light beam to the control circuit 28 as soon as the oscillator is activated since the beam is concentrated sufficiently to send light through the smoke.

FIG. 6 shows all the main components in the system in order to explain the general operation. The laser 10 generates a beam of light which first passes through a polarizing means 60 which may be a Nicol prism. The polarized beam 38 then passes through all the modulating prisms 40, to a second Nicol prism 61 which acts as an analyzer and is preferably set at an angle of 45 degrees to the first prism 60. The beam then moves to a photo transducer 62 which may be a photoconductive or photoelectric cell. Again, a negative lens 63 may be used to spread the concentrated laser beam over a wide area to lower spot heating. An amplifier 64 is coupled to the transducer but it is arranged to amplify only alternating currents so its output is zero under normal conditions. The amplifier output is connected to a plurality of filters 65 each tuned to pass a frequency which matches the frequency of one of the oscillators. An alarm, which may be audible or visual is connected to each filter circuit and is labeled to indicate the location of the operated sensor. If the second sensor detects an intruder or smoke, switch 48 is closed, the oscillator 51-2 sends high voltage to crystal 40-2 and the beam is modulated by a high frequence shift of the polarization angle. The average intensity of the beam is not changed. When the beam passes through the second Nicol prism 61, the frequency variations and transformed into intensity variations and the photocell 62 sends AC to the amplifier having a frequency f2. This current is amplified and sent on conductors 66 to all the filters 65 but only one 65-2 sends current to its output and the alarm 67.

It should be noted that the system described above has quite a high light efficiency. Each polarizing crystal cuts out about 55 percent of the light but there are only two of these crystals, no matter how many modulators are used and there is plenty of light left over to operate the final phototransducer 62. It should also be noted that if an intruder moves around and breaks several more light beams, all of the modulating frequencies will be carried on the same laser beam and a corresponding number of alarms will be set off.

Any type of oscillator 51 can be used, as noted above. Since the load is a capacitor and requires rather high voltage for its operation, a resonant circuit can be set up, at the output frequency, by the use of an output step-up transformer having enough secondary inductance to resonate with the crystal load.