ALARM CIRCUIT
United States Patent 3678511
The disclosed embodiment of the invention is a circuit which is responsive to one of a number of abnormal conditions for activating an alarm. The circuit includes a highly regulated power supply formed of a transistor connected in series between an unregulated voltage supply and the detector portion of the circuit. A feedback circuit, which senses voltage fluctuations, controls the level of conduction of the transistor, thereby controlling the voltage drop thereacross to provide a regulated output voltage. A smoke detector is formed of a light and a photocell mounted in a smoke chamber, the interior of which is shielded from ambient light. Any increase in the resistor of the photocell caused by the presence of smoke activates, through an amplifier circuit, a latching relay, which in turn activates an alarm. The amplifier circuit is also responsive to either the closing of normally open contacts or the opening of normally closed contacts to actuate the relay. Additional detectors and controls are connected in parallel with the relay contacts which energize the alarm.
US Patent References:
Timing circuits providing constant time delay independent of voltage supply variation
Sainsbury - January 1965 - 3165648
Smoke detecting radiation sensitive fire alarm system
Vassil - May 1967 - 3319069
SMOKE DETECTOR
Smith et al. - August 1969 - 3460124
RADIATION SENSITIVE SMOKE DETECTOR USING LIGHT CONDUCTING MEANS
Eaton et al. - March 1970 - 3504184
Voltage unbalance indicating system
Glarke - April 1952 - 2591511
Timing circuits providing constant time delay independent of voltage supply variation
Sainsbury - January 1965 - 3165648
Smoke detecting radiation sensitive fire alarm system
Vassil - May 1967 - 3319069
SMOKE DETECTOR
Smith et al. - August 1969 - 3460124
RADIATION SENSITIVE SMOKE DETECTOR USING LIGHT CONDUCTING MEANS
Eaton et al. - March 1970 - 3504184
Voltage unbalance indicating system
Glarke - April 1952 - 2591511
Application Number:
04/883486
Publication Date:
07/18/1972
Filing Date:
12/09/1969
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
340/541, 340/693.400, 340/630
International Classes:
G08B17/103; G08B19/00; G08B19/00; G08B17/10
Field of Search:
340/227,228,236,237,420 317/154,155,124
US Patent References:
| 2620385 | Photoelectrically controlled system for detecting suspended matter in fluids | December 1952 | Grant, Jr. |
Other References:
Transistor Manual, "Regulated D.C. Supply . . .," 1964, pp. 227-233 by General Electric. .
Reich, et al., Theory & Applications of Active Devices, D. V. Nostrand Co., Inc., New Jersey, 1966, pp. 180-183..
Primary Examiner:
Habecker, Thomas B.
Assistant Examiner:
Partridge, Scott F.
Claims:
The invention claimed is
1. A detector circuit, comprising: a step down transformer including primary and second windings, a low voltage AC input/output circuit connected across said secondary winding, a bridge rectifier including positive and negative terminals connected across said secondary winding, an unregulated DC input/output circuit connected across said bridge rectifier, a first transistor including emitter, collector and base electrodes, said first transistor emitter electrode connected to said positive terminal of said bridge rectifier, an incandescent light means connected between said first transistor collector electrode and said negative terminal of said bridge rectifier, an impedance means including first and second terminals having an impedance value which is proportional to the light impinging thereon, a means for enclosing said incandescent light means and said impedance means configured to allow the passage of air therethrough and to provide a light impervious shield, said impedance means disposed within said enclosing means for receiving light from said incandescent light means and being spaced apart from said incandescent light means to permit the passage of air therebetween, said impedance means first terminal connected to said first transistor collector electrode, a variable resistance means connected between said impedance means second terminal and said negative terminal of said bridge rectifier, a second transistor including emitter, collector and base electrodes, said second transistor base electrode connected to said impedance means second terminal, a first intrusion detection mean including a normally open set of electrical contacts connected between said second transistor base electrode and said negative terminal of said bridge rectifier, a third transistor including emitter, collector and base electrodes, said third transistor base electrode resistively connected to said first transistor collector electrode, a second intrusion detection means including a normally closed set of electrical contacts connected between said second transistor collector electrode and said third transistor base electrode, a relay including a coil and first and second sets of normally open electrical contacts, said relay coil connected between said third transistor collector electrode and said first transistor collector electrode, said first set of normally open relay contacts connected between said third transistor base electrode and said first transistor collector electrode, a normally open reset switch means connected between said third transistor base electrode and said negative terminal of said bridge rectifier, an indicator means including first and second terminals, said indicator means first terminal connected to said negative terminal of said bridge rectifier, said second set of normally open relay contacts connected between said indicator means second terminal and said positive terminal of said bridge rectifier, a fire detector including a set of normally open electrical contacts connected between said indicator means second terminal and said positive terminal of said bridge rectifier, a resistive voltage divider means including a potentiometer connected between said first transistor collector electrode and said negative terminal of said bridge rectifier, a fourth transistor including emitter, collector and base electrodes, said fourth transistor base electrode connected to said voltage divider potentiometer, said fourth transistor collector electrode connected to said first transistor base electrode, and a zener diode connected between said fourth transistor emitter electrode and said first transistor collector electrode.
1. A detector circuit, comprising: a step down transformer including primary and second windings, a low voltage AC input/output circuit connected across said secondary winding, a bridge rectifier including positive and negative terminals connected across said secondary winding, an unregulated DC input/output circuit connected across said bridge rectifier, a first transistor including emitter, collector and base electrodes, said first transistor emitter electrode connected to said positive terminal of said bridge rectifier, an incandescent light means connected between said first transistor collector electrode and said negative terminal of said bridge rectifier, an impedance means including first and second terminals having an impedance value which is proportional to the light impinging thereon, a means for enclosing said incandescent light means and said impedance means configured to allow the passage of air therethrough and to provide a light impervious shield, said impedance means disposed within said enclosing means for receiving light from said incandescent light means and being spaced apart from said incandescent light means to permit the passage of air therebetween, said impedance means first terminal connected to said first transistor collector electrode, a variable resistance means connected between said impedance means second terminal and said negative terminal of said bridge rectifier, a second transistor including emitter, collector and base electrodes, said second transistor base electrode connected to said impedance means second terminal, a first intrusion detection mean including a normally open set of electrical contacts connected between said second transistor base electrode and said negative terminal of said bridge rectifier, a third transistor including emitter, collector and base electrodes, said third transistor base electrode resistively connected to said first transistor collector electrode, a second intrusion detection means including a normally closed set of electrical contacts connected between said second transistor collector electrode and said third transistor base electrode, a relay including a coil and first and second sets of normally open electrical contacts, said relay coil connected between said third transistor collector electrode and said first transistor collector electrode, said first set of normally open relay contacts connected between said third transistor base electrode and said first transistor collector electrode, a normally open reset switch means connected between said third transistor base electrode and said negative terminal of said bridge rectifier, an indicator means including first and second terminals, said indicator means first terminal connected to said negative terminal of said bridge rectifier, said second set of normally open relay contacts connected between said indicator means second terminal and said positive terminal of said bridge rectifier, a fire detector including a set of normally open electrical contacts connected between said indicator means second terminal and said positive terminal of said bridge rectifier, a resistive voltage divider means including a potentiometer connected between said first transistor collector electrode and said negative terminal of said bridge rectifier, a fourth transistor including emitter, collector and base electrodes, said fourth transistor base electrode connected to said voltage divider potentiometer, said fourth transistor collector electrode connected to said first transistor base electrode, and a zener diode connected between said fourth transistor emitter electrode and said first transistor collector electrode.
Description:
This invention relates generally to a detector circuit for indicating the occurrence of an abnormal condition and more particularly to a circuit which is responsive to the occurrence of one or more of a number of abnormal conditions for actuating an indicating device, such as an alarm.
The present invention has particular application in residences as a safety system for providing an indication of an abnormal condition, such as an unauthorized intrusion or the presence of a fire. However, the circuit disclosed herein may be employed in any installation for detecting an abnormal condition.
The need has long existed for a home alarm device which is not only reliable, but inexpensive to the point of being attractive to all income groups. The normally available alarm circuits which are relatively inexpensive suffer from one major problem which makes them undesirable; namely, such known devices are not sufficiently reliable to provide adequate protection. The unreliability of such known devices vary considerably with changes in environmental conditions. If such devices are employed, for example, to detect the occurrence of a fire by detecting the presence of smoke in the surrounding atmosphere, the environmental conditions at one time may cause triggering of the device at a relatively low level of smoke in the surrounding atmosphere and at another time at a relatively high content of smoke in the surrounding atmosphere. Consequently, such devices may be triggered, for example, by the presence of cigarette smoke in the atmosphere under certain environmental conditions. This relatively low sensitivity of such devices is, of course, not desirable.
The lack of a high degree of sensitivity in such prior known devices is due primarily to inadequate regulation of supply voltage to the device. If the power supply voltage is maintained constant under all types of environmental conditions, the sensitivity of the detection circuit will be greater.
If a detector circuit is employed for detecting the occurrence of an unauthorized intrusion onto a premises, it is usually necessary to employ relatively long lead lengths between the detector unit and the various stations at which an intrusion may occur. The resistance and stray capacitance of such long leads can change the operating conditions of the detector circuit if such leads are connected to relatively low impedance points in the circuit. Such a change in the operating conditions of the detector circuit reduce its sensitivity and reliability. Since the lead length from a particular sensing unit to the detector circuit varies considerably from one installation to another, a detector circuit which has a relatively low input impedance must be designed specifically for each installation.
Accordingly, it is an object of the present invention to provide a circuit for detecting the presence of an abnormal condition or an unauthorized intrusion which is relatively inexpensive and has a high degree of sensitivity and reliability.
A feature of the present invention resides in the provision of a highly regulated power supply for the detector circuit, which power supply is relatively simple and inexpensive, but high effective for maintaining the supply voltage constant.
Another feature of the present invention resides in the provision of a high impedance circuit which forms the input to the detector circuit for various intrusion sensors.
These and other objects, features and advantages of the present invention will be more fully realized and understood from the following detailed description when taken in conjunction with the accompanying drawing, wherein the single FIGURE is a schematic diagram of the preferred embodiment of the present invention.
With reference to the drawing in detail, there is shown a detector circuit, generally designated with the reference numeral 10, and a power supply, generally designated with the reference numeral 12, which provides a highly regulated supply of voltage to the detector circuit 10. A transformer 14 is connected between a source of alternating current voltage and a full wave bridge rectifier circuit 16. The input to the transformer 14 may be 110 volts AC or 220 volts AC line voltage which is reduced by the action of the transformer to 12 volts AC. A pair of input terminals 18 and 20 are also connected to the rectifier circuit 16 to permit operation of the device by means of a 12 volt AC auxillary voltage source. A capacitor 22 is connected in parallel with the full wave rectifier circuit 16 and together therewith provides unregulated partially filtered DC voltage across lines 24 and 26. A pair of input terminals 28 and 30 are provided for connecting a 12 volt DC auxillary voltage source for operation of the circuit. Accordingly, the circuit can be operated from either a normal line voltage supply or from either a 12 volt DC or 12 volt AC auxillary voltage supply. Circuit arrangements which are well known in the art may be employed for connecting one of the auxillary supplies to the circuit if the line voltage should fail for various reasons.
The unregulated voltage appearing across the lines 24 and 26 is supplied to a regulation circuit generally indicated with the reference number 32, which provides a regulated voltage to the detector circuit 10. The regulation circuit 32 includes a PNP transistor 34 having its emitter and collector electrodes connected in series between the line 24 and a line 36. The voltage developed across the lines 26 and 36 constitutes the regulated voltage employed by the detector circuit 10. A resistor 38 is connected between the positive voltage on the line 24 and the base of the transistor 34, and a resistor 40 is connected between the positive voltage on the line 36 and the base of the transistor 34. The base of the transistor 34 is also connected to the collector of a transistor 42 having its emitter connected through a resistor 44 to the line 26. It can be readily appreciated that the biasing voltage on the base of the transistor 34 is established by the voltage divider action of the resistors 38 and 40 and the conduction level of the transistor 42.
A Zener diode 46 is connected between the line 36 and the emitter of the transistor 42. A voltage divider which is formed of a resistor 48, a potentiometer 50 and a resistor 52 is connected between the lines 26 and 36 and forms a biasing network for the transistor 42. The base of the transistor 42 is connected to the variable contact arm of the potentiometer 50 which provides a means for adjusting the voltage on the line 36. Adjustment of the voltage on the line 36 is necessitated by the fact that the operating characteristics of Zener diodes, which would be used as the Zener diode 46, vary considerably.
The instantaneous voltage developed on the line 36 is dependent upon the conduction level of the transistor 34, which is in turn controlled by the conduction level of the transistor 42. The Zener diode 46 supports a fixed value of voltage drop across its terminals, such that the voltage drop across the resistor 44 is equal to the voltage on the line 36 plus the voltage supported by the Zener diode 46. Accordingly, if the voltage on the line 36 increases by a predetermined amount, the voltage drop across the resistor 44 will increase by the same amount. However, the bias voltage on the base of the transistor 42 will increase only a fractional amount of an increase in the voltage on the line 36. As a result, if the voltage on the line 36 tends to become greater in value, the conduction level of the NPN transistor 42 will decrease. Such a decrease in the conduction level of the transistor 42 will cause the voltage on the base of the transistor 34 to increase, rendering the transistor 34 less conductive, thereby decreasing the voltage on the line 36 by an amount equal to the subsequent increase of that voltage. As a result, the voltage on the line 36 will be maintained constant regardless of any variation in the voltage developed across the lines 24, 26. The transistor 42 and its associated circuitry effectively forms a high gain comparitor circuit between the reference voltage developed across the Zener diode 46 and the voltage established at the base thereof. The action of the transistors 34 and 42 is effectively a feedback gain to compensate for voltage fluctuations which may appear on the line 36.
The detector circuit 10 includes a light source 54 and a photocell 56 mounted within a smoke chamber which is diagrammatically illustrated by the dotted line outline indicated with the reference numeral 58. The smoke chamber 58 is constructed to shield the photocell 56 from any ambient light, but permits the flow of external air therethrough which may contain smoke therein. The light source 54 is connected in series with a resistor 60 across the lines 26, 36 and the photocell 56 is connected in series with a potentiometer 62 across the lines 26, 36. The junction of the photocell 56 and the resistor 62 is connected to the base of a transistor 64. The emitter of the transistor 64 is connected through a resistor 66 to the negative potential on the line 26. The collector of the transistor 64 is connected through the normally closed contacts of an intrusion detector, generally designated with the reference numeral 68, to the base of a transistor 70. A relay solenoid coil 72 is connected from the collector of the transistor 70 to the line 26. A resistor 76 is connected from the line 36 to the base of the transistor 70. Normally open relay contacts 78, which are controlled by the relay coil 72, are connected in series with a resistor 80 across the resistor 76.
In the quiescent condition of the detector circuit, light emitted from the light source 54 impinges on the photocell 56 and maintains the resistance thereof at a sufficiently low value to maintain the transistor 64 conductive. The conduction of the transistor 64 maintains the bias on the base of the transistor 70 at a level to maintain the transistor 70 nonconductive. When a sufficient amount of the smoke enters the chamber 58 to reduce the amount of light impinging on the photocell 56 and increases the resistance thereof to a predetermined level, the transistor 64 is rendered nonconductive. When the transistor 64 changes from a state of conduction to a state of nonconduction, the voltage on the base of the transistor 70 increases causing it to be rendered conductive. Such conduction of the transistor 70 results in sufficient current flow through the relay coil 72 to close the relay contacts 78, thereby placing the resistor 80 in circuit to maintain the transistor 70 conductive. This arrangement effectively makes the relay 72 a latching relay.
A second set of relay contacts 82 are connected in series with an indicating device 84, such as a buzzer, across the lines 24, 26. When sufficient current is passed through the relay coil 72, the relay contacts 82 are closed to actuate the indicating device 84.
Additional inputs are provided for a variety of sensors and actuating devices. The normally open contacts of a intrusion detector, generally indicated with the reference numeral 86 are connected between the base of the transistor 64 and the line 26. If the normally open contacts of the intrusive detector 86 are closed, the transistor 64 will be rendered nonconductive, thereby forcing the transistor 70 to be rendered conductive to actuate the relay contacts 78 and 82. If the normally closed contacts of the intrusion detector 68 are open, the transistor 70 will be rendered conductive to actuate the relay contacts 78 and 82. Since the normally open contacts of the detector 86 and the normally closed contacts of the detector 68 are connected to relatively high impedance points in the circuit, any stray capacitance in the leads to those contacts will not adversely affect the operation of the circuit. As a result, such contacts can be employed with lead lengths up to several hundred feet in length. It has been found in practice that the operation of the detector circuit will not be adversely affected if the resistance of the leads to the contacts is maintained at a value no greater than 100 ohms. In addition, a fire detector or a "panic button" may be connected in parallel with the contacts 82 to actuate the indicating device 84. Such additional sensors and/or switches are diagrammatically illustrated by the block indicated with the reference numeral 88.
A switch 90 is connected between the base of the transistor 70 and the line 26 and is effective upon closure thereof to render the transistor 70 nonconductive. The witch 90 is employed to reset the detector circuit after it has been energized from one or more of the sensors connected thereto or from the action of the photocell 56.
The circuit illustrated in the drawing was constructed and successfully tested with the following values for the various components as shown therein:
R 38 100 ohms R 40 600 ohms R 44 470 ohms R 48 2000 ohms R 50 1000 ohms R 52 1000 ohms R 60 120 ohms R 62 500 ohms R 66 300 ohms R 74 30 ohms R 76 20,000 ohms R 80 5000 ohms C 22 100 microfarads C 92 50 microfarads
The principles of the invention explained in connection with the specific exemplification thereof will suggest many other applications and modifications of the same. It is accordingly desired that, in construing the breadth of the appended claims they shall not be limited to the specific details shown and described in connection with the exemplification thereof.
The present invention has particular application in residences as a safety system for providing an indication of an abnormal condition, such as an unauthorized intrusion or the presence of a fire. However, the circuit disclosed herein may be employed in any installation for detecting an abnormal condition.
The need has long existed for a home alarm device which is not only reliable, but inexpensive to the point of being attractive to all income groups. The normally available alarm circuits which are relatively inexpensive suffer from one major problem which makes them undesirable; namely, such known devices are not sufficiently reliable to provide adequate protection. The unreliability of such known devices vary considerably with changes in environmental conditions. If such devices are employed, for example, to detect the occurrence of a fire by detecting the presence of smoke in the surrounding atmosphere, the environmental conditions at one time may cause triggering of the device at a relatively low level of smoke in the surrounding atmosphere and at another time at a relatively high content of smoke in the surrounding atmosphere. Consequently, such devices may be triggered, for example, by the presence of cigarette smoke in the atmosphere under certain environmental conditions. This relatively low sensitivity of such devices is, of course, not desirable.
The lack of a high degree of sensitivity in such prior known devices is due primarily to inadequate regulation of supply voltage to the device. If the power supply voltage is maintained constant under all types of environmental conditions, the sensitivity of the detection circuit will be greater.
If a detector circuit is employed for detecting the occurrence of an unauthorized intrusion onto a premises, it is usually necessary to employ relatively long lead lengths between the detector unit and the various stations at which an intrusion may occur. The resistance and stray capacitance of such long leads can change the operating conditions of the detector circuit if such leads are connected to relatively low impedance points in the circuit. Such a change in the operating conditions of the detector circuit reduce its sensitivity and reliability. Since the lead length from a particular sensing unit to the detector circuit varies considerably from one installation to another, a detector circuit which has a relatively low input impedance must be designed specifically for each installation.
Accordingly, it is an object of the present invention to provide a circuit for detecting the presence of an abnormal condition or an unauthorized intrusion which is relatively inexpensive and has a high degree of sensitivity and reliability.
A feature of the present invention resides in the provision of a highly regulated power supply for the detector circuit, which power supply is relatively simple and inexpensive, but high effective for maintaining the supply voltage constant.
Another feature of the present invention resides in the provision of a high impedance circuit which forms the input to the detector circuit for various intrusion sensors.
These and other objects, features and advantages of the present invention will be more fully realized and understood from the following detailed description when taken in conjunction with the accompanying drawing, wherein the single FIGURE is a schematic diagram of the preferred embodiment of the present invention.
With reference to the drawing in detail, there is shown a detector circuit, generally designated with the reference numeral 10, and a power supply, generally designated with the reference numeral 12, which provides a highly regulated supply of voltage to the detector circuit 10. A transformer 14 is connected between a source of alternating current voltage and a full wave bridge rectifier circuit 16. The input to the transformer 14 may be 110 volts AC or 220 volts AC line voltage which is reduced by the action of the transformer to 12 volts AC. A pair of input terminals 18 and 20 are also connected to the rectifier circuit 16 to permit operation of the device by means of a 12 volt AC auxillary voltage source. A capacitor 22 is connected in parallel with the full wave rectifier circuit 16 and together therewith provides unregulated partially filtered DC voltage across lines 24 and 26. A pair of input terminals 28 and 30 are provided for connecting a 12 volt DC auxillary voltage source for operation of the circuit. Accordingly, the circuit can be operated from either a normal line voltage supply or from either a 12 volt DC or 12 volt AC auxillary voltage supply. Circuit arrangements which are well known in the art may be employed for connecting one of the auxillary supplies to the circuit if the line voltage should fail for various reasons.
The unregulated voltage appearing across the lines 24 and 26 is supplied to a regulation circuit generally indicated with the reference number 32, which provides a regulated voltage to the detector circuit 10. The regulation circuit 32 includes a PNP transistor 34 having its emitter and collector electrodes connected in series between the line 24 and a line 36. The voltage developed across the lines 26 and 36 constitutes the regulated voltage employed by the detector circuit 10. A resistor 38 is connected between the positive voltage on the line 24 and the base of the transistor 34, and a resistor 40 is connected between the positive voltage on the line 36 and the base of the transistor 34. The base of the transistor 34 is also connected to the collector of a transistor 42 having its emitter connected through a resistor 44 to the line 26. It can be readily appreciated that the biasing voltage on the base of the transistor 34 is established by the voltage divider action of the resistors 38 and 40 and the conduction level of the transistor 42.
A Zener diode 46 is connected between the line 36 and the emitter of the transistor 42. A voltage divider which is formed of a resistor 48, a potentiometer 50 and a resistor 52 is connected between the lines 26 and 36 and forms a biasing network for the transistor 42. The base of the transistor 42 is connected to the variable contact arm of the potentiometer 50 which provides a means for adjusting the voltage on the line 36. Adjustment of the voltage on the line 36 is necessitated by the fact that the operating characteristics of Zener diodes, which would be used as the Zener diode 46, vary considerably.
The instantaneous voltage developed on the line 36 is dependent upon the conduction level of the transistor 34, which is in turn controlled by the conduction level of the transistor 42. The Zener diode 46 supports a fixed value of voltage drop across its terminals, such that the voltage drop across the resistor 44 is equal to the voltage on the line 36 plus the voltage supported by the Zener diode 46. Accordingly, if the voltage on the line 36 increases by a predetermined amount, the voltage drop across the resistor 44 will increase by the same amount. However, the bias voltage on the base of the transistor 42 will increase only a fractional amount of an increase in the voltage on the line 36. As a result, if the voltage on the line 36 tends to become greater in value, the conduction level of the NPN transistor 42 will decrease. Such a decrease in the conduction level of the transistor 42 will cause the voltage on the base of the transistor 34 to increase, rendering the transistor 34 less conductive, thereby decreasing the voltage on the line 36 by an amount equal to the subsequent increase of that voltage. As a result, the voltage on the line 36 will be maintained constant regardless of any variation in the voltage developed across the lines 24, 26. The transistor 42 and its associated circuitry effectively forms a high gain comparitor circuit between the reference voltage developed across the Zener diode 46 and the voltage established at the base thereof. The action of the transistors 34 and 42 is effectively a feedback gain to compensate for voltage fluctuations which may appear on the line 36.
The detector circuit 10 includes a light source 54 and a photocell 56 mounted within a smoke chamber which is diagrammatically illustrated by the dotted line outline indicated with the reference numeral 58. The smoke chamber 58 is constructed to shield the photocell 56 from any ambient light, but permits the flow of external air therethrough which may contain smoke therein. The light source 54 is connected in series with a resistor 60 across the lines 26, 36 and the photocell 56 is connected in series with a potentiometer 62 across the lines 26, 36. The junction of the photocell 56 and the resistor 62 is connected to the base of a transistor 64. The emitter of the transistor 64 is connected through a resistor 66 to the negative potential on the line 26. The collector of the transistor 64 is connected through the normally closed contacts of an intrusion detector, generally designated with the reference numeral 68, to the base of a transistor 70. A relay solenoid coil 72 is connected from the collector of the transistor 70 to the line 26. A resistor 76 is connected from the line 36 to the base of the transistor 70. Normally open relay contacts 78, which are controlled by the relay coil 72, are connected in series with a resistor 80 across the resistor 76.
In the quiescent condition of the detector circuit, light emitted from the light source 54 impinges on the photocell 56 and maintains the resistance thereof at a sufficiently low value to maintain the transistor 64 conductive. The conduction of the transistor 64 maintains the bias on the base of the transistor 70 at a level to maintain the transistor 70 nonconductive. When a sufficient amount of the smoke enters the chamber 58 to reduce the amount of light impinging on the photocell 56 and increases the resistance thereof to a predetermined level, the transistor 64 is rendered nonconductive. When the transistor 64 changes from a state of conduction to a state of nonconduction, the voltage on the base of the transistor 70 increases causing it to be rendered conductive. Such conduction of the transistor 70 results in sufficient current flow through the relay coil 72 to close the relay contacts 78, thereby placing the resistor 80 in circuit to maintain the transistor 70 conductive. This arrangement effectively makes the relay 72 a latching relay.
A second set of relay contacts 82 are connected in series with an indicating device 84, such as a buzzer, across the lines 24, 26. When sufficient current is passed through the relay coil 72, the relay contacts 82 are closed to actuate the indicating device 84.
Additional inputs are provided for a variety of sensors and actuating devices. The normally open contacts of a intrusion detector, generally indicated with the reference numeral 86 are connected between the base of the transistor 64 and the line 26. If the normally open contacts of the intrusive detector 86 are closed, the transistor 64 will be rendered nonconductive, thereby forcing the transistor 70 to be rendered conductive to actuate the relay contacts 78 and 82. If the normally closed contacts of the intrusion detector 68 are open, the transistor 70 will be rendered conductive to actuate the relay contacts 78 and 82. Since the normally open contacts of the detector 86 and the normally closed contacts of the detector 68 are connected to relatively high impedance points in the circuit, any stray capacitance in the leads to those contacts will not adversely affect the operation of the circuit. As a result, such contacts can be employed with lead lengths up to several hundred feet in length. It has been found in practice that the operation of the detector circuit will not be adversely affected if the resistance of the leads to the contacts is maintained at a value no greater than 100 ohms. In addition, a fire detector or a "panic button" may be connected in parallel with the contacts 82 to actuate the indicating device 84. Such additional sensors and/or switches are diagrammatically illustrated by the block indicated with the reference numeral 88.
A switch 90 is connected between the base of the transistor 70 and the line 26 and is effective upon closure thereof to render the transistor 70 nonconductive. The witch 90 is employed to reset the detector circuit after it has been energized from one or more of the sensors connected thereto or from the action of the photocell 56.
The circuit illustrated in the drawing was constructed and successfully tested with the following values for the various components as shown therein:
R 38 100 ohms R 40 600 ohms R 44 470 ohms R 48 2000 ohms R 50 1000 ohms R 52 1000 ohms R 60 120 ohms R 62 500 ohms R 66 300 ohms R 74 30 ohms R 76 20,000 ohms R 80 5000 ohms C 22 100 microfarads C 92 50 microfarads
The principles of the invention explained in connection with the specific exemplification thereof will suggest many other applications and modifications of the same. It is accordingly desired that, in construing the breadth of the appended claims they shall not be limited to the specific details shown and described in connection with the exemplification thereof.