United States Patent 3696382

A smoke and heat alarm system employing a pulse-forming transmitter to produce a ringing perturbation in the household line voltage when actuated. This perturbation is sensed by a remote alarm device on the same line to produce a remote alarm. An optical smoke detector which is relatively insensitive to line voltage variations is employed in the transmitter of the alarm system.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
340/520, 340/538, 340/691.4
International Classes:
G08B25/06; H02J13/00; (IPC1-7): H04M11/04
Field of Search:
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US Patent References:

Other References:

Westinghouse SCR Designer's Handbook, First Edition, 1963, pp. 4-19, 4-20 TK2798M8 .
GE Transistor Manual, Sixth Edition, 1962, p. 340.
Primary Examiner:
Caldwell, John W.
Assistant Examiner:
Mooney, Robert J.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows

1. A smoke detector of the type which produces a local alarm signal as well as an excitation signal for actuation of a remote alarm device in response to the detection of smoke and comprising: a local alarm device and a selectively actuated current switch having a gate electrode, said device and switch being connected serially for application across an AC power line, detector means connected in parallel with said device and switch for application across the AC power line and comprising the combination of a light-sensitive resistor and a transistor having base, emitter, and collector electrodes, the light-sensitive resistor being connected to the gate electrode of the switch and to the collector-emitter circuit of the transistor whereby conduction of the transistor through the collector-emitter circuit thereof tends to subtract from current flowing from the resistor to said gate electrode, a light source connected for application across the AC power line and responsive to the voltage variations therein to produce a variable light output, said light output being directed by any smoke in the atmosphere proximate the detector to the light-sensitive resistor, means for connecting AC power from the line to the base electrode of the transistor when said smoke detector is connected across the AC power line whereby the conductivity of the transistor is varied according to any voltage variations in the line to shunt current away from the gate electrode and to maintain a relatively constant actuation of said switch irrespective of variations in said line voltage and light output, and remote alarm actuating means also connected for application across the power lines, said alarm actuating means comprising an SCR connected to the detector means to be rendered conductive thereby upon detection of smoke, and a reactive impedance connected in series circuit with the SCR to produce a ringing perturbation during positive half-cycles of the AC line voltage which ringing perturbations are transmitted over the AC power line for actuation of a remote alarm device connected thereto.

2. Apparatus as defined in claim 1 including a remote alarm device connected across said AC power line and responsive to the ringing perturbations produced by said remote alarm actuating means to generate an audible alarm signal.

This invention relates to alarm systems such as fire alarms and particularly to a system for actuating a remote alarm by causing a distinct perturbation in the line which supplies electrical power to that alarm.

It is desirable for adverse condition alarm systems, such as fire alarm systems, to have the ability to actuate a remote alarm device either singly or in combination with a local alarm. In a household fire alarm system, for example, a detector of excess smoke and heat may actuate both a local alarm device as well as a second device which is disposed in a remote part of the house or in an adjacent building.

In accordance with the present invention, a detector for an adverse condition such as excess smoke and heat is operable to actuate a remote alarm device by transmitting actuation signals to the remote alarm by way of the power line or wiring network which commonly supplies electrical power to both the detector and the remote alarm. The ordinary wiring network in the typical household, thus, serves as the medium by which the detector communicates with the remote alarm, eliminating the need for costly and complex interconnections between the remotely located devices.

In accordance with a more specific aspect of the invention, the alarm system comprises a detector and a transmitter which responds to actuation by the detector to produce a ringing perturbation in the power line to which the detector transmitter is connnected. This ringing perturbation in the normal line voltage serves to actuate a remote alarm device which is connected to the power line to produce the remote alarm signal. In an illustrative example hereinafter described, the ringing perturbation takes the form of a series of discrete pulses in the normal line voltage waveform, these pulses being of a predetermined number and amplitude to actuate a remote alarm device. In one specific example, a capacitive discharge is employed to produce the ringing perturbation in an AC 60-cycle power line, the number of pulses per positive half-cycle of the line voltage being between six and 45.

In accordance with a second aspect of the invention, a detector is provided for detecting the presence of excess smoke in the air by means of an optical detection arrangement employing the well-known Tindal effect. However, in the smoke detector of the subject invention, compensation means are employed for rendering the detector substantially insensitive to line voltage variations which cause a variation in the intensity of the light source employed in the optical detector means which intensity variations without compensation would cause a variation in the sensitivity of the smoke detection. In a specific embodiment hereinafter described, the compensation means utilizes a simple transistor circuit which is connected between the photoelectric sensor and a switch device to cumulatively modify the normal line voltage sensitivity curve of the photoelectric detector means so as to produce an area of substantial insensitivity, the modified detector means being constructed so as to operate over this area of relative insensitivity.

Further features and advantages of the subject invention will become apparent from a reading of the following specification which sets forth a specific and illustrative embodiment of the invention in detail. This specification is to be taken with the accompanying drawings of which:

FIG. 1 is a simplified schematic view of an alarm system employing the invention;

FIG. 2 is a schematic diagram of a detector-transmitter for use in the invention;

FIG. 3 is a waveform diagram indicating the nature of the ringing perturbation produced by the transmitter of FIG. 2;

FIG. 4 is a schematic diagram of a compensated smoke detector and local alarm;

FIG. 5 is a sensitivity curve for the circuit of FIG. 4; and,

FIG. 6 is a schematic diagram of remote alarm device for use with the detector-transmitter of FIG. 2.

Referring first to FIG. 1, a household fire alarm system 10 is shown to comprise a detector transmitter 12 which is connected across a common household 110-volt AC power line having conductors 14 and 16. At a point which is remote from the detector transmitter 12 a remote alarm device 18 is also connected across the 110-volt AC power line. Accordingly, when the detector-transmitter 12 senses a fire condition indicated by either excessive smoke or excessive heat or both a ringing perturbation is produced in the AC line voltage waveform between conductors 14 and 16. This ringing perturbation is communicated by way of the conductors 14 and 16 to the remote alarm device 18 to trigger the generation of a remote alarm signal. Detector-transmitter 12 may include an additional local alarm which operates simultaneously with the remote alarm device 18.

Referring to FIG. 2, the detector-transmitter 12 is shown in greater detail to include an excess smoke and heat detector 20 and a local alarm device 22 connected in series across conductors 14 and 16 of the AC power line. Detector 20 is responsive to a condition of excess smoke or excess heat to permit current to flow through the local alarm 22 to produce an alarm signal. In addition, detector 20 controls the voltage of point 23, either letting it follow the voltage of conductor 14 in the absence of an alarm condition or forcing it to closely follow the voltage of conductor 16 upon the existence of an alarm condition. In the circuit of FIG. 2, the conductor 16 is shown as the ground or reference voltage indicating that the voltages within the circuit are measured for purposes of discussion with reference to the voltage on conductor 16.

For the purpose of explaining the operation of the transmitting circuit, only the alarm condition will be described. In that case point 23 is at essentially the same voltage as line 16 and, consequently, point 23 will be defined as line 16.

The transmitter portion of detector-transmitter circuit 12 includes an inductor 26 and a capacitor 28 having a parallel resistor 30 connected in series with a fixed value resistor 32, a diode 34, and a variable resistor 36 between the power line conductors 14 and 16. Also connected in parallel-shunt relation with resistor 32, diode 34, and variable resistor 36 is an SCR 38 having a gate electrode connected to the point 42 between one end of resistor 32 and the anode of diode 34 for control purposes.

When detector 20 senses an excess heat or smoke condition and imposes the voltage of line 16 upon point 23, the transmitter begins to function. The transmitter only functions during the half cycle when line 14 is positive with respect to line 16. As the voltage of line 14 rises the voltage of point 40 also rises. Since resistors 32 and 36 act as a voltage divider with reference to point 42, the potential on the gate electrode of SCR 38 eventually becomes high enough to fire the SCR to provide a very low resistance current path from point 40 to ground. This abruptly charges capacitor 28 to the voltage which exists between line 14 and line 16 at that moment and in doing so draws current heavily from line 14 for a brief instant. It is this short duration excessive current which perturbs the voltage across line 14 to line 16 and causes a line voltage transient. When the SCR 38 is conductive, the voltage at point 40 abruptly returns to ground removing the gate drive on SCR 38. The reactive impedance characteristic introduced by inductor 26 and capacitor 28 causes the voltage at point 40 to go slightly negative at this point, thus, terminating the flow of current through the SCR 38 even though the line voltage is still in the positive half cycle. When the SCR 38 is turned off, resistor 30 discharges capacitor 28 causing the voltage at point 40 to gradually rise above ground again firing the SCR 38 and repeating the entire cycle.

It may be desirable to connect a germanium diode across the gate and cathode SCR 38 of FIG. 2 to stabilize the pulse number against variations due to temperature changes.

As indicated in FIG. 3, where anode to cathode voltage experienced by SCR 38 is shown as 46, where the line 14 to line 16 voltage is shown as 44, and where the detailed waveform of 44 at the time of discharge of SCR 38 is shown as 45, it is seen that voltage perturbations appear on the line voltage waveform every time that SCR 38 fires and have the natural ringing effect due to the reactance of the power line. These perturbations which will also be referred to as pulses are discernable throughout the household wiring system.

There are many electrical devices found in the typical household which produce perturbations in the line voltage when used. As an example, a vacuum cleaner produces a great deal of random frequency and amplitude variation noise in the line voltage as it is used. A light dimmer also produces as substantial line voltage perturbation during use. Accordingly, it is necessary to distinguish between line voltage perturbations or variations which are caused by these devices and that perturbation represented by the series of pulses 45 on 44 in FIG. 3 which are to actuate the remote alarm device 18 of FIG. 1. To give these perturbations a distinction from other accessory caused line voltage variations, the amplitude of the pulses 45 as well as the number of pulses per positive half-cycle of line voltage waveform 44 is controlled. In accordance with the invention, the number of pulses 45 per positive half-cycle of 60-cycle AC line voltage is preferably selected between six and 45. The lower limit of this number is selected to render the remote alarm device 18 insensitive to the pulses that are generated from solid state light dimmers which typically emit two pulses per cycle of 60-cycle line voltage. By setting the number of pulses at six, the use of two such solid-state light dimmers in the household wiring system would still be insufficient to produce a false alarm. The lower limit on the number of pulses 45 per positive half-cycle of line voltage may, however, be set as high as 10. The upper limit on the number of pulses 45 is set principally by amplitude considerations. Using a capacitive circuit of the type illustrated in FIG. 2 having a definite RC time constant, an attempt to generate a large number of pulses would result in a very low amplitude pulse. In practice, an upper limit of between 25 and 45 has been found to exist. In view of both upper and lower limit consideration the preferred number of pulses per positive half-cycle of 60-cycle AC line voltage is 14.

Referring now to FIG. 4, the detector 20 is illustrated in greater detail. In circuit 20 the series combination of a diode 48, a resistor 50 and a capacitor 52 are connected between the conductor 14 and 16 of the AC power line. Thus, during each positive half-cycle of the 110-volt AC voltage, current is caused to flow through the diode 48, resistor 50, and capacitor 52 to charge the capacitor. A reverse current flow is prevented by the diode 48. The voltage across capacitor 52 is applied to a normally open circuit thermostatic switch 54 which senses excess heat and a photoelectric cell 56 which operates in cooperation with a light source 64 to sense excess smoke in the air. The parallel combination of thermostatic element 54 and photocell 56 is connected in series with a current-limiting resistor 58 one end of which is connected directly to the gate electrode of an SCR 60. The series combination of SCR 60 and a horn 22 which operates as a local alarm is connected directly across conductors 14 and 16. Accordingly, when the resistance of photoelectric cell 56 is rendered sufficiently low or when the presence of excess heat causes the switch 54 to close, part of the voltage on capacitor 52 is applied through resistor 58 to the gate electrode to SCR 60 causing the SCR to become conductive. This permits current to flow through the local alarm device 22 sounding a loud alarm in the area of the detector 20. Point 23 of FIG. 2 is connected to point 61 of FIG. 4 causing the transmitter of FIG. 2 to become operative whenever the horn 22 of FIG. 4 is operative.

The smoke detector further includes the series combination of a light source 64 and a current-limiting resistor 66 connected directly across the conductors 14 and 16. The light source 64 is directed in such a fashion as to produce a beam which crosses the observation path of the photocell 56 but which is not directly in line with the observation path of the photocell 56. Accordingly, when the air is reasonably clear, little or no light is reflected from the source 64 to the photocell 56 and its resistance is high. However, when smoke is present in the air in sufficient quantity, that smoke causes a reflection of the light from light source 64 to the photocell 56 lowering the resistance thereof and producing the aforementioned effect of sounding the local alarm 22. This manner of smoke detection is well known in the art and, thus, is not described in great detail herein.

Because the ordinary incandescent lamp is sensitive to line voltage variations to produce variations in output illumination, the sensitivity of the smoke detector 20 can be variable along with variations in line voltage. It appears that the light intensity of the ordinary incandescent lamp varies approximately as the 3.5 power of the line voltage and, thus, rather extreme variations in sensitivity of the detector can occur from variations in line voltage. To compensate for this line voltage sensitivity, a voltage divider comprising resistors 68 and 70 is connected in parallel with capacitor 52. An NPN transistor 72 has the collector connected commonly to the resistor 58 and the gate electrode of SCR 60 and the emitter connected through resistor 74 to the ground conductor 16. The base electrode of transistor 72 is connected to the junction between voltage divider resistors 68 and 70. Because resistor 74 is in series with the emitter of transistor 72, there is a relatively straight and linear transfer characteristic between the voltage on the base of transistor 72 and its collector current. Since the collector current of transistor 72 subtracts from the gate drive of SCR 60 and since transistor 72 only begins to conduct above a given value of line voltage, the otherwise 3.5 power curve of smoke detector sensitivity versus line voltage is modified by the presence of transistor 72 as shown in FIG. 5 to produce a dip 77 in the otherwise exponential smoke detector sensitivity curve 76. Between points a and b of this dip in the sensitivity curve lies an area of substantial insensitivity to line voltage. The values of the components in the circuit of FIG. 4 are preferably chosen so as to produce operation within this intermediate region of insensitivity to line voltage, thus, producing an optical-type smoke detector which is relatively insensitive to variations in line voltage within the range defined by the points a and b. To accomplish this objective, transistor 72 plus resistor 74 or a device or devices having a similarly straight line current voltage characteristic must be used to subtract from the gate drive to SCR switch 60.

Referring now to FIG. 6, the circuitry of the remote alarm device 18 of FIG. 1 is shown in schematic detail to include a pulse counting and amplitude sensing circuit connected across the AC power line conductors 14 and 16. Again, the voltage on line 16 is taken as a ground or reference voltage. The series combination of a capacitor 78 and an inductor 80 is connected between conductors 14 and 16. A resistor 82 is connected in parallel with inductor 80 the midpoint between the capacitor 78 and the inductor 80 being identified as 84. The values of these components are selected such that the voltage at point 84 closely approximates the voltage on conductor 16 whenever unperturbed 60-cycle AC appears across the lines. However, when a high-frequency component appears on the line, a significant portion of that component also appears at point 84. An NPN transistor 86 has the base or control electrode connected to a tap on resistor 82 which is set so that normal line hash, such as that produced by a vacuum cleaner, is of insufficient amplitude to trigger transistor 86 into conduction whereas pulses 45 of FIG. 3 are of sufficient amplitude to cause the transistor 86 to conduct. Resistors 88 and 90 act as a voltage divider stepping down the high voltage supply achieved through diode 92 and resistor 94 and capacitor 122. The voltage on the collector of transistor 86 is relatively constant except when there are strong, relatively high frequency transients or perturbations on the line at which time the voltage on the collector of transistor 86 makes excursions to the voltage on conductor 16. These voltage excursions acting through capacitor 96, diode 98, and diode 100 serve to charge a capacitor 102 negatively with respect to conductor 16. Since resistor 104 is continuously discharging the capacitor 102, the greater the rate of incoming pulses or excursions, the more negative is the potential at point 106. The potential on point 106 is reflected through resistor combination 108 and 110 to the base of transistor 112 which controls the gate electrode current for SCR 114. When the line perturbation signal produced by the transmitter shown in FIG. 2 reaches the remote alarm, transistor 112 goes from an on or conductive condition to an off or nonconductive condition, thus, releasing the gate of SCR 114 to the influence of resistor 115.

Transistor 170 is maintained in the conduction state by resistor 171 at all times when the line voltage is positive, consequently the gate of SCR 114 is first released by both transistor 112 and transistor 170, and consequently first fires, when the voltage of line 14 goes from a positive value to a negative value. SCR 114 discharges the charge stored in capacitor 122 through the voice coil 116 of a speaker once each cycle of the line to activate the speaker and produce the remote alarm signal.

Accordingly, the circuit of FIG. 6 includes the elements 78, 80, 82 and 86 which respond to pulse amplitude and the elements 88, 90, 96, 98, 100, 102, and 104 which respond to pulse repetition rate to activate the voice coil 116 of the remote alarm speaker only when pulses of the proper amplitude and rate of occurrence are caused to appear as line voltage perturbations between conductors 14 and 16.

It is to be understood that the foregoing description of specific circuitry is intended to be illustrative in nature and is not to be construed as limiting the inventions.