Abnormal Condition Responsive Means With Periodic High Sensitivity

Abnormal condition responsive devices such as ionization or photoelectric smoke detectors can be set to be highly sensitive so that with a smoke detector the detection of a smoldering fire can be made. With such a highly sensitive smoke detector, the number of false alarms which might be brought about by other conditions, whether it is in the detector or in the circuit, greatly increases with the highly sensitive detection device. On the other hand, if the sensitivity is low, a smoldering type fire can go undetected for a considerable period of time resulting in a buildup of a highly dangerous condition.

The present invention is concerned with an abnormal condition responsive device or smoke detector wherein the sensitivity of the responsive device is periodically increased for a short period of time to make the device more sensitive to smoldering fires and yet during a greater portion of the operation time the sensitivity is quite low to minimize the possibility of false alarms. In order to provide the invention, a timing device periodically modifies the reference signal level needed to produce an alarm so that the sensing device has a high sensitivity for short periods of time. Additionally, during the high sensitivity period of time, the presence of an output signal is stored and if three successive output signals are provided with the high sensitivity period response, an output to the horn or annunciator is provided.

The single FIGURE is a schematic representation of the abnormal condition responsive device.

Referring to FIG. 1, the abnormal condition responsive device has a condition responsive sensor means or ionization chamber smoke detector 10 of the type shown in the Larry D. Larsen U.S. Pat. No. 3,849,685, issued Nov. 19, 1974. A sensing chamber 11 which is open to the atmosphere and a reference chamber 12 which is closed provide a voltage divider so that the voltage with respect to ground on electrode 13 varies with the intensity of the smoke in sensing chamber 11. A comparator or amplifier 14 has a signal voltage input 15, a reference voltage input 20 and an output 21. When the signal input voltage at 15 from sensor 10 drops to the reference voltage input, an output at 21 is available.

The normal or low level of sensitivity reference voltage signal or input is established by voltage divider 22. When the output of the ionization chamber 10 on input 15 compares with the normal reference voltage on input 20 established at 22, comparator 14 provides an output over a conductor 23 to gates 24 and 25 and a switch 30 to energize the annunciator or horn 31. The sensitivity of detector circuit as established by a low reference voltage at 22 for the normal operation is low to provide ionization chamber 10 which responds only to large magnitudes of smoke level to maintain the number of false alarms at a reasonable minimum. When the sensitivity of the circuit is high, various conditions both in the ionization chamber 10 and the circuit to which the ionization chamber is connected can result in the enerization of horn 31 when there is no smoke in the ionization chamber 10, causing a nuisance to the homeowner. The level of the reference voltage for the normal operation as established at 22, is set as a compromise between the level of sensitivity of ionization chamber 10 and the occurrence of nuisance or false alarms.

A conventional digital timer means 40 which is energized by a conventional oscillator circuit 41, provides for a short time pulse such as 10 to 100 milliseconds over conductor 42 at predetermined intervals such as one minute. The timing pulse is then placed on the input of the switching circuit 43 including a D type flip-flop circuit 46 similar to that described in the handbook COS/MOS Integrated Circuits Manual of the RCA Technical Series CMS-271, copyright 1972. The circuit identification is RCA CD4013 or equivalent, which is connected to apply a voltage over circuit 44 to the voltage divider 22 to modify the reference voltage applied to the reference input 20. When the pulse from the timing circuit occurs, the reference voltage established by the voltage divider 22 is modified and a higher level of voltage is applied to reference electrode 20 and a high level of sensitivity is established for a short time period. Ionization chamber 10 then responds to low magnitudes of smoke level and thus is more responsive to smoldering fire conditions.

A check logic circuit 50 has a NAND gate 51, at least two flip-flop circuits 52 and 53 (similar to 46) and a NAND gate 54. Circuit 50 upon receiving an input pulse from gate 51 activates flip-flop 52 and upon receiving a subsequent pulse, activates flip-flop circuit 53 until upon receiving the third pulse an output is provided over gate 54 to energize horn 31 through a circuit 55. When the switching circuit 43 is energized to modify the sensitivity of the detector to the high sensitivity, simultaneously a pulse is transmitted over circuit 60 to an input of gate s1 to activate the circuit 50. If an alarm output at 21 is provided when the high sensitivity reference voltage is applied to the comparator circuit 14, the output after passing through the inverter 61 is applied over circuit 62 to a second input of gate 51. When the simultaneous output over 60 and 62 are applied to gate 51, the first flip-flop 52 is activated. Subsequently upon a second output from the comparator and the pulse over 60, the second flip-flop 53 would be energized until upon receiving three simultaneous pulses indicating three abnormal conditions at detector 10, the output of circuit 50 would energize horn 31.

A reset gate 70 has a first input from the pulse circuit 60 over conductor 71 and second input from the output of comparator 14 over circuit 72. When these input are both available, the reset circuit is not activated; however, if a pulse over circuit 60 occurs and no abnormal condition is sensed by detector 10 to provide an output of the comparator 14, the reset circuit 70 is activated to reset the counting circuit 50 back to zero. The reset circuit then provides that the counter circuit 50 must receive three consecutive pulses of a low level abnormal condition of detector 10 before the annunciator or horn 31 is energized.

Under normal operation with a low level of sensitivity, smoke detector 10 operates in a normal manner in that when an output over electrode 13 drops to a predetermined reference value as established by the voltage on reference electrode 20, the comparator amplifier 14 provides an output over circuit 23 to energize the annunciator or horn 31. As the level of output over electrode 13 of detector 10 varies with the amount of smoke contained in chamber 11, with the low level of sensitivity as established by the reference voltage at 22, the number of false alarms due to other conditions will be maintained at a minimum.

To make the ionization chamber 10 responsive to smoldering fires which requires a detector having a high level of sensitivity, it would be possible to increase the sensitivity by the reference voltage at 22; however, with a constant high level of sensitivity the number of false alarms which provide quite a nuisance to the homeowner would greatly increase.

By means of the timer 40, each minute, a 15 millisecond pulse is placed over conductor 42 to activate the switching circuit 43. During this pulse, the counting circuit 50 is energized over conductor 60 to make it acceptable to the output of the comparator if an abnormal condition exists during that pulse. At the same time, a voltage is applied over circuit 44 to modify the reference voltage at electrode 20 to establish a high level of sensitivity for the detector 10. If during the time of the 15 millisecond pulse, the smoke condition in the sensing chamber 11 is low which might be the case with a smoldering fire, an output is provided and the output is applied to the counting circuit over circuit 62. If upon three subsequent timing pulses over the next two minutes there is an output from the ionization chamber 10, the three outputs will be counted by circuit 50 and an output to horn 31 will provide a horn operation indicative of the smoldering fire condition.

Not only does the present invention provide for a high level of sensitivity at ionization chamber 10 to respond to smoldering fires, but the assurance that a smoldering fire condition is taking place over a timing period is provided by requiring at least three subsequent outputs at the high level sensitivity of ionization chamber 10 before the annunciator or horn can be energized.