United States Patent 3717862

A fire detecting system embodying a plurality of ionization smoke detectors and a receiving unit for supplying electrical energy to the smoke detectors, for sounding an alarm in the event a detector is actuated and for applying predetermined voltages to the detectors to test both operability and sensitivity.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
250/381, 250/385.1, 340/514, 340/629
International Classes:
G08B17/11; (IPC1-7): G08B17/10; H01J39/28
Field of Search:
340/237S,214 250
View Patent Images:
US Patent References:
3564524N/A1971-02-16Walthard et al.

Primary Examiner:
Caldwell, John W.
Assistant Examiner:
Myer, Daniel
What is claimed is

1. A fire detecting system comprising a pair of conductors, a plurality of ionization detectors each having a closed ionization chamber and an open ionization chamber connected in series across said conductors and a circuit including a field effect transistor connected to said chambers to detect an impedance change upon the entrance of smoke in said open chamber, a receiving unit including a voltage supply connected to said conductors and a relay connected in series with one of said conductors, a third conductor extending from said receiving unit, each of said circuits being connected between one of said pairs of conductors and said third conductor, means in said receiving unit including an impedance and switch for selectively interconnecting the other of said pair of conductors and said third conductor directly and through said impedance, and means for indicating the voltage drop across said impedance, each of said circuits upon being actuated by its associated ionization chamber produces an increase in current in said conductors to actuate said relay, said indicating means indicating the number of detectors that have been actuated upon the insertion of said impedance between said third conductor and the other of said pair of conductors.

2. A fire detecting system according to claim 1 wherein said impedance is a resistor.

3. A fire detecting system according to claim 2 wherein each of said circuits includes switching means interconnected with said transistor and actuated thereby, said switching means being connected between said one of said pair of conductors and said third conductor, and a resistor connected between the side of said switching means connected to said one of said pair of conductors and the other of said pair of conductors.

4. A fire detecting system according to claim 1 including means for applying different voltages between said pair of conductors.

This invention relates to fire detection systems and more specifically to a novel and improved system embodying a plurality of ionization smoke detectors and means for testing the detectors without introducing smoke into each of the detectors.

Ionization smoke detectors utilized in this system each include a pair of ionization chambers connected in series across a voltage source. Each of the chambers includes a pair of electrodes and a radioactive source. One of the chambers in each detector is closed to ambient air while the other chamber is open to permit smoke to enter the chamber. A field effect transistor is utilized and the gate electrode is connected to the junction between the two chambers. The source-drain path of the transistor is connected in series with a load resistor and across said voltage source. A relay element such as a silicon controlled rectifier or the like has its conduction path also connected across the voltage source and the control electrode is connnected to the transistor and is responsive to an increase in current in the source-drain path and effectively short-circuits the voltage source when it is fired.

In prior fire detecting systems a plurality of ionization smoke detectors were connected in parallel across the terminals of a receiving unit which included a voltage source for applying a voltage to the series connected chambers, the transistor, and the relay element. The unit also included an alarm device for detecting an increase in current caused by the relay element and thereby produced an alarm. More specifically when smoke entered the open ionization chamber of at least one of the detectors, the ionization current in the open chamber varied and produced a change in impedance with a resultant change of potential at the junction of both of the chambers. This change was sensed by the gate electrode of the transistor and caused an increase in the source-drain current path which in turn caused the relay element to conduct and thereby produce an alarm.

In such prior fire detecting systems, the individual detectors were tested or checked by introducing smoke into the open chamber of each detector. Such a procedure, however, was difficult and time consuming and involved considerable time and money.

One object of the invention resides in the provision of a novel and improved fire detecting system having a plurality of ionization smoke detectors and a receiving unit and wherein the receiving unit is constructed and arranged to test the ionization smoke detectors simultaneously without the need for introducing smoke into any of the detectors.


In accordance with the invention, the fire detecting system includes a receiving unit which supplies both operating voltages as well as test voltages to the ionization chambers. In addition, the voltage across each of the relay elements of the detectors is fed through a diode to the receiving unit so that the erroneous operation of any detector can be immediately detected in the receiving unit.

The above and other objects of the invention will become more apparent from the following description and accompanying drawings forming part of this application.

In the drawings:

FIG. 1 is a circuit diagram of an ionization smoke detector for use in a fire detecting system in accordance with the invention; and

FIG. 2 is a schematic diagram of one embodiment of a complete fire detecting system in accordance with the invention.

In the drawings like reference numerals have been used to denote corresponding components of the two figures.


Referring to FIG. 1, the ionization smoke detector in accordance with the invention includes a closed chamber 10 having a pair of electrodes 11 and 12 and a radioactive source 13. The detector further includes an open ionization chamber 20 having electrodes 21 and 22 and a radioactive source 23. The chambers are connected in series across voltage supplying conductors 1 and 3 with the electrode 11 of the chamber 10 being connected to conductor 1, the electrodes 12 and 21 being connected to the junction 7 and the electrode 22 being connected to the conductor 3. A field effect transistor 4 has a gate electrode G connected to the junction 7, a drain electrode D connected to conductor 1 and a source electrode S connected through a load resistor 8 to a third conductor 2. The source electrode is also connected through a zener diode 9 to the control electrode of a silicon controlled rectifier 6. The anode of the rectifier 6 is connected through an indicating lamp 5 to the conductor 1 while the cathode is connected to the conductor 3. The anode of the rectifier 6 is also connected through a series resistor 14 and a diode 15 to the conductor 3.

During normal operation of the fire detection system, the conductors 2 and 3 are connected together and a predetermined voltage is applied across the conductors 1 and 2 so that a constant ionization current flows through the ionization chambers 10 and 20. When smoke enters the ionization chamber 20, the potential at junction 7 and therefore the gate electrode increases. This decreases the impedance of the source-drain path of the transistor 4 and produces a corresponding increase in current and an increase of the potential of the source electrode S. When the voltage of the source electrode exceeds the critical voltage of the diode 9, the latter conducts and applies a voltage to the control electrode of the rectifier 6 causing it to conduct, illuminating the lamp 5 and actuating the alarm device in a receiving unit supplying energy to the conductors 1, 2, and 3.

FIG. 2 shows a complete fire detecting system in accordance with the invention and includes a plurality of ionization detectors 101, 102, 103, 104, . . . , connected in parallel across conductors denoted by the numerals 100, 200, and 300, which correspond to the conductors 1, 2, and 3 of FIG. 1. For simplicity, only the detector 101 has been illustrated in detail while the other similar detectors have been merely shown in block form. The three conductors 100, 200, and 300 are connected to the receiving unit generally denoted by the numeral 40 which is constructed in accordance with the invention.

In the receiving unit 40, the conductor 100 is connected through the coil of relay 33 and a normally closed switch 32 to the positive side of a suitable voltage source such as the battery 18. The conductor 200 is connected directly to the negative terminal of the voltage source 18 while the conductor 300 is connected to the movable contactor of switch 31 which is provided with fixed contacts 31-1, 31-2, 31-3, and 31-4. The contact 31-1 is connected through a resistor 16 to the conductor 200. The contact 31-2 is connected directly to the conductor 200. The contacts 31-3 and 31-4 are connected to two different potential points on the voltage source 18. The potential points on the voltage source 18 are selected so that a low positive voltage corresponding to a nonoperating voltage is applied to the conductor 300 when the movable contactor of switch 31 engages contact 31-3 and a high voltage corresponding to an operating voltage is applied to the conductor 300 when the movable arm engages contact 31-4.

The coil of relay 34 is connected at one end to the conductor 100 and at the other end through self-holding contacts 31-1 to the conductor 200 so that the coil 34 can only be energized when the switch 32 is in the closed position. Contacts 33-1 forming part of relay 33 are connected in parallel with the contact 34-1 so that operation of the relay 33 by an increase in current in conductor 100 will close contacts 33-1 which energizes relay 34, the latter remaining in the energized condition by reason of the closure of contacts 34-1 until such time as the switch 32 is opened. The relay 34 is further provided with normally open contacts 34-2 which short-circuit the coil of relay 33 when the relay 34 is energized. A third set of normally open contacts 34-3 associated with relay 34 are connected in series with an alarm 17 and an associated voltage source so that closure of the contacts by energizing relay 34 will sound the alarm. A voltmeter 19 is connected between the conductors 200 and 300. The value of resistor 16 is selected so that it is equal to the resistors 14 in the respective detectors 101, 102, etc.

Under normal operating conditions the movable arm of switch 31 engages contact 31-2 to short-circuit conductors 200 and 300. Should smoke enter any one of the detectors and cause the silicon controlled rectifier 6 to fire, the associated lamp 5 will be illuminated and the increased load current will energize relay 33 which in turn will energize relay 34 and thereby sound the alarm 17. As pointed out above, the relay 34 will remain energized until the switch 32 is open even through the detector ceases to indicate the presence of smoke. The contact 34-2 which short-circuits the relay 33 prevents an excessive voltage drop which may occur from the simultaneous actuation of a plurality of detectors.

In order to test the detectors, the presence of smoke can be simulated by placing the movable contactor of switch 31 in engagement with contactor 31-4 to apply a high positive voltage to the conductor 300 which corresponds to the voltage across the conductors 100 and 300 should smoke be present in the vicinity of one or more of the detectors. This procedure increases the voltage across each open ionization chamber 20 and thereby increases the voltage at each junction 7 and the associated gate electrode G of the field effect transistor 4. Thus each detector will react in the same manner as if smoke entered its ionization chamber 20 and each of the silicon control rectifiers 6 will be driven into conduction.

Operation of all of the detectors can be determined by the magnitude of the voltage indicated by the voltmeter 19. More specifically, if the movable arm of switch 31 is moved to contact 31-1 should any one or more of the detectors be inoperative, the voltmeter will indicate the number of inoperative detectors. The inoperable detectors can be located by inspecting each detector to determine if the lamp 5 has been illuminated.

Should the voltmeter indicate zero voltage, it is unnecessary to inspect the detectors since it will be known that all detectors are operating properly.

If it is desired to test the detectors to insure that the detectors will not be actuated in the presence of minute amounts of smoke, the movable contactor of switch 31 is placed in contact with the contact 31-3 to apply a low voltage to the conductor 300 which would correspond to a nonoperating voltage. This causes a slight increase of the gate voltage of the field effect transistors of each detector. The increase in voltage, however, must not result in firing of the silicon controlled rectifier 6. Should one of the rectifiers be fired, then it will be known that the sensitivity of the detector is too high as the alarm will be sounded.

As described above, the fire detection system in accordance with the invention affords a high degree of convenience and economic savings both in maintenance and testing since the testing operation of each detector can be readily carried out at the receiving unit and without effecting the normal operation of each of the ionization smoke detectors.

While only one embodiment of the invention has been illustrated and described, it is apparent that alterations, modifications and changes may be made without departing from the true scope and spirit thereof as defined by the appended claims.