JPH0460277B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a mis-alarm prevention circuit that prevents a photoelectric smoke detector from becoming insensitive and unable to detect smoke, resulting in mis-alarms.

[Background technology]

Conventionally, photoelectric smoke detectors do not have a false alarm function, that is, a function that alerts them that their sensitivity has become insensitive and they are no longer able to detect smoke in the event of a fire; they simply issue an alarm when smoke exceeds a certain concentration. It outputs a signal. Therefore, the sensitivity has to be checked frequently, and there is a risk that the optical system for detecting smoke will become so dirty during the inspection that it will deteriorate and become inoperable in the event of a fire.

Generally, the internal circuit of a photoelectric smoke detector is configured as shown in FIG. 3, and the oscillation circuit 1 causes the light emitting element 2 to emit light momentarily at a period of about 3 seconds as shown in FIG. 4a. When the light emitting element 2 emits light, the light receiving element 3 receives the light and generates a small current, which is amplified by the amplification circuit 4 and output as a voltage Vs as shown in FIG. 4b. Since the optical system 5 is of a scattering type, normally only the light reflected from the wall of the optical system 5 enters the light receiving element 3, but when smoke enters at time t in Figure 4, it is reflected by smoke particles. Since light also enters the light receiving element 3, the voltage
Vs increases as smoke concentration increases. Voltage Vs
The relationship between the peak value of When it is off, there is smoke, and when it is off, there is no smoke, so the threshold is approximately
It becomes 0.6V. Therefore, during normal operation as shown by the solid line in FIG. 5, the smoke density operates at a%/m, but when the sensitivity deviates toward the false alarm side as shown by the broken line, it changes to b%/m. Further, since the voltage Vs when there is no smoke is A or B, in the conventional example, the transistor is off in either case, making it impossible to detect a false alarm.

[Purpose of the invention]

The purpose of the present invention is to automatically detect deterioration in the sensitivity of smoke detectors and output an alarm signal to prevent false alarms in the event of a fire, and to reduce the need for periodic sensitivity checks. It's about doing.

[Disclosure of the invention]

Embodiment In FIG. 1, an input terminal IN is connected to an amplification circuit 4, and a voltage Vs obtained by amplifying the output of a light receiving element 3 that detects smoke is inputted thereto. This voltage Vs is input to the transistor _Tr1 , which is the main switching element, and as in the conventional case, an alarm is issued when the smoke concentration reaches a certain level. The voltage Vs is applied to two FETs (FET ₁ ,
_FET2 ) is input to the differential amplification circuit 6 as a voltage comparison circuit, and is compared with a reference voltage _V1 obtained by dividing the circuit voltage by a resistor _R3 and a variable resistor VR, and Vs> _V1 . If so, FET ₁ is turned on and output is output from differential amplifier circuit 6. The reference voltage _V1 is set to determine the output voltage of the amplification circuit 4 obtained by reflection on the wall of the smoke monitoring space when there are no smoke particles in the smoke monitoring space, and
The output voltage of the amplifying circuit 4 obtained when the light emitting element 2 emits light is set to be lower than the normal voltage, but higher than the voltage when the sensitivity to the false alarm side decreases. The reason why FETs are used in the differential amplifier circuit 6 is because the voltage Vs is a low value of 0.6V or less, so it cannot be configured with transistors. It can be turned on. Further, the reason why the voltage V ₁ is adjusted by the variable resistor VR is that the voltage V ₁ varies due to variations in the characteristics of the optical system. The output of the differential amplification circuit 6 is input to the AND circuit 7, and when both the output of the differential amplification circuit 6 and the output of the oscillation circuit 1 that drives the light emitting element 2 are input to the AND circuit 7, the AND circuit is input. 7 outputs an output, and turns on the transistor Tr ₄ , which is the first switch element forming the discharge circuit 8. That is, when the light emitting element 2 emits light according to the signal shown in FIG. 2a and the voltage Vs has the waveform shown in FIG. 2b, t ₁
-t ₂ , t ₅ - t ₆ and t ₇ - t ₈ , the transistor
Both Tr ₂ and Tr ₃ turn on, so transistor Tr ₄ turns on. However, if the voltage Vs is noise, such as between t ₃ and t ₄ , transistor Tr ₃ does not turn on, so transistor Tr ₄ turns on. Does not turn on. In other words, even if FET ₁ is turned on due to a change in voltage Vs due to noise, it will not be affected at all as long as the time period is outside the light emission period. The switching circuit 9 is a CR time constant circuit 10 and a second switching element.
PUT, and when the voltage of the capacitor C exceeds a certain voltage, a switching operation is performed, and the capacitor C is discharged when the discharge circuit 8 is operated.

Operation When the optical system is normal, the voltage Vs is input to the differential amplifier circuit 6 and compared with the voltage V _{1. If Vs>V 1 ,} FET ₁ is turned on, and at the same time, the transistor Tr _{2 is} turned on.
is also turned on, and if transistor Tr ₃ is on at this time, transistor Tr ₄ is also turned on and capacitor C
The electric charge that has been charged in is discharged. Therefore,
Voltage V ₂ does not exceed voltage V ₃ as shown in FIG. 2c, and PUT of switching circuit 9 is not turned on.

Next, consider the voltage when there is no smoke due to dirt in the optical system, etc.
When the value of Vs becomes low, that is, when the sensitivity shifts to the side of false alarm, when the voltage Vs no longer exceeds the voltage V ₁ , FET ₁ no longer turns on, and transistors Tr ₂ and Tr ₄ also turn on. Instead, capacitor C is charged with a time constant R ₇ ×C, and PUT is turned on when voltage V ₂ exceeds V ₃ . A missed alarm can be realized by issuing an alarm when PUT is turned on. Missed alarms do not need to be as urgent as fire alarms, so the time constant
It is sufficient to increase R ₇ ×C and 1 to 5 minutes after the voltage Vs falls below V ₁ , and by increasing the time constant, false alarms can be reduced.

〔Effect of the invention〕

As described above, since the present invention has the above-described configuration, when the sensitivity for detecting smoke decreases to the side of a false alarm due to dirt on the optical system, etc., the amplification circuit when the light emitting element emits light is activated. By utilizing the fact that the output voltage becomes lower than the reference voltage, it is possible to detect a decrease in sensitivity; in this case, the output of the voltage comparator circuit does not invert and the first switch element does not turn on. The voltage at the terminals of the capacitor then increases, turning on the second switch element and allowing a false alarm to be issued. In other words, the present invention has the effect that a decrease in sensitivity can be notified by automatically detecting a decrease in sensitivity and turning on the second switch element. As a result, not only can false alarms be prevented in the event of a fire, but the number of periodic sensitivity checks can also be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an embodiment of the present invention, Fig. 2 is a voltage and current waveform diagram of the same main parts as above, Fig. 3 is a block circuit diagram of main parts of a general photoelectric smoke detector, and Fig. 4 is a block circuit diagram of the main parts of a general photoelectric smoke detector. The main part voltage and current waveform diagrams same as above, and FIG. 5 are characteristic diagrams same as above. DESCRIPTION OF SYMBOLS 1... Oscillation circuit, 2... Light emitting element, 3... Light receiving element, 4... Amplification circuit, 6... Differential amplification circuit, 7
...AND circuit, 8...Discharge circuit, 9...Switching circuit, 10...CR time constant circuit, C...Capacitor.

Claims (1)

[Claims] 1. A light emitting element that is driven by an oscillation circuit and intermittently irradiates light into the smoke monitoring space, a light receiving element that receives light scattered by smoke particles in the smoke monitoring space, and an amplification circuit that amplifies the output of the light receiving element. , a main switch element that turns on when the output voltage of the amplification circuit is above a predetermined voltage, and an output that issues an alarm when it is determined that the smoke density has exceeded a predetermined level based on the on/off of the main switch element. A false alarm prevention circuit for detecting a decrease in smoke detection sensitivity in a photoelectric smoke detector equipped with a switching circuit, which compares the output voltage of the amplification circuit with a predetermined reference voltage obtained by dividing the circuit voltage. a voltage comparator circuit that inverts the output when the output voltage exceeds a reference voltage; a first switch element that is turned on when the output of the voltage comparator circuit is inverted; and a first switch element that is constantly charged through a predetermined resistor. a capacitor that is discharged when the terminal voltage of the capacitor is turned on, and a second switch element that is turned on when the terminal voltage of the capacitor exceeds a predetermined voltage and issues a false alarm that indicates a decrease in smoke detection sensitivity; is set to be lower than the normal voltage of the amplification circuit output voltage when the light emitting element emits light when there are no smoke particles in the smoke monitoring space, and higher than the voltage when smoke detection sensitivity decreases. A false alarm prevention circuit for photoelectric smoke detectors featuring: