JPH04324595A - Light-sensitive type smoke detector - Google Patents

Abstract

PURPOSE: To prevent misalarming due to smoke resulting from cooking work, etc., by lowering the sensitivity of the smoke detector in the day and uses the detector to alarm a fire in its early stage by increasing the sensitivity in the night. CONSTITUTION: This smoke detector 10 has a threshold value which varies with external incident light. When there is the external light, the threshold value of the detector is increased to generate no alarm unless the concentration of smoke is high. When there is no external light, the threshold value of the detector is decreased. To generate the threshold value, plural series resistances R1, R2, and R3 are used. A semiconductor element 18 which senses radiant energy is connected to one resistance R2 in parallel and when the radiant energy is made incident, the photoconductor 18 increases in conductivity to short-circuit the parallel resistance R2, thereby increasing the threshold value. When there is no incident radiant energy, the semiconductor element 18 becomes substantially an open circuit to lower the threshold value.

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION This invention relates to fire detectors. More particularly, the present invention relates to light sensitive fire detectors.

0002

BACKGROUND OF THE INVENTION Smoke or combustion detectors are commonly available and found in many commercial buildings and residential buildings. These detectors are important for early warning of fires, especially at night when residents are asleep.

One type of smoke detector commonly used utilizes low-level indoor radiation sources to detect smoke. Other types keep the radiant energy source and photodetector separated;
Monitor smoke levels.

Known household or residential detectors usually have a predetermined threshold value and compare this with an electrical signal that varies in relation to the density of smoke or combustion material. If the changing electrical signal exceeds this threshold by more than a predetermined value, the detection device will issue an alarm.

[0005]

[Problem to be solved by the invention] Conventional detectors are
It doesn't take into account that cooking fumes can also set off unintentional false alarms in small homes. Therefore, there is a need for a smoke detector of a level that is less sensitive to combustion products associated with normal cooking operations, but provides adequate safety, especially during nighttime non-cooking periods.

[0006]

SUMMARY OF THE INVENTION A variable sensitivity smoke detector has a first level of sensitivity during the day and a second level of sensitivity during the night. The detector includes a base and a device mounted on the base to detect smoke.

[0007] This detector has variable sensitivity. It has a radiant energy sensing electrical circuit coupled to a smoke detection device. The radiant energy sensing circuit senses incoming radiation or radiant energy and responsively reduces the sensitivity of the smoke detection device.

The incident radiant energy may correspond to visible light. The circuit for sensing radiant energy may include optical devices such as phototransistors and cadmium sulfide CdS variable resistance cells. Furthermore, phototransistors can also be used as optical elements.

[0009] The threshold of a smoke detection device may be created using a plurality of resistors connected in series. By connecting an element sensitive to radiant energy in parallel with one of the resistors, the threshold is varied as a function of the incident light.

[0010] When radiant energy is applied to an element that senses radiant energy, one of the series resistors shorts out. As a result, a first threshold is created.

[0011] In the absence of incident radiant energy, the device exhibits high impedance characteristics. The parallel resistors then form a second lower threshold or reference value as part of a series voltage divider. In this way, a low reference value is used for nighttime use, and a high reference value is used for daytime use.

[0012] In order for a smoke detection device to issue an alarm, the output of the smoke detection device must exceed a predetermined threshold. The threshold is high during the day, so smoke levels must be high for the device to issue an alarm. On the other hand, at night, when there is no incoming radiant energy, the threshold is low and the device will issue an alarm even at low smoke densities.

[0013] Many other advantages and characteristics of the invention will be apparent from the invention and examples described in detail below, as well as from the claims and associated drawings, which fully disclose the details of the invention as part of this specification. It becomes clear.

While the invention may take many different forms, it is understood that the disclosure herein is illustrative of the principles of the invention and is not intended to limit the invention to the particular applications shown. Certain embodiments are shown in the drawings and described below.

[0015]

[Embodiment] FIG. 1 shows an ion type smoke detector 10 according to the present invention.
FIG. Detector 10 is mounted on base 12.

Ambient smoke is detected by a normal ionization type sensor 14.
It can be detected using Alternatively, an optical sensor may also be used.

The output electrical signal on line 16 is connected to comparator CA
connected to. Series resistors R1, R2, and R3 provide a reference voltage VREF to input pin 13 of comparator CA.

The output voltage VCENT from sensor 14 produced on line 16 is connected to reference voltage V by comparator CA.
Compared to REF. If the voltage on line 16 is greater than VREF, device 10 will issue an alarm.

A light sensitive cell, such as a CdS photocell 18, is connected in parallel to resistor R2. During the day, the resistance of the cell is very low. Resistor R2 is therefore short-circuited.

At night, the resistance of the CdS photocell 18 is very high. In this case, resistor R2 is not short-circuited and creates the reference voltage VREF of the comparator as part of a voltage divider.

FIG. 2 shows a resistor string formed by R1, R2, and R3. The reference voltage VREF of comparator CA is R1, R
2, created at node 20 by the substantial value of R3.

The resistance value of the CdS cell 18 changes depending on the incident radiant energy. As the level of radiant energy incident on cell 18 increases, the internal resistance of the cell decreases significantly.

During the day, the resistance value of R2 becomes zero in the resistor array, so the reference voltage increases. Therefore, during the day, the voltage at the central electrode of the ion chamber must be very high, making it difficult to generate an alarm.

At night, the reference voltage drops and the sensitivity of the detector increases. Therefore, the output voltage V from the central electrode of the ion chamber
If CENT increases slightly, it exceeds the reference voltage and an alarm is generated. When there is a lot of ambient light,

00
25]

[Equation 1] RCdS <<R2<R1<R3,
VREF (bright) = VS [R3/R1
+R3] (1)

[0026]
Resistor R2 is shorted when exposed to light. When there is little ambient light

[0027]

[Math. 2] RCdS >> R2 Also

[0028]

[Math. 3] VREF (dark) = VS [R3/R
1+R2+R3] (2)
ΔV=VREF −VCENT

It is clear from equations (1) and (2) that VREF (bright)>VREF (
dark). As VREF increases, ΔV increases. The sensitivity of the smoke detector is therefore reduced. The reverse is also true. As a result, the output voltage VCENT must increase further to create an alarm condition. The values of resistors R1, R2, R3 are chosen according to the above formula.

For example, the resistors R1, R2, and R3 have the following values. R1=820KΩ R2=180KΩ R3= 1MΩ

The resistance value of the CdS cell 18 is RCdS <500K when a 2-foot lamp or more light is input.
Changes to Ω.

From the foregoing, many changes and modifications may be made without departing from the spirit and scope of the invention. It is clear that no limitations are or should be given as to the specific equipment illustrated. Of course, the scope of the claims includes all such modifications.

[Brief explanation of drawings]

1 is a schematic diagram of the circuit of a smoke detector according to the invention; FIG.

FIG. 2 is a schematic diagram of a portion of FIG. 1 showing an example of the present invention.

[Explanation of symbols]

10 Smoke detector 12 Base 14 Sensor 16 line 18 Semiconductor cell 20 nodes

Claims (4)

[Claims] 1. A support member, a sensor mounted on the support member for detecting an ambient condition, wherein the sensor generates an electrical output representative of the detected ambient condition. a threshold voltage generation circuit for generating a threshold electrical output indicative of the threshold value, and a threshold voltage generation circuit connected to the threshold voltage generation circuit for varying the threshold in response to incident radiant energy. A variable threshold detector for detecting ambient conditions, including a radiant energy sensing element and a circuit that compares the electrical output to the threshold. 2. The detector of claim 1, wherein the sensor comprises an ionic smoke detector. 3. The detector of claim 1, wherein the threshold generation circuit includes a plurality of resistors. 4. The detector of claim 1, wherein the radiant energy sensing element comprises a variable resistance semiconductor.