JP2009282955A - Smoke sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a smoke sensor capable of simplifying or eliminating a means for preventing the incidence of disturbance light on a light-receiving unit. <P>SOLUTION: A current-voltage converting circuit 2 includes a first feedback circuit 5 for outputting such a voltage of an output voltage Vout as accords to the magnitude of a low-frequency component at or lower than a predetermined first cut-off frequency, and a shunt transistor Q1 for extracting a current according to the magnitude of the output of the first feedback circuit 5, from an input current Iin. The first feedback circuit 5 comprises a first integration circuit 9 for integrating the output voltage Vout of a conversion unit 3, and a sample-and-hold circuit 10 for sampling and holding the output of the first integration circuit 9 for a sensing period, during which a pulsating detection signal is input. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a smoke detector that detects and reports smoke generated during a fire.

  Conventionally, as this kind of smoke detector A, an LED (light emitting unit) 6 that has a detection space in the housing 20 as shown in FIG. 11A and outputs light intermittently toward the detection space. And a photodiode (light receiving unit) PD that is arranged at a position where direct light from the LED 6 is not incident and converts received light into current is known (for example, see Patent Document 1). In this smoke detector A, when smoke flows into the detection space, the light from the LED 6 is diffused and reflected by the smoke in the detection space, so that the amount of light received from the LED 6 at the photodiode PD increases. The amount of current output from the diode PD increases.

  The LED 6 and the photodiode PD constitute an optical block 25 together with a light projecting lens 23 disposed in front of the LED 6 and a light receiving lens 24 disposed in front of the photodiode PD. The housing 20 has an opening formed on the lower surface and a body 26 housing the optical block 25 so that light from the LED 6 is emitted toward the opening, and a bottomed cylindrical shape with an upper surface opening. A cover 27 coupled to the body 26 is provided so as to cover the opening. An opening window for taking in smoke is formed on the peripheral wall of the cover 27, and the detection space is formed in the cover 27. Here, in the cover 27, an insect net 28 for preventing insects from entering the detection space, and a labyrinth 21 for preventing disturbance light from entering the detection space are arranged so as to surround the detection space. The labyrinth 21 is complicated to prevent the incidence of various disturbance lights from fluorescent lamps and incandescent lamps, and to prevent the light of the LED 6 from entering the photodiode PD in the absence of smoke in the detection space. A complicated structure with an optical path is adopted.

  In this type of smoke detector A, as shown in FIG. 11 (b), current-voltage conversion for converting the input current from the photodiode PD into a voltage and outputting it to the circuit block 1 housed in the housing 20 A circuit (IV conversion circuit) 2 is provided. Further, the output voltage of the current-voltage conversion circuit 2 is input to the alarm determination circuit 14 which is a determination processing unit through the amplifier circuit 12 and the filter circuit 13, and when the change amount of the output voltage exceeds a predetermined fire determination level, The alarm circuit 15 (buzzer or the like) issues a report. The circuit block 1 includes a power supply circuit 16 that supplies power to each circuit, an interlocking circuit 17 that links other reporting means and the like, and a transistor Tr1 (see FIG. 12) connected in series with the LED 6. An LED drive circuit 18 that periodically emits light is provided.

  The current-voltage conversion circuit 2 used here has a conversion unit 3 formed by connecting a conversion resistor R2 between an inverting input terminal and an output terminal of an operational amplifier OP1, for example, as shown in FIG. When the input current Iin is input to the terminal, the output voltage Vout whose voltage value varies according to the variation of the input current Iin is output to the output terminal Tout. In the example of FIG. 12, since the reference voltage Vs is applied to the non-inverting input terminal, the output voltage Vout is expressed by Vout = Vs− (Iin × r2) if the resistance value of the conversion resistor R2 is r2. . In short, the current-voltage conversion circuit 2 uses the output voltage Vout in a steady state where the photodiode PD does not receive the light from the LED 6 as an operating point, and outputs the output voltage Vout based on the operating point according to the fluctuation of the input current Iin. It will be fluctuated.

  In recent years, since the installation is simple, the demand for the smoke detector A using a battery as a power source is increasing. When the battery is used as the power source of the smoke detector A, it is necessary to drive the smoke detector A intermittently in order to suppress the average power consumption of the smoke detector A and extend the life of the battery. In this case, the power supply to the current-voltage conversion circuit 2 shown in FIG. 13A is also intermittently performed. Therefore, the LED 6 outputs pulsed light while power is supplied to the current-voltage conversion circuit 2 as shown in FIG. Here, when smoke flows into the detection space and the photodiode PD receives light from the LED 6, the change amount ΔV of the output voltage Vout of the current-voltage conversion circuit 2 increases as shown by the solid line in FIG. The fire judgment level in the figure will be reached. On the other hand, if there is no smoke in the detection space, as shown by the broken line in FIG. 13C, the change amount ΔV of the output voltage becomes small and does not reach the fire determination level.

  In the current-voltage conversion circuit 2 as shown in FIG. 12, the dynamic range of the operational amplifier OP1 is defined between the power supply voltage VDD of the operational amplifier OP1 and the ground GND as shown in FIG. The output voltage Vout described above fluctuates within this dynamic range. Therefore, when the input current Iin exceeds a certain level, the output voltage Vout is saturated.

  For example, in the smoke detector A described above, although the labyrinth 21 is provided, the detection space cannot be completely blocked from the outside, so that a slight disturbance light may enter the photodiode PD. Normally, disturbance light has a small temporal variation, and when the photodiode PD receives the disturbance light, a current having a small temporal variation (hereinafter referred to as “low frequency component”) is output from the photodiode PD. Become. When the low frequency component included in the input current Iin exceeds a certain level, the output voltage Vout may be saturated. In particular, when the battery is used as the power source of the smoke detector A as described above, the output voltage Vout tends to be saturated because the power supply voltage of the operational amplifier OP1 is low and the dynamic range of the operational amplifier OP1 is relatively narrow.

  That is, if the input current Iin does not contain a low frequency component, the operating point of the output voltage Vout becomes the reference voltage Vs as shown in FIG. 14A. Therefore, if the input current Iin varies, the output voltage Vout also changes. On the other hand, if the input current Iin includes a low frequency component, the operating point of the output voltage Vout decreases as shown in FIG. When Iin increases, the output voltage Vout may be saturated in the middle. In particular, when the low frequency component is large and the operating point of the output voltage Vout is reduced to near the ground GND as shown in FIG. 14C, the output voltage Vout is saturated regardless of the fluctuation of the input current Iin. Therefore, the output voltage Vout does not follow the increase in the input current Iin.

  For example, if the resistance value r2 of the conversion resistor R2 is 1 MΩ and the reference voltage Vs is 1 V, the input current Iin is 1 μA and the voltage drop across the conversion resistor R2 is 1 V. As a result, the output voltage Vout of the current-voltage conversion circuit 2 Saturates to 0V. In this state, even if the photodiode PD receives light from the LED 6 and the pulsed input current Iin is input to the current-voltage conversion circuit 2, the output voltage Vout of the current-voltage conversion circuit 2 is saturated. There is a possibility that the change ΔV of the output voltage Vout does not reach the fire determination level without being fluctuated as described above.

  Therefore, as the current-voltage conversion circuit 2, when the input current Iin includes a low frequency component, feedback is applied between the output terminal Tout and the input terminal Tin to saturate the output voltage Vout due to the low frequency component. The one that can be suppressed has been proposed.

  In addition to the conversion unit 3 described above, the current-voltage conversion circuit 2 receives an output voltage Vout of the conversion unit 3 and outputs an integration voltage Vdc corresponding to the integral value component of the output voltage Vout as shown in FIG. The circuit 8 includes a shunt resistor R1 inserted between the output of the integrating circuit 8 and the input terminal Tin of the conversion unit 3. Thus, by extracting a current according to the magnitude of the integrated voltage Vdc from the input current Iin and flowing it through the shunt resistor R1, the influence of the integrated value component on the output voltage Vout can be suppressed, so that the input current Iin In the case where the low frequency component is included, the low frequency component is subtracted from the input current Iin, so that the influence of the low frequency component on the output voltage Vout can be suppressed.

  In the current-voltage conversion circuit 2, the magnitude of the current flowing through the shunt resistor R1 is determined by the potential difference between both ends of the shunt resistor R1 and the resistance value of the shunt resistor R1. Since the potential difference between the both ends of the shunt resistor R1 changes according to the magnitude of the low frequency component included in the input current Iin, the larger the low frequency component becomes in the input current Iin as the output voltage Vout of the conversion unit 3 becomes saturated. If it is included, the potential difference between both ends of the shunt resistor R1 will also be saturated, and no more current can flow through the shunt resistor R1. The current at this time becomes the upper limit of the magnitude of the low frequency component that can suppress the influence on the output voltage Vout.

However, in the smoke detector A described above, since the disturbance light is prevented from entering the detection space by the labyrinth 21, the disturbance light that is stronger as the low frequency component included in the input current Iin exceeds the upper limit is generated by the photodiode PD. When the current-voltage conversion circuit 2 configured as described above is employed, the saturation of the output voltage Vout can be sufficiently prevented.
Japanese Patent No. 2783945 (page 1-2)

  By the way, in the current-voltage conversion circuit 2 configured as described above, if an input current Iin including a low frequency component exceeding the upper limit is input, output is performed as shown in FIGS. 14B and 14C. There is a possibility that the operating point of the voltage Vout is lowered and the output voltage Vout is saturated. If a resistor having a small resistance value is used as the shunt resistor R1, the upper limit of the size of the low frequency component capable of suppressing the influence on the output voltage Vout can be widened by drawing a large amount of current through the shunt resistor R1. However, the thermal noise of the shunt resistor R1 itself increases. If the thermal noise of the shunt resistor R1 increases, the input conversion noise of the current-voltage conversion circuit 2 also increases, and there is a problem that the SN ratio, which is the ratio between the noise and the signal component to be detected, is reduced. The resistance value of the shunt resistor R1 must be set large to some extent. As a result, there is a possibility that the output voltage Vout is saturated, and the following problems occur, for example.

  That is, in the above-described smoke detector A, the structure of the labyrinth 21 that prevents the incidence of ambient light into the detection space is complicated, and the cost for manufacturing the labyrinth 21 hinders the cost reduction of the entire smoke detector A. Therefore, it is desired to reduce the cost of the smoke detector A by simplifying the structure of the labyrinth 21 as much as possible or omitting the labyrinth 21 itself. However, if the labyrinth 21 is simplified or omitted, disturbance light received by the photodiode PD becomes strong, and the low frequency component included in the input current Iin can suppress the influence on the output voltage Vout. As a result, the output voltage Vout may be saturated.

  The present invention has been made in view of the above reasons, and an object of the present invention is to provide a smoke detector that can simplify or omit means for preventing disturbance light from entering the light receiving unit.

  According to the first aspect of the present invention, a light emitting unit that outputs pulsed light during a predetermined sensing period toward the detection space, and direct light from the light emitting unit is not incident and diffusely reflected by smoke flowing into the detection space. A light receiving unit that is placed at a position where light from the light emitting unit is incident, receives light and converts it into current, and an input current that is input from the light receiving unit to the input terminal changes in voltage value according to the change in the input current. A current-voltage conversion circuit having a conversion unit that converts the output voltage into an output terminal and outputs the output voltage from an output terminal; and a determination processing unit that determines the presence or absence of smoke in the detection space based on the output voltage. A first feedback circuit that outputs a low frequency component equal to or lower than a first cutoff frequency that is lower than the frequency of a pulsed detection signal generated when the light receiving unit receives light from the light emitting unit in the output voltage; Circuit ground and the input terminal And a control terminal connected to the output of the first feedback circuit, and a shunting transistor for drawing a current having a magnitude corresponding to the output of the first feedback circuit from the input current. And

  According to this configuration, a low frequency component equal to or lower than the first cut-off frequency can be extracted to the shunting transistor, so that a large low frequency component can be extracted as compared with a case where a current is extracted to the shunt resistor. Therefore, even when the disturbance light received by the light receiving unit is strong and the low frequency component contained in the input current is relatively large, the influence of the low frequency component on the output voltage generated at the output terminal of the conversion unit can be suppressed. . As a result, it is possible to simplify or omit the means for preventing disturbance light from entering the light receiving unit.

  According to a second aspect of the present invention, in the first aspect of the invention, the current-voltage conversion circuit outputs a voltage corresponding to a low frequency component equal to or lower than a second cutoff frequency lower than the frequency of the detection signal in the output voltage. A second feedback circuit; and a shunt resistor that is inserted between the output of the second feedback circuit and the input terminal and extracts a current having a magnitude corresponding to the output of the second feedback circuit from the input current. It is characterized by.

  According to this configuration, since the low frequency component below the second cutoff frequency can be extracted to the shunting resistor, the influence on the output voltage can be suppressed compared to the case where the low frequency component is extracted only to the shunting transistor. The upper limit of the size of the low frequency component can be widened. Therefore, even when a larger low-frequency component is included in the input current, the influence of the low-frequency component on the output voltage generated at the output terminal of the conversion unit can be suppressed.

  According to a third aspect of the invention, in the second aspect of the invention, the first feedback circuit is lower than the second cutoff frequency during the sensing period and is higher than the second cutoff frequency during a period other than the sensing period. Thus, it has a frequency switching means for switching the first cut-off frequency.

  According to this configuration, since the first feedback circuit has the frequency switching means for switching the first cut-off frequency, it is possible to extract a relatively wide range of low frequency components to the shunting transistor during a period other than the sensing period. In the sensing period, the detection signal can be prevented from being pulled out by the shunting transistor. Further, even during the sensing period, if the low frequency component is equal to or lower than the second cut-off frequency, it can be extracted to the shunt resistor by the second feedback circuit.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the first feedback circuit includes an integration circuit that outputs an integral value component of the output voltage, and the frequency switching means is configured to output the integration circuit output and the dividing circuit. A sample-and-hold circuit having a first switch inserted between the control terminal of the diverting transistor and operating the sample-and-hold circuit by turning off the first switch during the sensing period, so that the held integration The output voltage of the circuit is applied to the control terminal of the shunting transistor.

  According to this configuration, during the sensing period, the first switch is turned off to cut off the output of the integration circuit and the control terminal of the shunting transistor, so that the steady state such as flicker noise generated in the integration circuit Noise can be prevented from affecting the input, and the SN ratio is improved.

  According to a fifth aspect of the present invention, in the fourth aspect of the invention, a second switch is connected between the input terminal and the output terminal of the conversion unit, and the second switch is the first switch. It is characterized by being turned on when the switch is on.

  According to this configuration, when the second switch is turned on, the gain between the input terminal and the output terminal of the conversion unit is reduced, and the oscillation of the system due to the first switch being turned on can be suppressed. .

  According to a sixth aspect of the present invention, in the fourth aspect of the invention, the first switch has an off-resistance value set smaller than a resistance value between the control terminal of the shunting transistor and circuit ground. It is characterized by that.

  According to this configuration, since a minute current can flow through the off resistance of the first switch even during the sensing period in which the first switch is turned off, leakage that occurs between the control terminal of the shunting transistor and the circuit ground. It is possible to prevent the potential at the control terminal of the shunting transistor from being lowered by the current. Therefore, fluctuations in the output voltage due to a decrease in the potential of the control terminal of the shunt transistor can be suppressed.

  According to a seventh aspect of the present invention, in the third aspect of the present invention, the first feedback circuit includes an integrating circuit that outputs an integrated value component of the output voltage, and the frequency switching means is a control terminal of the shunting transistor. A low-pass filter circuit having a capacitor connected between the output circuit and the circuit ground, and a parallel circuit of a resistor and a third switch connected between the output of the integrating circuit and the control terminal of the shunt transistor, In the sensing period, the low-pass filter circuit is operated by turning off the third switch.

  According to this configuration, since the current can flow through the resistor even in the sensing period in which the third switch is turned off, the control of the shunt transistor is performed by the leakage current generated between the control terminal of the shunt transistor and the circuit ground. It is possible to prevent the terminal potential from being lowered. Therefore, fluctuations in the output voltage due to a decrease in the potential of the control terminal of the shunt transistor can be suppressed. In addition, since the first cut-off frequency in the sensing period can be set with high accuracy, the first cut-off frequency can be set high so that the detection signal is not extracted by the shunting transistor. A relatively wide range of low frequency components can be extracted to the shunting transistor.

  The invention according to claim 8 is the invention according to claim 3, wherein the first feedback circuit includes an integration circuit having a time constant determined by a first resistor and a capacitor, and the frequency switching means includes A series circuit of a second resistor and a fourth switch connected in parallel with the resistor is provided, and the fourth switch is turned off during the sensing period.

  According to this configuration, it is possible to reduce the size and power consumption of the circuit as compared with the case where the sample hold circuit is provided. In addition, since the first cut-off frequency in the sensing period can be set with high accuracy, the first cut-off frequency can be set high so that the detection signal is not extracted by the shunting transistor. A relatively wide range of low frequency components can be extracted to the shunting transistor.

  According to a ninth aspect of the present invention, in any one of the second to eighth aspects, the second feedback circuit includes an active filter that outputs a voltage having a phase opposite to that of the input current. The feedback circuit comprises an active filter that outputs a voltage having the same phase with respect to the input current.

  According to this configuration, since the first and second feedback circuits are realized by the active filter, it is possible to have a high gain with respect to the low frequency component, and to reliably suppress the fluctuation of the output voltage due to the low frequency component. can do.

  According to a tenth aspect of the present invention, in the invention according to any one of the first to ninth aspects, a plurality of the diversion transistors are provided, and the selection transistors are respectively inserted between the diversion transistors and the input terminals. And a switch control circuit that controls on / off of the selection switch in accordance with the output of the first feedback circuit so that the number of selection switches that are turned on increases as the output of the first feedback circuit increases. It is characterized by that.

  According to this configuration, as the current drawn to the shunting transistor increases, the number of shunting transistors used for drawing current increases, so that a large current can be drawn, but the channel width ( The ratio (W / L) between W) and channel length (L) can be kept small, and the stationary noise per shunting transistor can be kept small.

  The invention of claim 11 is the operation mode of the invention of claim 9, wherein the first feedback circuit and the second feedback circuit share an operational amplifier, and the operational amplifier uses the operational amplifier for the first feedback circuit. And a mode switching means for switching between operation modes in which the operational amplifier is used in the second feedback circuit.

  According to this configuration, since the first feedback circuit and the second feedback circuit share the operational amplifier, the circuit can be reduced in size as compared with the case where each feedback circuit is provided with the operational amplifier. it can.

  The invention of claim 12 is characterized in that, in the invention of claim 9, the power supply voltage of the first feedback circuit is set higher than the power supply voltages of the other circuits.

  According to this configuration, the upper limit value of the output of the first feedback circuit is higher than when the power supply voltage of the first feedback circuit is the same as the power supply voltage of the other circuits. The upper limit of the magnitude of the current that can be drawn can be increased.

  In the present invention, since the low frequency component equal to or lower than the first cutoff frequency can be extracted to the shunting transistor, even when the disturbance light received by the light receiving unit is strong and the low frequency component included in the input current is relatively large, The influence of the low frequency component on the output voltage generated at the output terminal of the converter can be suppressed. As a result, there is an advantage that it is possible to simplify or omit the means for preventing the disturbance light from entering the light receiving unit.

(Embodiment 1)
The smoke detector A of the present embodiment has a detection space in the housing 20 as in the conventional configuration shown in FIG. 12, and a light emitting unit that intermittently outputs pulsed light toward the detection space; A light receiving unit that is arranged at a position where direct light from the light emitting unit is not incident and converts received light into current, and a circuit block 1 that detects smoke in the detection space based on an input current from the light receiving unit. . In the smoke detector A, when smoke flows into the detection space, the amount of light received from the light emitting unit at the light receiving unit increases due to diffuse reflection of the light from the light emitting unit with the smoke in the detection space, The amount of current output from the light receiving unit increases. The smoke detector A exemplified here uses a battery as a power source, and is intermittently driven in order to reduce the average power consumption and extend the life of the battery.

  The circuit block 1 of the present embodiment includes a current-voltage conversion circuit that converts an input current input from the light receiving unit into an output voltage whose voltage value varies according to the variation of the input current, and outputs the output voltage. And a determination processing unit (notification determination circuit 14 in FIG. 11B) that determines the presence or absence of smoke in the detection space based on the output voltage.

  As shown in FIG. 2, the current-voltage conversion circuit 2 converts the input current Iin inputted from the input terminal Tin into an output voltage Vout whose voltage value varies according to the variation of the input current Iin, and outputs the output voltage Vout from the output terminal Tout. A conversion unit 3 that outputs, a second feedback circuit 4 that outputs a voltage corresponding to the magnitude of a low-frequency component equal to or lower than a predetermined second cutoff frequency fc2 among the output voltage Vout output from the conversion unit 3; 2 and a shunt resistor R1 inserted between the output of the feedback circuit 4 and the input terminal Tin of the converter 3. Furthermore, in the present embodiment, the first feedback circuit 5 that outputs a voltage corresponding to the magnitude of the low frequency component of the output voltage Vout that is equal to or lower than the predetermined first cutoff frequency fc1, and the first feedback circuit 5 A shunting transistor Q1 is provided that draws a current corresponding to the magnitude of the output from the input current Iin.

  As shown in FIG. 1, in the conversion unit 3, the conversion resistor R2 is connected between the inverting input terminal and the output terminal of the operational amplifier OP1, as in the conventional configuration shown in FIG. 15, and the non-inverting input of the operational amplifier OP1. The reference voltage Vs is applied to the terminal. A photodiode PD (see FIG. 11) as a light receiving unit is connected to the input terminal Tin of the conversion unit 3, and an input current Iin is input from the photodiode PD. Here, the conversion unit 3 of the present embodiment has a capacitor C1 connected in parallel to the conversion resistor R2 and also functions as a low-pass filter, and passes only the input current Iin having a predetermined cutoff frequency fc0 or less. Circuit constants of the conversion resistor R2 and the capacitor C1 are set. This cut-off frequency fc0 is represented by fc0 = 1 / (2π × r2 × c1) using the resistance value r2 of the conversion resistor R2 and the constant c1 of the capacitor C1, and at least the photodiode PD is an LED 6 (light emitting portion). It is set so as to pass a pulsed input current (hereinafter referred to as a detection signal) Iin generated when receiving light from (see FIG. 11).

  Therefore, the current-voltage conversion circuit 2 uses the output voltage Vout (here, the reference voltage Vs) when the input current Iin from the photodiode PD is zero as the operating point, and uses the operating point as a reference according to the fluctuation of the input current Iin. Therefore, the output voltage Vout is changed.

  The second feedback circuit 4 integrates the inverting amplifier circuit 7 that inverts and amplifies the output voltage Vout of the conversion unit 3 and the output voltage Vout that is inverted and amplified by the inverting amplifier circuit 7 and corresponds to an integrated value component of the output voltage Vout. And a second integration circuit 8 that outputs an integration voltage Vdc.

  The second integration circuit 8 connects the inverting input terminal of the operational amplifier OP2 to the output of the inverting amplifier circuit 7 via the resistor R3, and connects the capacitor C2 between the inverting input terminal and the output terminal of the operational amplifier OP2. Configured as a low-pass filter having a time constant determined by the resistor R3 and the capacitor C2. The integration circuit 8 has a time constant set so as to have a second cutoff frequency fc2 that blocks at least the detection signal generated when the photodiode PD receives light from the LED 6 (that is, lower than the frequency of the detection signal). The

  The inverting amplifier circuit 7 is for making the output of the integrating circuit 8 in phase with the output voltage Vout of the conversion unit 3, and is connected to the output terminal Tout of the conversion unit 3 via the resistor R4 to invert the operational amplifier OP3. An input terminal is connected, and a resistor R5 is connected between the inverting input terminal and the output terminal of the operational amplifier OP3. A reference voltage Vs is applied to the non-inverting input terminals of both operational amplifiers OP2 and OP3.

  Accordingly, when the detection signal and the low frequency component generated when the photodiode PD receives the light from the LED 6 are included in the input current Iin of the conversion unit 3, it is output from the second integration circuit 8. The integrated voltage Vdc is a voltage corresponding to the low frequency component. At this time, the phase of the input current Iin is once inverted by the converter 3 and further inverted once by the inverting amplifier circuit 7 and the integrating circuit 8 respectively. An antiphase integrated voltage Vdc appears. Here, since the reference voltage Vs is applied to the input terminal Tin of the conversion unit 3, a potential difference obtained by subtracting the integrated voltage Vdc from the reference voltage Vs is generated between both ends of the shunt resistor R1. That is, since a current corresponding to the magnitude of the integrated voltage Vdc can be drawn from the input current Iin by flowing the current to the shunt resistor R1, if the input current Iin contains a low frequency component, this low frequency The component is removed from the output voltage Vout by subtracting from the input current Iin.

  If a resistor having a small resistance value is used as the shunt resistor R1, the thermal noise of the shunt resistor R1 itself increases and the input conversion noise of the current-voltage conversion circuit 2 also increases. Since there is a problem that the S / N ratio, which is the ratio with the signal component (input current Iin) from the diode PD, is lowered, the resistance value of the shunting resistor R1 is set to be somewhat large.

  By the way, in the smoke detector A of the present embodiment, the first feedback circuit 5 includes a first integration circuit 9 that integrates the output voltage Vout of the conversion unit 3 and an output of the first integration circuit 9 by sample-holding. And a sample and hold circuit 10 for performing the above operation.

  The first integrating circuit 9 is configured by connecting the inverting input terminal of the operational amplifier OP4 to the output terminal Tout via the resistor R6, and connecting the capacitor C3 between the inverting input terminal and the output terminal of the operational amplifier OP4. And functions as a low-pass filter having a time constant determined by the resistor R6 and the capacitor C3. The integration circuit 9 has a time constant set so as to have a first cutoff frequency fc1 (that is, fc2 <fc1) higher than the second cutoff frequency fc2 of the second integration circuit 8 described above. The reference voltage Vs is applied to the non-inverting input terminal of the operational amplifier OP4.

  The shunting transistor Q1 is inserted between the input terminal Tin and the circuit ground, and allows a current corresponding to the output of the first integrating circuit 9 to flow from the input terminal Tin of the conversion unit 3 to the circuit ground. Then, it is composed of an N-channel MOSFET. The shunting transistor Q1 has a drain connected to the input terminal Tin of the converter 3 and a source connected to the circuit ground, and a gate connected to the output of the integrating circuit 9 via the sample hold circuit 10 (an output terminal of the operational amplifier OP4). Is provided in a connected form.

  The sample and hold circuit 10 is connected between a normally closed first switch SW1 inserted between the output of the integrating circuit 9 and the gate of the shunting transistor Q1, and between the gate of the shunting transistor Q1 and the circuit ground. By turning off the first switch SW1 at a predetermined timing, the output of the integrating circuit 9 at the predetermined timing is maintained as the output voltage of the capacitor C4.

  With the above-described configuration, the integration circuit 9 integrates the output voltage Vout of the conversion unit 3, so that a low frequency component having a frequency equal to or lower than the first cutoff frequency fc1 in the output voltage Vout of the conversion unit 3 appears in the output of the integration circuit 9. It will be. At this time, the phase of the input current Iin is once inverted by the converter 3 and further inverted by the integrating circuit 9, so that a low frequency component in phase with the input current Iin appears at the output of the integrating circuit 9. Here, since the output of the integrating circuit 9 is applied to the gate of the shunting transistor Q1 via the sample and hold circuit 10, when the switch SW1 of the sample and hold circuit 10 is on, the drain-source region of the shunting transistor Q1 is turned on. Current flows in accordance with the magnitude of the output of the integrating circuit 9 (low frequency component of the output voltage Vout). Therefore, a low frequency component having a frequency equal to or lower than the first cut-off frequency fc1 included in the input current Iin can be extracted to the shunting transistor Q1, and the gain of the low frequency component can be lowered in the current-voltage conversion circuit 2 as a whole.

  Here, when the switch SW1 of the sample hold circuit 10 is turned off, the output of the integrating circuit 9 and the gate of the shunting transistor Q1 are cut off, but the output of the integrating circuit 9 is maintained as the voltage across the capacitor C4. Therefore, a current corresponding to the magnitude of the output of the integrating circuit 9 immediately before the switch SW1 is turned off can continue to flow between the drain and source of the shunting transistor Q1. In other words, when the sample-and-hold circuit 10 is activated by turning off the switch SW1 of the sample-and-hold circuit 10, the upper limit value of the frequency of the current that can flow between the drain and source of the shunting transistor Q1 (first cutoff frequency fc1). However, the DC component can be removed from the output voltage Vout by continuously drawing it out to the shunting transistor Q1.

  In the present embodiment, the timing at which the switch SW1 of the sample hold circuit 10 is turned off is a period during which the LED 6 of the smoke detector A outputs pulsed light and detects the presence or absence of smoke flowing into the detection space (hereinafter referred to as sensing). (Referred to as “period”). That is, the smoke detector A of the present embodiment is for converting a detection signal generated when the photodiode PD receives light from the LED 6 during the sensing period into a voltage and outputting it as an output voltage Vout. Therefore, the switch SW1 is turned off during the sensing period so that the detection signal during the sensing period is not pulled out by the shunting transistor Q1.

  More specifically, the first cutoff frequency fc1 of the first integration circuit 9 is set closer to the frequency of the detection signal than the second cutoff frequency fc2 of the second integration circuit 8. When the switch SW1 is on, the detection signal is extracted by the shunting transistor Q1, and the gain of the detection signal may be reduced as a whole of the current-voltage conversion circuit 2. Therefore, in the present embodiment, the switch SW1 is turned off during the sensing period to operate the sample and hold circuit 10, thereby avoiding the detection signal being pulled out by the shunting transistor Q1, and the current-voltage conversion circuit 2 as a whole as described above. A high gain of the detection signal is ensured.

  Here, when the smoke detector A is intermittently driven as described above, the power supply to the current-voltage conversion circuit 2 is also intermittently performed and the power supply to the current-voltage conversion circuit 2 is performed. The sensing period is set in the inside. The switch SW1 of the sample-and-hold circuit 10 is turned off only during the sensing period, the current voltage from the start of power supply to the current-voltage conversion circuit 2 until the sensing period starts, and after the sensing period ends. The switch SW1 is turned on until the power supply to the conversion circuit 2 is stopped.

  Therefore, during the sensing period in which the switch SW1 is turned off, the output of the first feedback circuit 5 is fixed to a value immediately before the switch SW1 is turned off, so that there is no fluctuation in the DC component included in the input current Iin. Although it can be continuously extracted by the shunting transistor Q1, regarding the low frequency component with fluctuation included in the input current Iin, even if the low frequency component is a low frequency component equal to or lower than the first cutoff frequency fc1, It cannot be pulled out to the shunting transistor Q1.

  However, even during the sensing period, a low frequency component having a frequency equal to or lower than the second cutoff frequency fc2 can be extracted as the output of the second feedback circuit 4 to be extracted to the shunt resistor R1.

  According to the smoke detector A having the above-described configuration, during the period other than the sensing period, the low frequency component of the input current Iin that is equal to or lower than the first cut-off frequency fc1 is fed back through the first feedback circuit 5, and the shunting transistor Since it is extracted by Q1, the operating point of the output voltage Vout settles to the reference voltage Vs even if the input current Iin includes a low frequency component. Further, in the sensing period, the low frequency component of the input current Iin having a frequency equal to or lower than the second cut-off frequency fc2 is fed back through the second feedback circuit 4 and is extracted by the shunt resistor R1. Even if a low-frequency component is included, the operating point of the output voltage Vout settles at the reference voltage Vs. At this time, the switch SW1 of the sample hold circuit 10 is turned off, and the output of the first feedback circuit 5 is held at a value immediately before the sensing period, so that the detection signal is not pulled out by the shunting transistor Q1, and the detection signal The output voltage Vout corresponding to can be output.

  That is, by using the shunt resistor R1 and the shunt transistor Q1 as means for extracting the low frequency component of the input current Iin, a larger current component can be obtained compared to the case where the low frequency component is extracted only by the shunt resistor R1. Can handle pulling. In addition, by setting the first cut-off frequency fc1 higher, it is possible to remove a relatively wide range of low-frequency components from the output voltage Vout during a period other than the sensing period, while switching the switch SW1 of the sample hold circuit 10 on and off. In the sensing period, it is possible to prevent the detection signal to be detected from being attenuated.

  Further, since the first cutoff frequency fc1 of the first feedback circuit 5 is set higher than the second cutoff frequency fc2 of the second feedback circuit 4, when power supply to the current-voltage conversion circuit 2 starts The output of the first feedback circuit 5 responds before the second feedback circuit 4. Therefore, most of the low-frequency components below the first cut-off frequency fc1 can be extracted to the shunting transistor Q1 without adding means for shutting off the second feedback circuit 4, and as a result, the current voltage The conversion circuit 2 can be downsized.

  Moreover, by using the sample hold circuit 10 as frequency switching means for reducing the first cutoff frequency fc1 of the first feedback circuit 5 during the sensing period, the output of the integrating circuit 9 and the shunting transistor Q1 Therefore, the noise (such as flicker noise) generated in the integrating circuit 9 does not affect the input current Iin, and the S / N ratio can be improved.

  A specific example of the current-voltage conversion circuit 2 having the above configuration will be described. Even if the photodiode PD receives light from a fluorescent lamp that is lit with a commercial power source (60 Hz AC power source) using a copper iron ballast. The first cut-off frequency fc1 of the first feedback circuit 5 is set higher than at least 120 Hz so that the output voltage Vout of the current-voltage conversion circuit 2 does not fluctuate due to the flickering of the fluorescent lamp light. . Further, by setting the second cutoff frequency fc2 of the second feedback circuit 4 to be higher than 120 Hz, the influence of the light of the fluorescent lamp can be influenced through the second feedback circuit 4 even in the sensing period in which the switch SW1 is turned off. The received low frequency component of 120 Hz or less is fed back, and the low frequency component is extracted to the shunt resistor R1.

  By the way, in FIG. 1, the 2nd switch SW2 connected in parallel with the conversion resistance R2 of the conversion part 3 is provided. The switch SW2 is a normally closed switch that is turned off at the same timing as the first switch SW1, and has a function described below.

  That is, if only the first switch SW1 is provided, the cutoff frequency fc1 of the first feedback circuit 5 is shifted to the high frequency side when the first switch SW1 is turned on. In the frequency characteristics of the gain of the entire current-voltage conversion circuit 2 shown in (2), when the first switch SW1 is turned on, the gain on the low frequency side is crushed as shown by the solid line, and the cutoff frequency fc0 of the conversion unit 3 A gain peak occurs between the first feedback circuit 5 and the first cut-off frequency fc1, and the system easily oscillates. That is, since the output voltage Vout is in a state of being easily oscillated, if the first switch SW1 is turned off when the output voltage Vout is low as shown by a broken line in FIG. 4C, the output voltage Vout is consequently obtained. There is a problem that the rise of is delayed.

  On the other hand, in the present embodiment in which the second switch SW2 connected in parallel with the conversion resistor R2 is provided, the second switch SW2 is turned on together with the first switch SW1 from when the power is turned on to when the sensing period starts. Thus, as shown in FIG. 3B, while the first switch SW1 is on, both ends of the conversion resistor R2 are connected, the gain of the conversion unit 3 is crushed, and the above-described gain peak is eliminated. be able to. As a result, it is possible to suppress the oscillation of the system due to the first switch SW1 being turned on.

  Furthermore, in this embodiment, since the integrating circuits 8 and 9 of the first and second feedback circuits 4 and 5 are not passive circuits but active circuits having operational amplifiers OP2 and OP4, low frequency The component can have a high gain, and feedback can be applied up to the open gain of the operational amplifiers OP2 and OP4. That is, in the configuration using the passive filter, when the first feedback circuit 5 is activated, a voltage drop (the amount of drop varies depending on the gain) occurs in the output voltage Vout, and then the sample hold circuit 10 is activated. The second feedback circuit 4 operates and the output voltage Vout fluctuates in the same manner. As a result, the fluctuation also appears in the final output. For this reason, erroneous detection of the detection signal may occur during the fluctuation. In order to suppress the fluctuation, it is conceivable that the sample hold circuit 10 is not operated until just before the timing at which the detection signal is generated. In this case, the current consumption of the entire circuit increases. On the other hand, since the active filter is used in the present embodiment, it is possible to suppress a decrease in the output voltage Vout due to a low frequency component and reliably avoid saturation of the output voltage Vout.

  Here, it is desirable that the power supply voltage of the first feedback circuit 5 be higher than the power supply voltage of circuits other than the feedback circuit 5 in the current-voltage conversion circuit 2. As a result, the output voltage of the first feedback circuit 5 (that is, the gate voltage of the shunting transistor Q1) can be increased, and therefore a relatively large input current Iin can be drawn to the shunting transistor Q1. . As a result, even when a low-frequency component having a relatively large amplitude is included in the input current Iin, the low-frequency component can be removed from the output voltage Vout, and the magnitude of the low-frequency component that can suppress the influence on the output voltage Vout. There is an advantage that the upper limit of the thickness is further increased.

  Even during the sensing period in which the first switch SW1 is turned off and the sample hold circuit 10 is operating, the shunting transistor is affected by the leakage current generated between the gate of the shunting transistor Q1 and the circuit ground. The gate voltage of Q1 may gradually decrease with time. At this time, since the current drawn between the drain and source of the shunting transistor Q1 also gradually decreases, the output voltage Vout may vary accordingly. As a countermeasure, it is conceivable to use an element (for example, an analog switch) whose off-resistance is smaller than the resistance value between the gate and the source of the shunting transistor Q1 for the first switch SW1. As a result, even during a sensing period in which the switch SW1 is off, a small current flowing through the off-resistance of the switch SW1 can suppress a decrease in the gate voltage due to the leakage current, and suppress fluctuations in the output voltage Vout. be able to.

  As described above, in the smoke detector A of the present embodiment, the input current Iin including a large low frequency component is input to the input terminal Tin of the conversion unit 3 when strong disturbance light is incident on the photodiode PD. Even in this case, the influence of the low frequency component on the output voltage Vout generated at the output terminal Tout of the conversion unit 3 can be suppressed. Therefore, the labyrinth 21 can be simplified, and the smoke detector A can be made thinner as shown in FIG. The smoke detector A in FIG. 5 uses the front of the housing 20 (below when the housing 20 is attached to the ceiling) as a detection space, and the light from the LED 6 diffused and reflected by the smoke of the photodiode PD flowing into the detection space. Smoke is detected by receiving light.

(Embodiment 2)
The smoke detector A of this embodiment is different from the smoke detector A of Embodiment 1 in that a plurality of shunt transistors Q1 are provided as shown in FIG.

  That is, in the present embodiment, a plurality of shunt transistors Q1 are connected in parallel between the input terminal Tin and circuit ground, and the output of the first feedback circuit 5 is connected to the gate of each shunt transistor Q1. ing. Here, selection switches SW11, SW12,... Are inserted between the drains of the respective shunt transistors Q1 and the input terminal Tin, and the selection switches SW11, SW12,. The control is performed by the switch control circuit 11.

  The switch control circuit 11 monitors the output of the first feedback circuit 5 applied to the gate of the shunting transistor Q1, and the number of selection switches SW11, SW12,... That are turned on as the output of the feedback circuit 5 increases. Are switched on and off in accordance with the magnitude of the output of the feedback circuit 5 so as to increase. Accordingly, as the current drawn from the input current Iin increases, the number of shunt transistors Q1 connected in parallel between the input terminal Tin and the circuit ground increases, and the number of shunt transistors Q1 used for drawing current increases. Become.

  In short, in the shunting transistor Q1 that draws current from the input current Iin, it is necessary to increase the ratio (W / L) of the channel width (W) to the channel length (L) in order to increase the applicable current value. However, if W / L is increased, the thermal noise of the shunting transistor Q1 increases even if the amount of current drawn is the same. On the other hand, in the present embodiment, a plurality of shunt transistors Q1 are connected in parallel when drawing a large current while keeping W / L of each shunt transistor Q1 small and thermal noise small. Therefore, there is an advantage that a relatively large current can be handled.

  Other configurations and functions are the same as those of the first embodiment.

(Embodiment 3)
The smoke detector A of the present embodiment is provided with a low-pass filter circuit 10 ′ instead of the sample hold circuit 10 as frequency switching means for switching the first cutoff frequency fc1 of the first feedback circuit 5 as shown in FIG. This is different from the smoke detector A of the first embodiment.

  The low-pass filter circuit 10 ′ includes a resistor R7 inserted between the output of the first integrating circuit 9 and the gate of the shunting transistor Q1, and a capacitor connected between the gate of the shunting transistor Q1 and the circuit ground. C5 and a normally closed third switch SW3 connected in parallel to the resistor R7. This low-pass filter circuit 10 'passes a low-frequency component having a frequency equal to or lower than a predetermined cutoff frequency determined by the resistor R7 and the capacitor C5.

  With the above configuration, when the third switch SW3 is on, the output of the integrating circuit 9 is directly applied to the shunting transistor Q1, while when the third switch SW3 is off, the output of the integrating circuit 9 is low-pass. This is applied to the shunting transistor Q1 through the filter circuit 10 ′. Therefore, the first cut-off frequency fc1 of the first feedback circuit 5 is lowered when the third switch SW3 is turned off.

  In the present embodiment, the timing when the switch SW3 of the low-pass filter circuit 10 ′ is turned off is a period (sensing period) in which the LED 6 of the smoke detector A outputs pulsed light and detects the presence or absence of smoke flowing into the detection space. ). That is, the smoke detector A of the present embodiment is for converting a detection signal generated when the photodiode PD receives light from the LED 6 during the sensing period into a voltage and outputting it as an output voltage Vout. Therefore, the switch SW3 is turned off during the sensing period so that the detection signal during the sensing period is not pulled out by the shunting transistor Q1.

  Other configurations and functions are the same as those of the first embodiment.

(Embodiment 4)
As shown in FIG. 8, the smoke detector A of the present embodiment uses a first integration circuit 9 as a frequency switching means for switching the first cut-off frequency fc1 of the first feedback circuit 5, instead of the sample-and-hold circuit 10. The difference from the smoke detector A of the first embodiment is that a series circuit of the second resistor R8 and the fourth switch SW4 is provided.

  A series circuit of the resistor R8 and the normally closed fourth switch SW4 is connected in parallel with the first resistor R6 of the first integrating circuit 9. Thereby, when the fourth switch SW4 is on, the first cutoff frequency fc1 of the integrating circuit 9 is determined by the time constant determined by the resistor R6, the resistor R8 and the capacitor C3, while the fourth switch SW4 is In the off state, the first cutoff frequency fc1 of the integrating circuit 9 is determined by a time constant determined by the resistor R6 and the capacitor C3. Accordingly, the first cut-off frequency fc1 of the first feedback circuit 5 is lowered when the fourth switch SW4 is turned off.

  In the present embodiment, the timing when the switch SW4 of the first integration circuit 9 is turned off is a period (sensing) in which the LED 6 of the smoke detector A outputs pulsed light and detects the presence or absence of smoke flowing into the detection space. (Period). That is, the smoke detector A of the present embodiment is for converting a detection signal generated when the photodiode PD receives light from the LED 6 during the sensing period into a voltage and outputting it as an output voltage Vout. Therefore, the switch SW4 is turned off during the sensing period so that the detection signal during the sensing period is not extracted by the shunting transistor Q1.

  Other configurations and functions are the same as those of the first embodiment.

(Embodiment 5)
As shown in FIG. 9, the smoke detector A of the present embodiment has the second feedback circuit 4 and the first feedback circuit 5 that are also used as the operational amplifier OP2. Different from A.

  That is, in this embodiment, the operational amplifier OP2 of the second feedback circuit 4 is also used as the first feedback circuit 5, and the operation mode in which the second feedback circuit 4 is operated and the first feedback circuit 5 are used. Are provided with a plurality of switches (mode switching means) SW5 to SW10. More specifically, a switch SW5 is inserted between the output terminal of the operational amplifier OP2 and the shunt resistor R1, a switch SW6 is inserted between the output terminal Tout and the resistor R3, and is inverted with respect to the output terminal Tout. A switch SW7 is inserted between the input (resistor R4) of the amplifier circuit 7, and a switch SW8 is inserted between the output of the inverting amplifier circuit 7 and the resistor R3. Further, a series circuit of a resistor R6 ′ and a switch SW9 is connected in parallel with the resistor R3 connected to the inverting input terminal of the operational amplifier OP2, and between the inverting input terminal and the output terminal of the operational amplifier OP2. Is connected to a capacitor C3, and a series circuit of a capacitor C2 'and a switch SW10 is connected in parallel with the capacitor C3.

  Here, as shown in FIG. 10, the switches SW6 and SW9 are turned on simultaneously with the first switch SW1 to operate the first feedback circuit 5. At this time, the other switches SW5, SW7, SW8 and SW10 are turned off. In this state, the output terminal Tout is connected to the inverting input terminal of the operational amplifier OP2 through the parallel circuit of the resistor R3 and the resistor R6 ′, and the output terminal of the operational amplifier OP2 is connected to the gate of the shunting transistor Q1. The capacitor C3 is connected between the inverting input terminal and the output terminal of the operational amplifier OP2.

  On the other hand, the switches SW5, SW7, SW8, and SW10 operate the second feedback circuit 4 by simultaneously turning on as shown in FIG. 10, and at this time, the other switches SW1, SW6, and SW9 are turned off. To do. In this state, the output terminal Tout is connected to the inverting input terminal of the operational amplifier OP2 via the inverting amplifier circuit 7 and the resistor R3, and the output terminal of the operational amplifier OP2 is connected to the shunt resistor R1. A parallel circuit of a capacitor C3 and a capacitor C2 ′ is connected between the inverting input terminal and the output terminal.

  In short, in any state, the inverting amplifier circuit OP2 constitutes an integrating circuit, but the cut-off frequency is determined by the parallel circuit of the resistor R3 and the resistor R6 ′ and the capacitor C3 when the switches SW1, SW6, SW9 are on. When the first cut-off frequency fc1 is set and the switches SW5, SW7, SW8, and SW10 are on, the second cut-off frequency fc2 is set by the parallel circuit of the capacitor C3 and the capacitor C2 ′ and the resistor R3.

  According to the configuration of the present embodiment described above, since the operational amplifier OP2 is shared by the second feedback circuit 4 and the first feedback circuit 5, an operational amplifier is individually provided for each of the feedback circuits 4 and 5. Compared with the case of providing, it is possible to achieve downsizing and low power consumption.

  Other configurations and functions are the same as those of the first embodiment.

  In each of the above embodiments, the description has been made on the assumption that the input current Iin flows into the input terminal Tin of the current-voltage conversion circuit 2 when the photodiode PD receives light. However, the input current Iin with respect to the input terminal Tin is described. It is also possible to presuppose a configuration in which the input current Iin flows out from the input terminal Tin when the photodiode PD receives light. In this case, the shunt resistor R1 and the shunt transistor Q1 do not function to draw the input current Iin from the input terminal Tin but function to add the input current Iin to the input terminal Tin. Specifically, the shunting transistor Q1 is composed of a P-channel MOSFET connected between the input terminal Tin and the reference power supply, and a current corresponding to the output of the first feedback circuit 5 is supplied from the reference power supply to the input terminal Tin. Configured to flow through.

  Further, in each of the above embodiments, the photodiode PD is illustrated as the light receiving unit, but the present invention is not limited to this example, and an element such as CdS or thermistor can be used for the light receiving unit. That is, the smoke detector A of the present invention can be used not only for an element that generates a photoelectromotive force itself, such as a photodiode PD, but also for a light receiving unit that includes a passive element that does not generate a photoelectromotive force such as CdS or thermistor. Is possible.

It is a schematic circuit diagram which shows the structure of Embodiment 1 of this invention. It is a block diagram which shows a structure same as the above. It is a characteristic view which shows the gain of the current voltage converter circuit same as the above. It is a timing chart which shows operation | movement same as the above. It is a perspective view which shows a smoke detector same as the above. It is a schematic circuit diagram which shows the structure of Embodiment 2 of this invention. It is a schematic circuit diagram which shows the structure of Embodiment 3 of this invention. It is a schematic circuit diagram which shows the structure of Embodiment 4 of this invention. It is a schematic circuit diagram which shows the structure of Embodiment 5 of this invention. It is a timing chart which shows operation | movement same as the above. A conventional smoke detector is shown, (a) is a schematic block diagram, (b) is a block of a circuit block. It is a schematic circuit diagram which shows a current-voltage conversion circuit same as the above. It is a timing chart which shows operation | movement same as the above. It is explanatory drawing which shows an output voltage same as the above. It is a schematic circuit diagram which shows another prior art example.

Explanation of symbols

2 Current-voltage conversion circuit 3 Conversion unit 4 Second feedback circuit 5 First feedback circuit 6 LED (light emitting unit)
8 Second integration circuit 9 First integration circuit 10 Sample hold circuit 10 'Low-pass filter circuit 11 Switch control circuit A Smoke detector C5 Capacitor Iin Input current OP2 Operational amplifier PD Photodiode (light receiving unit)
Q1 shunt transistor R1 shunt resistor R6 first resistor R7 resistor R8 second resistor SW1 first switch SW2 second switch SW3 third switch SW4 fourth switch SW5 to SW10 switch (mode switching means)
SW11, SW12 selection switch Tin input terminal Tout output terminal Vout output voltage

Claims (12)

  A light emitting unit that outputs pulsed light during a predetermined sensing period toward the detection space, and light from the light emitting unit that is diffusely reflected by smoke flowing into the detection space is incident without direct light from the light emitting unit entering A light receiving unit that receives light and converts it into current, and converts the input current that is input from the light receiving unit to the input terminal into an output voltage that varies in voltage according to the fluctuation of the input current. A current-voltage conversion circuit having a conversion unit that outputs from an output terminal; and a determination processing unit that determines the presence or absence of smoke in the detection space based on the output voltage, and the current-voltage conversion circuit includes the output voltage A first feedback circuit that outputs a low-frequency component that is lower than a first cut-off frequency lower than the frequency of a pulse-like detection signal generated when the light-receiving unit receives light from the light-emitting unit, a circuit ground, and the input terminal Inserted between the second By the control terminal to the output of the feedback circuit is connected, smoke detector, characterized in that it comprises a shunt transistor pulling a current having a magnitude corresponding to the output of the first feedback circuit from the input current.   The current-voltage conversion circuit outputs a voltage corresponding to a low frequency component equal to or lower than a second cutoff frequency lower than the frequency of the detection signal in the output voltage, and an output of the second feedback circuit The smoke detector according to claim 1, further comprising a shunt resistor inserted between the input terminal and the input terminal for extracting a current having a magnitude corresponding to the output of the second feedback circuit from the input current.   The first feedback circuit has frequency switching means for switching the first cutoff frequency so as to be lower than the second cutoff frequency during the sensing period and higher than the second cutoff frequency during periods other than the sensing period. The smoke detector according to claim 2.   The first feedback circuit has an integration circuit that outputs an integral component of the output voltage, and the frequency switching means is a first inserted between the output of the integration circuit and the control terminal of the shunting transistor. In the sensing period, the sample and hold circuit is activated by turning off the first switch, and the held output voltage of the integrating circuit is applied to the control terminal of the shunting transistor. The smoke detector according to claim 3.   A second switch is connected between the input terminal and the output terminal of the conversion unit, and the second switch is turned on when the first switch is on. Item 5. The smoke detector according to item 4.   The smoke detector according to claim 4, wherein the first switch has an off-resistance value set smaller than a resistance value between a control terminal of the shunting transistor and circuit ground.   The first feedback circuit includes an integration circuit that outputs an integrated value component of the output voltage, and the frequency switching unit includes a capacitor connected between a control terminal of the shunting transistor and a circuit ground, an integration circuit A low-pass filter circuit having a resistor and a parallel circuit of a third switch connected between the output of the circuit and the control terminal of the shunting transistor, and turning off the third switch during the sensing period; 4. The smoke detector according to claim 3, wherein a low-pass filter circuit is operated.   The first feedback circuit includes an integration circuit having a time constant determined by a first resistor and a capacitor, and the frequency switching means includes a second resistor and a fourth resistor connected in parallel with the first resistor. 4. The smoke detector according to claim 3, further comprising a series circuit of switches, wherein the fourth switch is turned off during the sensing period.   The second feedback circuit is composed of an active filter that outputs a voltage having an opposite phase to the input current, and the first feedback circuit is composed of an active filter that outputs a voltage having the same phase as the input current. The smoke detector according to any one of claims 2 to 8, wherein the smoke detector is provided.   A plurality of the shunt transistors are provided, a selection switch inserted between each shunt transistor and the input terminal, and a selection switch that is turned on as the output of the first feedback circuit increases. The smoke control device according to any one of claims 1 to 9, further comprising: a switch control circuit that performs on / off control of the selection switch in accordance with an output of the first feedback circuit so that the number of the first feedback circuit increases. sensor.   The first feedback circuit and the second feedback circuit share an operational amplifier, an operation mode using the operational amplifier for the first feedback circuit, and an operation mode using the operational amplifier for the second feedback circuit; The smoke detector according to claim 9, further comprising mode switching means for switching between. The smoke detector according to claim 9, wherein the power supply voltage of the first feedback circuit is set higher than the power supply voltages of the other circuits.

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