JP3267184B2 - Photoelectric smoke detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric smoke detector for detecting smoke by detecting light scattered by smoke.

[0002]

2. Description of the Related Art Generally, as shown in FIG. 3, a photoelectric smoke detector of this type receives a scattered light X due to smoke of light emitted from a light emitting element and converts the light into an electric signal.
And a sample-and-hold circuit 2 for sampling and holding the output voltage of the photoelectric conversion circuit 1 and an analog output circuit 3 for adjusting the output voltage level of the output of the sample-and-hold circuit 2 based on a reference voltage and outputting it as an analog signal corresponding to the density. And

As shown in FIG. 4, the photoelectric conversion circuit 1 includes a light receiving element 11, a reference voltage source 12, operational amplifiers A1 and A2, resistors R1 to R3, and a capacitor C1. When light is applied to the light receiving element 11, a weak photocurrent (photocurrent signal) flows. This photocurrent is connected to the operational amplifier A1 and the current −
The voltage signal is converted into a voltage signal by a voltage conversion resistor R1, and DC amplified by an operational amplifier A2. The amplification factor of the operational amplifier A2 is determined by the ratio between the input resistance R2 and the feedback resistance R3. A high-pass filter is formed by the capacitor C1 and the resistor R2, and the operational amplifier A1 converted into a voltage signal
The low frequency component is cut from the output of. That is, the leak current flowing through the light receiving element 11 when the light emitting element is not emitting light is cut off, and only the signal component not including the leak current is DC-amplified by the operational amplifier A2 and output to the sample hold circuit 2.

[0004]

However, in the above-described photoelectric smoke detector, the capacitor C1 and the resistor R2 constituting the high-pass filter of the photoelectric conversion circuit 1 are designed so that the photocurrent signal is not attenuated. Must be set to Therefore, when the frequency of the photocurrent signal is low, the capacitor C1 having a large capacitance value is required, and the capacitor C1 cannot be integrated (one-chip) and becomes an external component. There was a problem that would.

The present invention has been made in view of the above points, and an object of the present invention is to provide a photoelectric smoke sensor which can be integrated into a circuit regardless of the frequency of a photocurrent signal. .

[0006]

According to the first aspect of the present invention,
A photoelectric conversion circuit that receives scattered light due to smoke emitted from the light emitting element by a light receiving element and converts the scattered light into an electric signal; a sample and hold circuit that samples and holds an output voltage of the photoelectric conversion circuit; And an output voltage level of the output of the photoelectric smoke detector having an analog output circuit that outputs an analog signal according to the density, in the photoelectric conversion circuit, when the light emitting element is not emitting light Detecting a leak current flowing through the light receiving element and subtracting the leak current from a light current flowing through the light emitting element when the light emitting element emits light.
Characterized in that the feedback section is provided .

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of a photoelectric conversion circuit of a photoelectric smoke detector according to an example of an embodiment of the present invention. FIG. 2 is a circuit diagram showing a specific example of a feedback unit in FIG. is there. The photoelectric smoke detector of the present embodiment is different from the conventional photoelectric smoke detector shown in FIG. 3 in that the photoelectric conversion circuit 1 includes a light receiving element 11, a reference voltage source 12, operational amplifiers A1, A2, and resistors R1 to R3. The current-voltage conversion circuit section 4 and the feedback section 5 are provided. The feedback unit 5 detects the voltage across the resistor R1 when the light emitting element does not emit light, and operates the operational amplifier A so that this voltage becomes zero.
1 is fed back, that is, the input of the operational amplifier A1 is fed back so that the leak current flowing to the light receiving element 11 when the light emitting element does not emit light becomes zero. Further, when the light emitting element emits light and the light receiving element 11 is irradiated with light, a signal obtained by subtracting the leak current from the photocurrent signal is output.

A specific description will be given with reference to the circuit diagram of FIG.
The feedback unit 5 includes an operational amplifier A3 and an analog switch SW1.
, A capacitor C2 and an n-type MOS transistor M1. The voltage across the resistor R1 is input to the input terminal of the operational amplifier A3, and the output terminal of the operational amplifier A3 is connected to the n-type MOS transistor M1 via the analog switch SW1.
Connected to the gate. The gate of the n-type MOS transistor M1 is grounded by a capacitor C2. The anode of the n-type MOS transistor M1 is connected to the input terminal of the operational amplifier A1, and the cathode is grounded.
The analog switch SW1 is turned on / off by the control signal LEDB.

Here, the analog switch SW1 is turned on when the control signal LEDB is at a high level, and turned off when the control signal LEDB is at a low level. And the control signal LE
DB is a low-level signal only when the light emitting element is emitting light. That is, the analog switch SW1 is turned on when the light emitting element is not emitting light, and is turned off when the light emitting element is emitting light.

Next, the operation of this embodiment will be described. When the light receiving element 11 is not receiving light, the analog switch S
W1 is turned on, and the voltage across the resistor R1, that is, the leakage current is detected by the operational amplifier A3, and the gate of the n-type MOS transistor M1 is driven so that the leakage current becomes zero. Therefore, at this time, the leak current Id of the light receiving element 11 flows from the drain terminal to the source terminal of the n-type MOS transistor M1.

When the light emitting element emits light from this state, the analog switch SW1 is turned off, and n
Since the gate voltage of the type MOS transistor M1 is kept constant, the n-type MOS transistor M1 keeps flowing the same amount of leak current Id from the drain terminal to the source terminal as when the analog switch SW1 was turned on. .
At this time, when the light receiving element 11 receives the scattered light X, a current (Ipd + Id) obtained by adding the leak current Id to the photocurrent signal Ipd due to the reception of the scattered light X flows, and the leak current Id is n. It flows to the ground via the type MOS transistor M1. Therefore, the voltage converted and output by the current-voltage conversion circuit unit 4 is the photocurrent signal Ip
d, and the leakage current Id has been removed.

Therefore, according to the present embodiment, the capacitor C1 constituting the high-pass filter for removing the leakage current Id is not required in the current-voltage conversion circuit section 4, and the capacitor C2 in the feedback section 5 is not required. Since the capacitor is used for holding a voltage and the capacitance value can be reduced, the entire circuit can be integrated (one chip).

[0013]

As described above, according to the first aspect of the present invention, a photoelectric conversion circuit for receiving scattered light due to smoke of light emitted from a light emitting element by a light receiving element and converting the scattered light into an electric signal, A photoelectric type having a sample-and-hold circuit that samples and holds an output voltage of a photoelectric conversion circuit, and an analog output circuit that adjusts an output voltage level of an output of the sample-and-hold circuit and outputs an analog signal corresponding to a density. In the smoke detector, the photoelectric conversion circuit detects a leak current flowing in the light receiving element when the light emitting element is not emitting light, and detects a leak current flowing in the light emitting element when the light emitting element is emitting light. subtract the leakage current
Since a feedback section is provided , a capacitor constituting a high-pass filter for removing leakage current is not required, and a photoelectric smoke sensor that can be integrated into a circuit regardless of the frequency of a photocurrent signal can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of a photoelectric conversion circuit of a photoelectric smoke detector according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific example of the above.

FIG. 3 is a block diagram showing a schematic configuration of a photoelectric smoke detector according to a conventional example.

FIG. 4 is a circuit diagram showing a schematic configuration of a photoelectric conversion circuit according to the above.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoelectric conversion circuit 2 Sample hold circuit 3 Analog output circuit 4 Current-voltage conversion part 5 Feedback part 11 Light receiving element 12 Reference voltage source A1 to A3 Operational amplifier SW1 Analog switch R1 to R3 Resistance C1, C2 Capacitor M1 n-type MOS transistor

Claims (1)

(57) [Claims] 1. A photoelectric conversion circuit for receiving light scattered by smoke emitted from a light emitting element by a light receiving element and converting the scattered light into an electric signal, and a sample and hold circuit for sampling and holding an output voltage of the photoelectric conversion circuit. And an analog output circuit that adjusts the output voltage level of the output of the sample and hold circuit and outputs an analog signal according to the density. A feedback unit for detecting a leak current flowing through the light receiving element when the light emitting element is not emitting light and subtracting the leak current from a light current flowing through the light emitting element when the light emitting element emits light. And photoelectric smoke detector.