JPH09198585A - Led current control circuit for smoke sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the control of a current made to flow to an LED and to hardly receive the influence of temperature characteristics or environment factor characteristics in an LED drive circuit used in a photoelectric type smoke sensor. SOLUTION: The LED drive circuit is used in the photoelectric type smoke sensor provided with the LED 21 for radiating light pulses and a detection circuit system for detecting scattered light reflected by smoke. At the time, the current made to flow to the LED 21 is controlled by control signals generated by measuring the current I2 made to flow to the LED 21 and comparing it with a reference current I1 set by a reference circuit provided with the temperature characteristics or the environment factor characteristics opposite to the ones of the LED 21 and the detection circuit system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED current control circuit for smoke detectors, and more particularly, to an LE in which temperature characteristics or environmental factor characteristics conflict with those of LEDs and detection circuit systems.
It relates to a D drive circuit.

[0002]

2. Description of the Related Art FIG. 4 shows the structure of a general photoelectric smoke detector. The LED 11 is driven by the drive circuit 12 intermittently. The temperature characteristic of the drive circuit 12 is controlled by the reference circuit 13. LED when no smoke
The light pulse from 11 does not reach the photodiode 14. On the other hand, when smoke is present, the light pulse from the LED 11 is reflected and scattered by the smoke and enters the photodiode 14. The presence of smoke is detected by detecting the light receiving current of the photodiode 14 with the detection circuit 15.

[0003]

The output of the conventional reference circuit 13 is not the reference current but the reference voltage, and this reference voltage is used to control the transistor that drives the LED. However, since there is no feedback in the conventional example, the LED drive circuit is sensitive to the change in the parameter of the circuit element driving the LED.

The present invention has been made in view of the above problems, and an object of the present invention is to improve the control of the current flowing through the LED and make it less susceptible to the effects of temperature characteristics or environmental factor characteristics. It is in.

[0005]

According to the first aspect of the present invention, in order to solve the above-mentioned problems, as shown in FIG.
The current I2 flowing through the LED is measured and compared with the reference current I1 set by the reference circuit having a temperature characteristic or an environmental factor characteristic that is opposite to that of the LED and the detection circuit system. It is characterized by controlling an electric current.

According to the second aspect of the invention, as shown in FIG. 2, when the current flowing through the LED cannot be directly measured, the current gain of the LED driving means is set sufficiently high so that the LED can be controlled by the control signal. It is characterized in that the total of the control current flowing through the driving means and the current flowing through the LED through the LED driving means is measured. Thereby, even when the current flowing through the LED cannot be directly measured, the approximate value of the current flowing through the LED can be measured.

Further, according to the third aspect of the invention, as shown in FIG. 3, when a low-voltage power source that cannot use a source follower or an emitter follower is used in the output stage of the control signal, this power source is used. And a transistor having a source or an emitter connected to the control signal output stage.

[0008]

1 is a circuit diagram of an LED drive circuit of the present invention used when the power supply of an LED and the power supply of a control circuit are the same. The cathode of the LED 21 is connected to the collector of the driving transistor 22, and the anode thereof is connected to the resistor 23. The other end of the resistor 23 is connected to the positive power supply. The anode of the LED 21 is connected to the non-inverting input of the conductivity control amplifier 24. The inverting input of the amplifier 24 for controlling conductivity is connected to the resistor 25 and the reference current generator 26. The other end of the resistor 25 is connected to the positive power supply. The reference current source 26 is a resistor 25.
Is connected between and. The output of the conductivity control amplifier 24 is connected to the base of the transistor 22. The emitter of the transistor 22 is connected to ground.

In this circuit, the current I flowing through the LED 21 is
2 is dominated by the current flowing in the base of the bipolar transistor 22, and the current I2 flowing in this LED 21 flows in the resistor 23. The voltage of the resistor 23 is compared by the amplifier 24 with the voltage of the resistor 25 through which the reference current I1 from the current source 26 is flowing. Amplifier 2
The current output of 4 flows to the base of the transistor 22,
It controls the current I2 flowing in the ED21. Current source 26
The reference current I1 has an environmental characteristic that is the opposite of the environmental characteristic of the system. This reference current I1 is
Dominates the current I2 flowing through 21. Therefore, the light output is a function of the reference current I1 and thus cancels the influence of the environmental characteristics of the system.

FIG. 2 shows a modified example of the circuit of FIG. 1 which is used when the LED driving power source is different from the control circuit power source. The anode of the LED 31 is connected to the power supply. LED
The cathode of 31 is connected to the collector of the transistor 32. The emitter of transistor 32 is connected to ground via resistor 33. The reference current source 34 is connected between the power supply for the control circuit and the resistor 35. The other end of the resistor 35 is connected to the ground. The non-inverting input of the amplifier 36 for controlling conductivity is the resistor 35 and the current source 3
It is connected to four connection points. Amplifier 3 for conductivity control
The inverting input to 6 is connected to the emitter of transistor 32. The output of the conductivity control amplifier 36 is connected to the base of the transistor 32.

In this circuit, the LED 31 is driven by a bipolar transistor 32. If the beta (current amplification factor) of the transistor is sufficiently high, the emitter current of the transistor 32 becomes the majority of the current of the LED 31. The emitter current is converted into a voltage through the resistor 33. This voltage is applied to the conductivity control amplifier 3
6 is compared with a reference voltage produced by current source 34 and resistor 35. The output of amplifier 36 drives the base of transistor 32.

If operation at low power supply voltages is required, the improved circuit shown in FIG. 3 can be used.
FIG. 3 shows a circuit of the LED driving circuit of the present invention used when low voltage operation is desired. The cathode of the LED 41 is connected to the collector of the transistor 42.
The anode of is connected to a positive power supply. The emitter of transistor 42 is connected to ground via resistor 43. The reference current source 44 includes a power supply for the control circuit and a resistor 45.
Connected between The other end of the resistor 45 is connected to the ground. The inverting input of the operational amplifier 46 is connected to the connection point between the resistor 45 and the reference current generator 44.
The non-inverting input to the operational amplifier 46 is the PMOSFET 47.
Connected to the gate. The drain of the PMOSFET 47 is connected to the base of the transistor 42, and P
The source of MOSFET 47 is connected to the positive power supply.

In this circuit, the LED 41 is driven by a bipolar transistor 42. Transistor 4
An emitter current of 2 produces a voltage with resistor 43.
This voltage is compared by the operational amplifier 46 with the reference voltage generated by the current source 44 and the resistor 45.
Since the operational amplifier 46 has no source follower stage for the output buffer, the transistor 42 is driven through the FET 47 whose source is connected to the power supply.

[0014]

According to the first aspect of the invention, in the LED drive circuit of the photoelectric smoke detector, the current flowing through the LED is measured, and the LED drive circuit has a temperature characteristic or an environmental factor characteristic that is contrary to the LED and the detection circuit system. The LE is controlled by the control signal generated by comparing with the reference current set by the reference circuit.
Since the current flowing through D is controlled, it is possible to drive the LED so as to contradict the influence of the temperature characteristic or the environmental factor characteristic of the LED and the detection circuit system, and the range of environmental conditions such as ambient temperature. The circuit can be operated with.

According to the second aspect of the present invention, when the current flowing in the LED cannot be directly measured, the LED driving means having a high current gain is used.
An approximation of the current flowing in the LED can be measured as the sum of the control current and the LED current.

According to the third aspect of the present invention, when the source follower or the emitter follower cannot be used because the power supply voltage is low, the low voltage operation is achieved by using the output stage including the transistor whose source or emitter is connected to the power supply. Can be possible.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an LED drive circuit of the present invention in the case where the LED and the control circuit have the same power supply.

FIG. 2 is a circuit diagram of an LED drive circuit of the present invention when the power supplies of the LED and the control circuit are different.

FIG. 3 is a circuit diagram of an LED drive circuit of the present invention when low voltage operation is required.

FIG. 4 is a schematic configuration diagram of a conventional general photoelectric smoke detector.

[Explanation of symbols]

 21 LED 22 Transistor for LED driving 23 Resistor 24 Amplifier for conductivity control 25 Resistor 26 Reference current source

Claims (3)

[Claims] 1. An LED drive circuit for use in a photoelectric smoke detector, comprising: an LED that emits a light pulse; and a detection circuit system that detects scattered light reflected by smoke.
The current flowing in the LED is controlled by the control signal generated by measuring the current flowing in the LED and comparing it with the reference current set by the reference circuit having a temperature characteristic or an environmental factor characteristic that is opposite to that of the LED and the detection circuit system. An LED current control circuit for a smoke sensor, comprising: 2. The LED driving means for controlling the current flowing through the LED by the control signal has a high current gain,
LE and the control current flowing through the LED drive means by the control signal
The smoke sensor LED current control circuit according to claim 1, wherein an approximate value of the current flowing through the LED is measured by measuring the total current flowing through the LED via the D driving means. 3. A control signal output stage is provided with a low-voltage power supply in which a source follower or an emitter follower cannot be used, and a transistor having a source or an emitter connected to this power supply is provided in the control signal output stage. The LED current control circuit for a smoke sensor according to claim 1 or 2.