JP3111878B2 - LED current control circuit for 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 an LED current control circuit for a smoke detector, and more particularly to an LE having a temperature characteristic or an environmental factor characteristic that is inconsistent with the characteristic of an LED and a detection circuit system.
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 intermittently driven by the drive circuit 12. The temperature characteristics of the drive circuit 12 are controlled by the reference circuit 13. LED when no smoke is present
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 detection circuit 15 detects the light receiving current of the photodiode 14 to detect the presence of smoke.

[0003]

The output of the conventional reference circuit 13 is not a reference current but a reference voltage, and this reference voltage is used to control a transistor for driving an LED. However, since there is no feedback in the conventional example, the LED drive circuit is sensitive to changes in the parameters of the circuit element driving the LED.

[0004] The present invention has been made in view of the above problems, and an object of the present invention is to improve control of a current flowing through an LED so as to be less affected by 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 control signal generated by measuring the current I2 flowing through the LED and comparing it with a reference current I1 set by a reference circuit having a temperature characteristic or an environmental factor characteristic opposite to that of the LED and the detection circuit flows through the LED. It is characterized by controlling the 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 made sufficiently high, so that the LED is controlled by the control signal. It is characterized in that a total of a control current flowing through the driving means and a current flowing through the LED through the LED driving means is measured. Thus, even when the current flowing through the LED cannot be directly measured, an approximate value of the current flowing through the LED can be measured.

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

[0008]

FIG. 1 is a circuit diagram of an LED driving 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 is connected to the resistor 23. The other end of the resistor 23 is connected to a positive power supply. The anode of the LED 21 is connected to a non-inverting input of an amplifier 24 for controlling conductivity. The inverting input of the conductivity control amplifier 24 is connected to a resistor 25 and a reference current generator 26. The other end of the resistor 25 is connected to a positive power supply. The reference current source 26 includes a resistor 25
And ground. The output of the conductivity control amplifier 24 is connected to the base of the transistor 22. The emitter of 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 through the base of the bipolar transistor 22, and the current I 2 flowing through the LED 21 flows through 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 flows. Amplifier 2
4 flows to the base of transistor 22 and
It governs the current I2 flowing in the ED21. Current source 26
Has an environmental characteristic opposite to that of the system. This reference current I1 is
21 dominates the current I2. Thus, the light output is a function of the reference current I1, and therefore cancels out the influence of the environmental characteristics of the system.

FIG. 2 shows a modification of the circuit shown in FIG. 1, which is used when the power supply for driving the LED is different from the power supply for the control circuit. The anode of the LED 31 is connected to a power supply. LED
The cathode of 31 is connected to the collector of 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 conductivity control amplifier 36 is connected to the resistor 35 and the current source 3.
4 are connected. 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 to a voltage through the resistor 33. This voltage is supplied to the amplifier 3 for controlling conductivity.
6 is compared with a reference voltage generated by a current source 34 and a 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 the 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.
Anode is connected to a positive power supply. The emitter of the transistor 42 is connected to ground via a 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. An inverting input of the operational amplifier 46 is connected to a connection point between the resistor 45 and the reference current generator 44.
The non-inverting input to the operational amplifier 46 is a PMOSFET 47
Connected to the gate. The drain of the PMOSFET 47 is connected to the base of the transistor 42.
The source of the MOSFET 47 is connected to a positive power supply.

In this circuit, the LED 41 is driven by the bipolar transistor 42. Transistor 4
An emitter current of 2 produces a voltage with resistor 43.
This voltage is compared by an operational amplifier 46 to a reference voltage generated by a current source 44 and a 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 present invention, in the LED drive circuit of the photoelectric smoke detector, the current flowing through the LED is measured, and the LED and the detection circuit have a temperature characteristic or an environmental factor characteristic that is inconsistent with the LED and the detection circuit system. The control signal generated by comparing with the reference current set by the reference circuit LE
Since the current flowing through D is controlled, it is possible to drive the LED in a manner contradictory to the influence of the temperature characteristic or the environmental factor characteristic of the LED and the detection circuit system, and to control the range of environmental conditions such as ambient temperature. Can operate the circuit.

According to the second aspect of the present invention, when the current flowing through the LED cannot be directly measured, the LED driving means having a high current gain can be 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 power supply voltage is low and the source follower or the emitter follower cannot be used, the output stage composed of the transistor having the source or the emitter connected to the power supply is used, so that the low voltage operation is achieved. Can be made possible.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an LED drive circuit according to the present invention when an LED and a control circuit have the same power supply.

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

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

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

[Explanation of symbols]

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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G08B 17/02-17/12

Claims (3)

(57) [Claims] An LED driving circuit used in a photoelectric smoke sensor having an LED that emits a light pulse and a detection circuit that detects scattered light reflected by smoke.
The current flowing to the LED is controlled by a control signal generated by measuring the current flowing to the LED and comparing it with a reference current set by a reference circuit having a temperature characteristic or an environmental factor characteristic that is opposite to the LED and the detection circuit system. An LED current control circuit for a smoke detector. 2. An LED driving means for controlling a current flowing through an LED according to a control signal has a high current gain.
The control current flowing through the LED driving means and the LE
2. The LED current control circuit for a smoke detector 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 includes 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 the power supply is provided in the control signal output stage. The LED current control circuit for a smoke detector according to claim 1 or 2.