JPH10133613A - Luminance adjusting circuit for led display unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adjust luminance up to a low-luminance part with one external dimming knob by providing a voltage adjusting process circuit which turns on a 2nd switching (TR) below an actuation voltage and supplies a current to a resistor for voltage division to lower the voltage applied to an LED. SOLUTION: The voltage V0 is varied by adjusting a variable resistor 3 for dimming with the external dimming knob to vary the duty cycle of a pulse output by MPU in a pulse generating circuit 1. Then when the voltage V0 drops to an arbitrary voltage V2 , software varies the duty cycle of the pulse output of the pulse generating circuit 1 to its limit and the voltage adjusting circuit 2 is actuated at this time to turn on the switching TR Q2 with its output, thereby lowering the voltage V1 applied to the LED by a current limiting resistor R1 and the resistor R2 for voltage division. Here, the duty cycle of the pulse generating circuit 1 is adjusted again.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a brightness adjusting circuit for a large number of LED (light emitting diode) displays used in, for example, an instrument panel and an operation display panel in a cockpit of an aircraft. The present invention relates to a brightness adjustment circuit of an LED display.

[0002]

2. Description of the Related Art For example, a large number of LEDs in a cockpit
Since the display needs to be adjusted in luminance, a luminance modulation circuit of a dynamic lighting system is provided. That is, LE
The brightness is adjusted by changing the duty cycle of the pulse voltage applied to D with a manual variable resistor.

FIG. 4 is an example of a conventional luminance adjustment circuit. In the figure, the dimming variable resistor 3 can be varied by an external dimming knob, and by adjusting this, the DC voltage V ₀ input to the pulse generation circuit 1 is changed. The pulse generation circuit 1 using an MPU (microprocessor) changes the duty cycle of the output pulse according to the change of the voltage V ₀ by software and outputs the result. That is, the pulse width is controlled by software so that the pulse width decreases as V ₀ decreases, and the pulse is output from the pulse generation circuit 1. Thus, the switching transistor Q ₁ only when the high-level pulse is turned on, LED is dynamically lit. R ₁ is a current limiting resistor the LED. As described above, by adjusting the dimming variable resistor 3, the LE is adjusted.
The brightness of D is adjusted.

[0004]

However, when the luminance is adjusted by such a conventional circuit, the variable range of the duty cycle of the pulse output from the pulse generating circuit 1 is limited, and in particular, the luminance is reduced. There is a limit. This is because the processing speed of software is determined by the system clock of the MPU. As a countermeasure, there is a method of using a high-speed processing MPU, but it is difficult because of the price and the processing speed of peripheral devices, and there is a limit to the high speed.

FIG. 5 is a characteristic example diagram of the above-mentioned conventional luminance adjustment circuit. The curve shown in the figure is a characteristic in which the duty cycle of the pulse is set by software so that the luminance decreases as the voltage V ₀ decreases. However, there is a limit in pulse generation, and there is a disadvantage that it is difficult to control the luminance at a luminance lower than point a.

[0006] To solve this drawback, by replacing the current limiting resistor R ₁ in FIG. 4 to the variable resistor LED
Can be achieved by adjusting both the duty cycle and the LED current to lower the brightness than the point a in FIG. However, new that this case, adjustment for light variable resistor 3 and the current limiting resistor R ₁ is an external adjusting both must be adjusted with two external dimming knob, adjustment is not easy to Problems arise.

SUMMARY OF THE INVENTION An object of the present invention is to solve the disadvantage of the prior art, namely, the difficulty of controlling brightness at low brightness,
Another object of the present invention is to provide a brightness adjustment circuit of an LED display that can adjust brightness to a low brightness portion with one external light control knob.

[0008]

A brightness adjusting circuit for an LED display according to the present invention comprises a dimming variable resistor for changing a constant voltage by a dimming knob and outputting the same, and a dimming variable resistor. A pulse generating circuit in which the duty cycle of the output pulse changes according to the DC voltage of the switching transistor, a switching transistor that is turned on / off by the output of the pulse generating circuit, and a constant voltage applied through a current limiting resistor, and the switching transistor In a brightness adjustment circuit of an LED display including an LED whose applied voltage is controlled on / off, a voltage dividing resistor having one end connected to an output terminal of the current limiting resistor; A second switching transistor connected between the other end and the ground and controlled to be on / off, and an output voltage of the dimming variable resistor; Starting with the output voltage of the dimming variable resistor when the duty cycle of the output pulse of the pulse generation circuit has reached the lower limit, and turning on the second switching transistor for a time equal to or less than the starting voltage. And a voltage adjustment processing circuit for reducing a voltage applied to the LED by flowing a current through the voltage dividing resistor.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention.
In FIG, 2 is a voltage adjustment processing circuit, Q ₂ is a switching transistor, R ₂ is dividing resistors. As in the prior art, the variable resistor 3 for dimming is adjusted by an external dimming knob to change the voltage V _0, and the MP in the pulse generating circuit 1 is changed.
U changes the duty cycle of the pulse output.
Thus, viewed switching transistor to Q ₁ at the high level of the output pulse is turned on, LED is dynamically lit. At this time, the switching transistor Q ₂ is turned off.

Here, the voltage V ₀ is equal to an arbitrary voltage V ₂ (0 <
When the voltage falls to V ₂ <V _cc ), the duty cycle of the pulse output by the pulse generation circuit 1 is limited by software, and at this time, the voltage adjustment processing circuit 2 is started, and the switching transistor Q ₂ is activated by the output. Turn on, lowering the applied LED of voltages V ₁ by the current limiting resistor R ₁ and the dividing resistor R _2. Therefore, by adjusting the duty cycle of the output pulse of the pulse generation circuit 1 again, the brightness can be adjusted with only one of the dimming variable resistors 3.

FIGS. 2 and 3 are graphs showing characteristic examples of the brightness adjusting circuit of the present invention. As shown in FIG. 2, when the voltage V ₀ is _gradually reduced from V _cc and the duty cycle of the pulse is reduced to reach the voltage V ₂ which is the limit of the low brightness adjustment, as shown in FIG. pulse generating circuit 1 by changing the V ₁
The low luminance adjustment can be performed by again performing the adjustment based on the duty cycle of the output.

[0012]

As described in detail above, by implementing the present invention, it is possible to adjust the luminance lower than the conventional luminance lower limit. In addition, there is an extremely large effect in practical use in that the low brightness adjustment can be performed with only one knob of the dimming variable resistor.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a characteristic example diagram of the present invention.

FIG. 3 is a characteristic example diagram of the present invention.

FIG. 4 is a circuit diagram illustrating a conventional configuration.

FIG. 5 is a diagram illustrating a conventional characteristic example.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 pulse generation circuit 2 voltage adjustment processing circuit 3 dimming variable resistor R ₁ current limiting resistor R ₂ voltage dividing resistor Q ₁ , Q ₂ switching transistor

Claims (1)

[Claims] A variable resistor for dimming and outputting a constant voltage by a knob for dimming; and a pulse generating circuit for changing a duty cycle of an output pulse by a DC voltage from the variable resistor for dimming. A constant voltage is applied through a switching transistor that is turned on / off by an output of the pulse generation circuit and a current limiting resistor, and the applied voltage is turned on / off by the switching transistor.
In a brightness adjustment circuit of an LED display including an LED that is controlled to be turned off, a voltage-dividing resistor having one end connected to an output terminal of the current-limiting resistor; and the other end of the voltage-dividing resistor and ground. A second switching transistor connected and controlled between on and off, and an output voltage of the dimming variable resistor is input, and a duty cycle of an output pulse of the pulse generation circuit reaches a lower limit value. It starts with the output voltage of the dimming variable resistor, and while the starting voltage is lower than the starting voltage, the second switching transistor is turned on to supply a current to the voltage dividing resistor to reduce the voltage applied to the LED. A brightness adjustment circuit for an LED display, comprising: a voltage adjustment processing circuit.