The working principle of anti-myopia dimming desk lamp
The commercial anti-myopia desk lamp is expensive , and it is very convenient to use the method provided by this party to transform the ordinary dimming table lamp at home, and only need to install a circuit with low cost. The installed circuit is shown in Figure 157, and the original dimming lamp control circuit is in the dotted frame. The installation circuit includes four parts: a DC power supply circuit, an infrared emission circuit, an infrared receiving and amplifying circuit, and an optical control circuit.
Working principle:
(1) DC power supply circuit. The AC 220V mains power is stepped down by C1, R1, rectified by VD1, stabilized by VD and filtered by R2, C2 to provide 7.5V/100mA of direct current for subsequent circuits. Among them, MY provides transient overvoltage protection for low-voltage circuits.
(2) Infrared transmitting circuit. IC1A and R4, BC, C3, C4 form an infrared modulation signal oscillator, the output of which is shaped by IC1B and the delay integration circuit composed of IC1C, R5, and C5 generates a 32768Hz sawtooth wave. Due to the T-shaped network of R6, R7, and C6 The filtering can passivate the sawtooth waveform, so as to obtain a series of approximate sine wave signals of the same frequency. VT!, R3, VD2 form the infrared pulse transmitting circuit. Modulate the emitted infrared rays through VT1, R3, and VD2, so as not to cause the subsequent LM567 circuit to malfunction.
(3) Infrared receiver amplifier circuit. If the person sitting in front of the lamp is not sitting upright and the head is too close to the writing, the modulated infrared rays emitted by the above circuit will be reflected by the human body, so that VD2, R8, VT2, RP will be received by VD2, R8, VT2, and RP. Linear amplifier, its maximum amplification gain can be more than 90dB, but this circuit does not need to be too high, and then through C9 input IC2 pin ③ for light control judgment.
(4) Light control circuit. Because the R11 and C12 connected to the ⑤ and ⑥ pins of the phase-locked loop circuit LM567 determine that the center frequency of the capture band is 32.768kHz, so when the signal frequency input from the ③ pin of IC2 is the same as the center frequency set by IC2, IC2 The logic terminal ⑧ pin becomes low level, which makes the yellow LED light-emitting tube light up. The interface with the light control can effectively isolate the 220V AC power supply and the DC low-voltage circuit, so the photoresistor RG is sensitive to light, and its resistance changes sharply, so that the bidirectional trigger diode cannot be triggered and turned on, and the light bulb is extinguished.
The sensing function of the light-emitting diode and the photoresistor in this circuit is very important. When making, the light-emitting diode and the photoresistor can be placed directly against each other, and the two are sealed together with an opaque material. In order to facilitate reliable sensing and prevent false interference.
Component selection and debugging:
IC1 is the six-inverter circuit CD4069B, IC2 is the audio phase-locked loop decoding circuit LM567, BC can be from the old Select a good quartz crystal in the electronic watch, the peak voltage of the varistor is 18V, VD is a 1W/7.5V voltage regulator, VD2 is TLN104, and VD3 is TLP104. R1 can use 1W/501K carbon film resistor, C1 should choose 1μF high-quality capacitor with withstand voltage above 400V. When debugging, the 7.5V DC power supply should be connected after R2 in the figure first, and it is not necessary to consider the joint debugging of the photoresistor RG. After the circuit boards except VD2 and VD3 are soldered and checked correctly, the device uses four long wires to lead out VD2 and VD3 and place them in the proper position of the dimming desk lamp to dig holes, isolate and fix them. Then adjust RP with the palm distance of VD2 and VD3 about 35cm. If the LED is always on, it means that the gain of the amplifier composed of IC1D, IC1E, and IC1F is too high. You can adjust R9 to reduce the gain. Then adjust the RP back and forth, so that the LED is on when the head is within 35cm from VD2 and VD3, and the LED is off when the head is a little farther away from VD2 and VD3. If the LED is still on, it means that the infrared signal emitted by VD2 is too strong. At this time, you can increase R3 and try again. The best effect should be that when the distance between the head and the writing is ≤35cm, the reflected infrared modulation signal will turn off the dimming desk lamp, and when the distance between the head and the book is greater than 35cm, the lamp will not be extinguished, so as to prevent myopia. Purpose.
