Solutions for anti-peep door mirrors (cat's eyes)

Door mirrors are commonly known as cat's eyes. Looking out through the door mirror from indoors, one can clearly see all the scenes outside the door with a field of view of about 120 degrees. However, from outside the door, one cannot see anything inside the room through the door mirror with the naked eye. However, criminals can easily see the situation inside the room from outside the door through an optical instrument composed of optical lenses.

　　This paper uses infrared detection technology and the optical switching characteristics of liquid crystal boxes to present a new solution for anti-peep door mirrors, providing a new solution for the anti-peep technology of door mirrors and has broad market prospects.

　　1. Structure Introduction

　　The anti-peep door mirror consists of three parts: optical system, liquid crystal box and its control circuit, infrared detection and its control circuit.

　　1.1 Optical part

　　Ordinary door mirrors are composed of 4 lenses, from the outside to the inside, there are 3 concave lenses close together and a convex lens far away. The 3 concave lenses close together can be equivalent to a group of concave lenses with a very short focal length, called objective lenses. The convex lens is another group, called eyepieces, as shown in Figure 1.

　　1.2 Liquid crystal cell

　　The liquid crystal box is between the objective lens and the eyepiece, and has two electrodes leading out. When there is no voltage on the electrodes, the liquid crystal box is "open", and light can pass through the liquid crystal box freely, which has no effect on the optical path of the door mirror. When an AC voltage of a certain amplitude is applied to the electrodes, light cannot pass through the liquid crystal box, and the liquid crystal box is "closed" at this time, blocking the optical path of the door mirror, thereby playing a role in preventing peeping.

　　The alternating current applied to the electrodes is provided by an oscillator, the effective value of the voltage is 2 to 3 V, and the frequency is tens to hundreds of Hz.

　　1.3 Infrared detection part

　　The infrared detector is a reflective optical coupler, consisting of a pair of infrared transmitting and receiving tubes, installed on one side of the eyepiece. The transmitting tube emits infrared rays. When no one is close to the room, the receiving tube cannot receive the reflected signal from the human body; when someone is looking out from the side of the eyepiece, the receiving tube receives the reflected signal from the human body, and generates a response signal after being processed by the amplifier and relay.

　　2 LCD Box Design

　　2.1 Design

　　Because the door mirror is circular, the basic structure of the liquid crystal box is circular, and its diameter is the same as the inner diameter of the door mirror. Since the liquid crystal box needs to have two control electrodes introduced, a gap is left under the circle to connect the pins, as shown in Figure 2.

　　2.2 Film version design

　　In the manufacturing process of liquid crystal boxes, the required electrode pattern needs to be made on the conductive layer of the conductive glass. This process is currently completed using photolithography technology. Photolithography requires a film plate with a specific pattern. It is through the platemaking method that a black and white pattern corresponding to the electrode pattern is made on the film. The black area can block light, while the transparent area allows light to pass through.

　　Several important processes in the production process of liquid crystal boxes are photolithography, PI coating, friction, and screen printing. The photolithography process is to carve electrode patterns on the glass. The film plate used for exposure needs to be designed. Because the liquid crystal box is composed of two pieces of glass, there are two photolithography film plates. PI coating requires a relief plate, and a film plate is needed to make the relief plate. The screen printing process requires screen printing, and the screen used for screen printing also requires two film plates, one is the Seal plate for printing the sealing frame, and the other is the Dot plate for printing the conductive dots. Therefore, a total of 5 film plates need to be designed. The 5 film plates of the liquid crystal box required for the anti-theft door mirror are shown in Figure 3.

　　3 Circuit Design

　　As shown in Figure 4, the NAND gate U2, capacitor C6 and resistors R5 and R6 form a multivibrator. When voltage is applied to pin 14 of the NAND gate U2, the oscillator starts to work and can output a square wave to the liquid crystal box, making the liquid crystal box in the "on" state. Otherwise, the liquid crystal box will be "closed".

　　LED and D1 form a reflective optical coupler (i.e., infrared detection head). When no one is close to the detection head in the room, the infrared emitted by the LED cannot be reflected to D1, and the photosensitive tube D1 is in a blocking state. The transistor Q1 is in a cut-off state due to no base bias. The power supply voltage is added to the base of the transistor Q2 through D2 and R3 to make it conductive. In this way, current flows through the two pins 1 and 8 of the relay, and its 2nd pin is connected to the 4th pin and disconnected from the 3rd pin. The oscillation circuit stops oscillating due to lack of power supply, and the liquid crystal box is "closed". People outside cannot observe the room through the optical instrument. On the contrary, when someone approaches the detection head in the room, the 2nd pin of the relay is disconnected from the 4th pin and connected to the 3rd pin, and the oscillator starts to oscillate, thereby making the liquid crystal box "open", and the people inside can observe the situation outside.

　　4 Conclusion

　　After the anti-peep door mirror was assembled and the circuit was debugged, it was observed. When observed from the objective lens side, it was completely dark, because the liquid crystal box was in a light-blocking state. When observed from the eyepiece side, the infrared detection head immediately worked, making the liquid crystal box "open", and the condition on the objective lens side could be freely observed. Therefore, the anti-peep door mirror has met the use requirements from design, processing to assembly.