Infrared receivers are divided into level type and pulse type. The pulse type is mostly, and the level type is rare.

　　 level type, receiving continuous 38K signal, can output continuous low level, unlimited time. Its internal amplification and pulse shaping are directly coupled, so it can receive and output continuous signals.

　　 Pulse type can only receive intermittent 38K signals. If it receives a continuous 38K signal, it will remain high after a few hundred milliseconds, unless the distance is very close (within 20 or 30 cm). Generally pulse type is used for remote control, but only for special occasions, such as serial port modulation output. Since the serial port may continuously output data 0, it is necessary to use the level type.

　Photoresistors are special resistors made of semiconductor materials such as vulcanized pads or selenized pads. Under the light of a specific wavelength, the incident light is strong and the resistance decreases, while the incident light is weak and the resistance increases. The photoresistor is also called a light tube, its characteristic is that its resistance value drops rapidly under the irradiation of a specific light, and it can be used to detect visible light. The resistance value drops rapidly. Its working principle is based on the internal photoelectric effect. The stronger the light, the lower the resistance value. As the light intensity increases, the resistance value decreases, and the light resistance value can be as small as 1KΩ or less.

　　Optical characteristics of photoresistor:

　　 Photoresistor represents the relationship between photocurrent and light intensity. The illuminance characteristic curve of the photoresistor is mostly nonlinear, and the illuminance characteristic of different photoresistors is different. Photoresistors are not suitable for linear measuring elements, and are generally used as switch-type photoelectric converters.