Single chip microcomputer: infrared remote control communication principle

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Infrared remote control communication principle

In the field of practical communication , The transmitted signal usually has a wide spectrum , And a large amount of energy is distributed in the relatively low frequency band , So it is called baseband signal , This kind of signal is not suitable for direct transmission in the channel . To facilitate transmission 、 Improve anti-interference ability and effectively use bandwidth , It is usually necessary to modulate the signal to a frequency range suitable for the channel and noise characteristics for transmission , This is called signal modulation . The received signal shall be demodulated at the receiving end of the communication system , Restore the original baseband signal . This part is about the communication principle , Let's get to know .

The infrared communication in the infrared remote controller we usually use , It is usually used 38 K The left and right carriers are modulated , Now I will introduce the principle to you , Let's first look at the principle of the transmission part .

modulation ： It is to use the signal to be transmitted to control the amplitude of a high-frequency signal 、 phase 、 The process of changing parameters such as frequency , That is, one signal is used to load another signal . For example, when our infrared remote control signal is to be sent , Go first 38 K modulation , As shown in the figure below .

The original signal is a data we want to send “0” Bit or bit data “1” position , And so called 38 K A carrier wave is one with a frequency of 38 K The square wave signal of , The modulated signal is the waveform we finally transmit . We use raw signals to control 38 K carrier , When the signal is data “0” When ,38 K All carriers are sent out without reservation , When the signal is data “1” When , Do not send any carrier signals .

That's in principle , How can we realize this function from the perspective of circuit ？

38 K carrier , We can use 455 K Crystal oscillator , after 12 Divide the frequency to get 37.91 K, It can also be realized by time base circuit NE555 To produce , Or using a single-chip computer PWM To produce . When the signal output pin outputs a high level ,Q2 end , No matter 38 K How to control the carrier signal Q1, None of the vertical branches on the right are conductive , Infrared tube L1 No messages will be sent . When the signal output is low , that 38 K The carrier will pass Q1 Let it go , stay L1 produce 38 K The carrier signal of . The point here is , Most home appliance remote controls 38 K The duty cycle of is 1/3, Also have 1/2 Of , But relatively few .

For normal communication , The receiving end should first monitor the signal 、 Zoom in 、 wave filtering 、 Demodulation and a series of circuit processing , Then the baseband signal is output . But the integrated receiver of infrared communication HS0038B, All these circuits have been integrated together , We just need to connect this circuit , We can directly output the baseband signal we want , Pictured .

The gain of the internal amplifier of the infrared receiving head is very large , It's easy to cause interference , Therefore, a filter capacitor must be added to the power supply pin of the receiving head , The value given in the official manual is 4.7 uF, We use it directly here 10 uF, The manual also requires series connection between the power supply pin and the power supply 100 Ohm resistance , Further reduce interference .

chart 16-6 The circuit shown , Used to receive graphs 16-5 The waveform sent by the circuit , When HS0038B It was detected that there was 38 K Infrared signal , Will be in OUT Pin out low level , When there is no 38 K When ,OUT The pin will output a high level . Then we put the OUT The pin is connected to the MCU IO On the mouth , By programming , You can get the data sent by infrared communication .

Think about it ,OUT Is the pin output data restored to baseband signal data ？ When our single chip microcomputer receives the baseband signal data , How to judge what data is received , What agreement should be followed ？ As we learned before UART、I2C、SPI And other communication protocols are baseband communication protocols , And infrared 38 K Only the baseband signal is modulated and demodulated , Make the signal more suitable for transmission in the channel .

Because our infrared modulated signal is half duplex , And only one signal source can be allowed in the same time space , Therefore, the infrared baseband signal is not suitable for I2C perhaps SPI Communication protocol , We mentioned earlier UART Although it is 2 line , But when communicating , In fact, one line is enough , So infrared can be used in UART Communicate in . Of course , This communication is not unlimited , For example HS0038B It is indicated in the data book of , Want to let HS0038B Identify to 38 K The infrared signal of , So this 38 K The carrier of must be greater than 10 A cycle , This limits that the bit rate of the baseband signal of infrared communication must not be higher than 3800, If the signal output from the serial port is used directly 38 K Modulated words , The baud rate cannot be higher than 3800. Of course, there are many other baseband protocols that can use infrared to modulate , Next, we introduce a common infrared communication protocol for remote control ——NEC agreement .

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