[PYTHON] I learned how the infrared remote control works with Raspberry Pi

I had never been aware of the mechanism of remote controls for TVs and air conditioners that are familiar to me, but I learned how to use infrared remote controls for electronic work. I also tried to check how communication is actually done using Raspberry Pi.

** What I used **

• Raspberry Pi3 model B+
• Infrared remote control
• Infrared remote control light receiving module

Infrared remote control communication format

First of all, there are roughly three communication formats for infrared remote controllers.

• NEC format
• Used all over the world
• Consumer Electronics Association
• Used in many home appliances in Japan
• Unique format
• Companies create their own formats and use them (Sony, etc.)

Since the remote control used this time was in NEC format, we will focus on this.

NEC format

In the NEC format, T = 562 [μs] is one time unit. The data '0' and '1' are represented in this way.

• '1': 1T ON, 1T OFF
• '0': 1T ON, 3T OFF

And the whole communication is roughly divided into four codes.

• Reader code
• 16T ON, 8T OFF
• Custom code
• 16-bit code consisting of '0' and '1'
• Data code
• 8-bit code consisting of '0' and '1' + code that inverts the 8-bit '0' and '1'
• Repeat code
• Send the same code repeatedly

Now, let's actually use Raspberry Pi and check if this is the case.

Infrared remote control analysis

We will analyze the infrared remote controller labeled Car mp3, which is often included in electronic work starter kits. <img width="150" alt="carMp3.jpg ", src="https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/551611/1c27be81-e637-922e-e676-f516271f3ba6.jpeg "> First, we will make a circuit.

1. 1. Creating a circuit

The infrared receiver module used this time is VS1838B. The pins of VS1838B are OUT on the left, GND in the middle, and VCC on the right. Since Min of VCC is 2.7V and Max is 5.5V, connect it to 3.3V without inserting a resistor. This time, connect OUT to PIN 21.

2. Source code creation

This is the source code that outputs the time (μs) when the signal is ON / OFF.

irAnalysis.py

import RPi.GPIO as GPIO
import time

IR_PIN = 21

def timeMeasure():
    maxTime = 1 #sec
    prevState = GPIO.input(IR_PIN)
    startTime = time.perf_counter()
    while time.perf_counter() - startTime < maxTime:
        nowState = GPIO.input(IR_PIN)
        if nowState != prevState:
            endTime = time.perf_counter()
            elapsedTime = (endTime - startTime) * 1000000 #microsec
            return round(elapsedTime)
    return None

def analysis():
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(IR_PIN, GPIO.IN)
    GPIO.wait_for_edge(IR_PIN, GPIO.BOTH)
    try:
        while True:
            elapsedTime = timeMeasure()
            if elapsedTime is None:
                break
            else:
                print(elapsedTime)

    except KeyboardInterrupt:
        GPIO.cleanup(IR_PIN)
    GPIO.cleanup(IR_PIN)

if __name__ == '__main__':
    analysis()

3. 3. Output analysis

I ran the program I mentioned earlier and pressed number 0 on the infrared remote controller. Since the remote controller was in NEC format, check the output while comparing it with the format.

First of all, in NEC format, the reader code can be received first.

Reader code

• 16T ON, 8T OFF

Because T is 562 μs 16T 16*562=8992(μs)

8T 8*562=4496(μs) It becomes.

The first two lines of output look like this.

python

9338
4294

The values are close to each other.

Next is the custom code.

Custom code

• 16-bit code consisting of '0' and '1'

'0': 1T ON, 3T OFF When ON is 562 μs and OFF is 562 μs

'1': 1T ON, 1T OFF When ON is 562 μs and OFF is 1686 μs

So this time the custom code is 0000 0000 1111 1111.

From the third line onward to the middle of the output

python

484 #'0'
461
543 #'0'
478 
546 #'0'
478
543 #'0'
480
545 #'0'
486
548 #'0'
471
538 #'0'
479
546 #'0'
478
544 #'1'
1604
546 #'1'
1596
544 #'1'
1600
544 #'1'
1600
536 #'1'
1602
544 #'1'
1601
485 #'1'
1578
496 #'1'
1596

Next is the data code.

Data code

• 8-bit code consisting of '0' and '1' + code that inverts the 8-bit '0' and '1'

The data code is 0110 1000, followed by the inverted 1001 01111.

python

520 #'0'
466
529 #'1'
1587
328 #'1'
1543
335 #'0'
426
449 #'1'
1589
527 #'0'
468
512 #'0'
442
463 #'0'
460
------Invert from here--------
521 #'1'
1573
506 #'0'
467
539 #'0'
484
532 #'1'
1586
460 #'0'
438
527 #'1'
1603
479 #'1'
1575
464 #'1'
1588

Repeat code

• Send the same code repeatedly

It seems that 1T, 71T, 16T, 4T are repeated first, and then 1T, 172T, 16T, 4T are repeated. This is a little different from the format, but it may be different for each product.

python

472
40106 
9203
2024
---------------
234
96804
9194
2128
----repetition----
466
96856
9161
2097
----repetition----
451
96858
9212
2168

It was found that the communication was performed according to the NEC format, although the numerical values were slightly different except for the repeat code.