Penulisan Rancangan Artikel Jurnal

Judul Skripsi :

UNIVERSAL REMOTE CONTROL FOR AIR CONDITIONERS with BACKLIT LCD

Penulis                 :             ANTONIUS SETYO (5235122716)

Tahun                   :             2012

Email                   :             anthonychoppers@gmail.com

Bidang Studi        :             JURUSAN TEKNIK ELEKTRO
                                           FAKULTAS TEKNIK
                                           UNIVERSITAS NEGERI JAKARTA

Dosen   : 

    Dr. Bambang DharmaPutra, M.Pd

Introduction

Acquiring data

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At first, i had to get somehow the IR data sent by the remote control. The simplest method was with a TSOP1838 IR receiver, the same i used for the long range IR beam break detector circuit.. With the proper trigger setting on my oscilloscope, i managed to capture the IR beam...

This is the TSOP 1838	I connected the oscilloscope to capture the data sent	With the proper trigger settings, here is what the remote sends upon button press

 

Air condition remote controls have a very long protocol

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Air condition remote controls with built-in LCD have a long protocol because they carry all the settings

There is a great difference between typical TV, radio, DVD and other remote control protocols with the air-condition remote controls, and more specifically the remote controls with the LCD screen. That is because the settings are stored on the remote control itself rather than the air-condition unit. This means that each time a button is pressed, the remote control must send not only this button press, but the complete set of parameters such as temperature, fan settings, swing, operation and others.

It was rather impossible to analyze the protocol directly from the oscilloscope's screen because of its length. So what I've done was to connect the oscilloscope with the PC through USB. With a special software i download the data from the oscilloscope to the PC. Luckily, this software can export a long set of measurements in excel format. I made a small software that reads these data, filters the results and draws an inverted rectangular waveform according to the data:

My oscilloscope can be connected with the PC through USB	The software exports a long series of serial data of the waveform in excel format	I made a small software which can analyze this serial data, filter the results and export a bitmap with the filtered waveform

Identifying the transmission protocol modulation

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The oscilloscope's screen shows the signal inverted and i did not recognize the modulation type immediately. But when i saw the filtered and inverted waveform on the screen, i immediately recognized the modulation type: It is Differential Pulse Position Modulation. I've written a detailed theory regarding the PPM and the D-PPM modulation in this page.

So, now i know exactly which parameters to measure: i need to measure the duration of the short and long pulse interval, the pulse duration, the initial pulse interval and the carrier frequency of the signal:

The short pulse duration is 320uSec	The long pulse duration is 1.16mSec	The pulse duration is 540uSec	The initial pulse duration is 3.1mSec

As for the carrier signal, i need to change the receiver circuit. the TSOP has built-in filters and outputs only pulses without the carrier signal. So i replaced it with a simple IR diode like the one i used in this IR Short Distance Beam Cut Detector circuit.

I used a simple IR diode to detect the carrier signal	Now the carrier frequency is visible	By decreasing the horizontal time/div of the oscilloscope, the carrier wave is measurable. It is 37.3KHz

Now i know everything regarding the modulation. I can proceed with the protocol identification...

The last byte was the hardest

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At first, i thought that the last byte represents the temperature change, because if you notice, the last byte is change whenever the temperature is changed. One degree of change in the temperature equals to one change in the last byte. But then i noticed that the last byte changes not only with the temperature. As a matter of fact, any change that is done the last byte changes as well. So, the last byte is something like error correction. But what kind of error correction?



I tried to XOR bytes to see if there is a logical result, but i failed. After many observations and tests, i remembered that the the last byte is changed by one when the temperature is changed by one. There is one mathematical function which can achieve this; The addition!

At first, i inverted all the bits because it seemed to be more logical. Then i added the bytes 7-8 and 9 for all the previous samples. To my surprise, when i subtracted this result from the last byte, i always had a constant result of decimal 57, with one exception: The OFF command. This command gave me a different result. Obviously, the 6^th byte had to be included in the addition.

So i added all bytes fro 6 to 9. To my surprise, everything looked prettier now! By subtracting the result of this addition from the last byte, i always had constant subtraction result of 21! Normally, this would be the end! To re-create the last byte, i only had to add the bytes form 6 to 9 and then add another 21 and that's all. But since my curiosity always overcomes my practical instincts, i had to find out where this "21" comes from.

The bytes 10 to 13 have no interest since they are all 0 (remember that i invert all bits). So the secret should be hidden somewhere in the first bytes. Unfortunately, none of them is 21. And if i add all these bytes, the result was very high...

BYTE 1: 11000100 = 35
BYTE 2: 11010011 = 203
BYTE 3: 01100100 = 38
BYTE 4: 10000000 = 1
BYTE 5: 00000000 = 0

Addition result = 277

And then, after and hour of tests and research, it just came to me! What if i just ignore the 9^th bit of the previous addition which results from the 8-bit overflow???

277 - 256 = 21

Bytes 1-4: Device ID. In my case this is 00100011 11001011 00100110 00000001

Byte 5: 0

Byte 6: 00100100 during normal operation, 00100000 to turn off the Air-Condition

Byte 7: 0000xxxx - MODE:

  00000001 - HEAT

  00000010 - DRY

  00000011 - COOL

  00000111 - FAN

  00001000 - AUTO
Byte 8: 0000xxxx - TEMPERATURE. The temperature is calculating by subtracting this value from the number 31. For example, if this value is 00000000, the temperature is 31 (31-0=31), if it is 15 (00001111), the temperature is 16 (31-15=16).

Byte 9: 00yyyxxx - FAN and SWING:

  00yyy000 - AUTO

  00yyy010 - LOW

  00yyy011 - MID

  00yyy101 - HIGH

  00000xxx - NO SWINGING

  00111xxx - SWING ENABLED
Bytes 10 through 13:Reserved for SLEEP and TIMER functions (i have not worked yet with these bytes)

Byte 14: Error correction. To generate this byte, simply add all the previous bytes (1 through 13) and ignore any overflow from the 8^th bit and above.

References :

• Dosen-dosen pengajar Fakultas Teknik Universitas Sam Ratulangi, Kumpulan Catatan Kuliah Dasar dan Teknik Pengontrolan, Teknik Elektro Universitas Sam Ratulangi, Manado, 2000 – 2004.
  
• http://www.jaycar.com.au/productView.asp?ID=AR1731
• Reverse Engineering oleh Fidji Air Conditioning IR Remote Controls
• Jan F. Kreider. Handbook of heating, ventilation, and air conditioning. CRC press. ISBN 0-8493-9584-4.
• Winnick, J (1996). Chemical engineering thermodynamics. John Wiley and Sons. ISBN 0-471-05590-5.