Thursday, October 06, 2011

IR remotes

As part of my home automation project, I also wanted to control my home entertainment system. The controller can be a web interface or an app on android phone, or even a universal remote. Which means, I have to be able to send IR signals from the arduino board that my TV, amplifier and set-top box can understand. For this I will first have to read the signal coming from the remotes, store the various signals and replay them from the arduino board depending on my needs.

Before going too far into the details of receiving and sending IR signals, it would help to understand how a remote control actually works. There is already a lot of material on the internet on how remotes work. So I will try to keep it short and simple.

The IR transmission from remote to your device works on a really simple protocol. The transmitter which is just an LED that emits light in the infrared spectrum, pulses the light in a quick succession and the receiver which is a phototransistor, converts the light signal back to electric pulses. Now to transmit information, the transmitter should send a bunch of 0s and 1s. Normally one would expect (at least I thought), a 1 would be represented by the LED being on and 0 would be represented by the LED being off. However all the data would then have to start with a 1 because only then will the receiver be able to start "listening" to the data.

One way to solve the dilemma of data always having to start with 1, is to have some kind of start signal (the header), to let the receiver know that the data is about to start, and that it should prepare for listening to the data.

Now what happens if there are more than 1 consecutive 0s or 1s? If we know that the pulse width is certain milliseconds then we can identify the consecutive 1s or 0s. But the problem however is the timing. The receiver's and the transmitter's timers should be perfectly synchronized. Otherwise 4 consecutive 1's from the transmitter could be mistaken for only 3 consecutive 1s, if the timers drift a little bit.

So to solve the problem, the IR transmitters and receivers actually use a combination of high and low to represent either a 1 or 0. The 1 is usually represented by longer high and smaller low. While a 0 is represented by smaller high and low. For example in the following drawing the 1 is represented by 1200 milliseconds of high and 600 milliseconds of low. A 0 is represented by 600 ms of high and 600 ms of low. The header is represented by 2400 ms of high and 600 ms of low.

This is the Sony protocol, used by all Sony remotes. Other remotes may have slightly different timings for 1s and 0s, but that is generally how most remotes work. Now there is just one more twist. The IR signal is finally modulated with a high frequency signal, usually between 38kHz and 40kHz. An IR signal modulated with 40kHz looks like this.

Finally, most remotes repeat the same signal at least 2 times. The sony protocol specified that it should be repeated at least 3 times, and the signal should repeat every 45ms.

That is all we need to know to get things started with automating home entertainment systems!
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