I had this silly idea a few weeks back: to simply attach an LED to an 3,5” audio jack and make a mobile phone application with which you can control all your devices, simply by playing a generated sound file.
So when I was in Shenzhen the other day I decided to buy an audio jack and some IR LEDs to keep myself busy in the hotel room. I attached a LED to the audio jack and played some mp3 on full volume. By using my phone’s digital camera to look at the LED I could see it clearly light up, so at least the voltage of the audio out was high enough to drive a LED.
First problem I’ve encountered: the RC5 signal (and many other IR protocols) is modulated at 36Khz and there’s no way my PC, let alone my mobile phone, will play a 36Khz tone. I slept on this for a few nights and suddenly had an idea: I use two LEDs! One will shine on a positive signal and the other on a negative signal. This way I’ll be able to get the LEDs to flash up to a theoretic frequency of 44.1Khz. In practice this limit is lower because audio-out has a high pass (and low pass) filter, but with some luck I’ll be able to get a decent 18Khz AC signal to achieve the required 36Khz modulation.
I quickly wrote a program (in D, of course) to generate a .wav file with a modulated RC5 signal to turn TVs on or off. The RC5 signal consists of 14 data bits, each data bit is sent as two modulated bits: a data bit 1 is sent as 01 (a pause followed by the modulated signal) and a data bit 0 is sent as 10 (signal followed by a pause.)
Unfortunately, when I played this generated .wav file and held my laptop in front of my TV, nothing happened. In order to debug this, I bought a cheap O-scope on taobao: only 350 RMB! Below you’ll see the IR signal with the 14 bits:
The signal appears to be correct, so there must be something else wrong. Are the modulated pulses too wide, not wide enough? Is my TV not using the RC5 protocol?
To be continued…