The infrared impulse

Imagine a movie party with friends. Just as the 237-minute Love Exposure is about to start, you feel the need to have a remote controller. You remember the spare TV remote lying around. You also happen to have an infrared phototransistor from a Hauppauge HVR-1100 PCI card. So you quickly try to come up with a way to receive the signals using stuff you readily have at home.

Far-fetched? Well, it did happen to me. The movie was some great stuff, so I decided to sit and watch it instead. But the next day, I finished this remotely useful contraption. (Of course proper USB IR receivers are cheap on eBay and well supported on Linux, but hey.)

Long story short, I connected the phototransistor directly into the sound card's line in by soldering the leads to a stereo jack. The sound card is becoming my favorite method of sampling the outside world into the computer.

And sure enough, pressing the button "1" on the remote produces this waveform:

By comparing the timing to some common IR protocols I found out the signal is Manchester-encoded RC-5. After running-average type lowpass filtering, thresholding and Manchester decoding in Perl, we get these 14 bits:

The first "11" is a start bit sequence, then follows a toggle bit that tells whether this is a repeat or a new keypress. The rest is an address field (11110b = 1Eh) and the command itself (000001b = "number 1").

Yay, no new hardware needed!