Controlling a device via the IR receiver port

When I first set up MythTV, I changed channels on the Huawei DC730 cable box using LIRC and the simple transmitter circuit. This means the Linux kernel was toggling a serial port pin at 38 kHz to generate the IR carrier frequency. This may sound ridiculous, but it actually works well. As long as the LED was properly aligned, the signal always got through.

The DC730 comes with an infrared receiver which connects via a standard headphone jack. It allows the unit to be put in some convenient out of the way place, with just the tiny receiver placed in a strategic position for receiving light from the remote. Of course, this same port can also be used to electrically control the DC730.

I could guess the pinout. The exposed sleeve needed to be ground. To minimize the risk of shorts, it would be best to power the receiver via the tip. This leaves the sleeve (middle section) for the IR signal. I confirmed all this with a multimeter. The DC730 supplies just under 5V via the tip. The sleeve is also near 5V, but the voltage is lower, and attempts to draw some current via a resistor show that there's much less of a voltage drop when drawing current from the tip.

Knowing this, it's easy to try using the receiver. It works just like most other IR receivers. The output is normally high, and it goes low when an IR signal modulated at the right frequency is detected. The internals are probably similar to this Hauppauge receiver. It could probably work with my PVR-250 card, though I never tried this because the lirc_i2c kernel module isn't included in Ubuntu and I don't have the Hauppauge remote for the ir-kbd-i2c module.

For supplying a signal from the computer, I chose to use an optocoupler. This is not really necessary because cable ground ties together DC730 and computer grounds. However, I have plenty of optocouplers, and I might as well have the extra security and ground loop avoidance. I chose a TIL-113, which has a darlington on the output. This allows for a lighter load on the serial port, and the slower operation of the darlington is not a problem with slow remote control signals. On the serial port side, the circuit is much like the simple LIRC transmitter. The DC730's internal pullup resistor wasn't sufficient to get a clean signal, so I added a pullup resistor. On the software side, all that's needed is the softcarrier=0 parameter for the lirc_serial module. This tells the module to simply keep the output high instead of pulsing it at the carrier frequency.

While it's nice to know that the kernel won't have to generate the carrier, the main advantages are more practical. There is no need to worry about alignment and IR leakage anymore. Setup involves simply plugging in a cable.