6 January 2015

A MythTV Remote Control without LIRC - Power Switch Addon


Overview
This is a little add-on for my MythTV remote control. It adds a hardware power switch feature. This means that you can switch the PC on even if it is completely shut down. There is just one condition, which is that the remote control receiver must have power, supplied through the USB port. On most modern mainboards, this is an option in the BIOS settings. The additional hardware is relatively simple, but the added benefit is substantial, so I think it is worth the effort.

Software
Of course there is new software that comes along with it, but this is nothing major. I added a new key translation table entry for the power button on the remote control and assigned key code 240, which is reserved in the HID specification. Instead of sending a key, when this button is detected pin 4 of port B is activated for 0.5 seconds.

Hardware
Port B pin 4 is of course connected to the USBasp pin MISO. This pin can drive the LED side of an optocoupler. The output side can be connected directly to the PC's power button. I built a Y-cable in order to preserve the cases power button function.

Here is a schematic of the add on board.

The photo at the top shows my version built on perfboard, the yellow LED was mainly added for development purposes and is completely useless for normal operation.

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