I put this in this forum because it is not equipment or remote specific - any equipment with a 12V trigger output will do. I used it with my Pronto NG and a Pioneer Elite SC-05 Receiver.

I have a Pronto universal remote set up to control everything in my Home Theater including powering OFF my HTPC -- but the only way to turn on the computer was to walk up to the equipment and push the power switch (gross!) So I designed a small circuit that powers on the HTPC from a +12V trigger. I have the SC-05 setup to turn on Trigger #1 when I switch to the HTPC input. The circuit is totally passive (Relay Logic ) and uses two relays you can buy at Radio Shack. Here is the circuit diagram and a picture of the competed switch (sharp eyes will notice the diode is missing - this is Rev 1.2, I added the diode in Rev 1.3).


HTPC Switch Rev 1.3 schematic.jpg
HTPC Switch Rev 1.2.jpg


The way it works is:

The relays are connected in parallel with the normal computer power switch (which therefore continues to work normally). When the 12V trigger comes on, the power switch header pins are shorted through the relay contacts (just as the manual power switch does). Once the computer powers up, the 12V fan header actuates relay 2 which opens the circuit. As long as the computer is on, relay two is actuated and the circuit does nothing (I use IR to turn the computer off). Therefore it is ok to switch the 12V trigger on and off while the computer is powered on (by switching to another input for example) - nothing happens. Pressing the manual power switch while the computer is on powers it off normally.

The 1K resistor is only there because I originally used it as an inductive snubber, but then got paranoid about allowing any negative voltage back into the SC-05 12V trigger output (although I hope Pioneer protects their outputs with snubbing diodes) and so I added the diode. The only downside of the diode is the input circuit becomes polarity sensitive - you have to make sure you get the cathode (banded) end of the diode connected to the tip connector of the jack. If you use the diode you can leave out the resistor. If you use the resistor you need to pick a relay that draws no more than 38 mA to avoid exceeding the 50 mA trigger output. You could also use a 5V relay and a dropping resistor chosen to drop 7V at the relay current.

The whole thing is built on a 1.5 inch square of Radio Shack perf board and just sits in a empty spot in the computer. The jack I chose fits perfectly in an unused round audio cutout in the back panel. You will also need a three foot monoplug-to-monoplug cable to hook up the computer to the receiver. Don't forget to go into the receiver setup and turn on the 12V trigger for the HTPC input.

If you wanted to have your HTPC always come on when you power on your Home Theatre, you could switch to HTPC in your power on macro.