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Circuit to turn on an HTPC with a +12V trigger.
This thread has 5 replies. Displaying all posts.
Post 1 made on Sunday June 21, 2009 at 12:58
Long Time Member
February 2008
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.

Last edited by dabrams on June 25, 2009 08:40.
Post 2 made on Sunday June 21, 2009 at 16:54
Advanced Member
May 2009
A small addition to any DC powered circuit where polarity is an issue is to install a full wave bridge using the AC inputs as designed and the DC outs will always be the correct polarity even if using reversed DC polarity INPUTS.

I put these in every custom device I build that uses external wall wart style power supplies. That way you can use AC or DC output generic supplies of either polarity. Add a voltage regulator IC and 2 caps and you can use any voltage supply- AC or DC- as long as the input voltage is at least 2 volts higher than the output voltage needed.
OP | Post 3 made on Sunday June 21, 2009 at 18:19
Long Time Member
February 2008
Giz - good suggestion.  You are right, that a FWB allows you to always get the right polarity if you don't know your source. That is not actually the problem here. The 12V trigger is ALWAYS ground on the ring and +12 (or 0V when off) on the tip. The problem is once I added the diode you have to actually make sure you wire up the jack correctly. Before I added the snubber the circuit had nothing you had to pay attention to as far as polarity goes making it more idiot proof.

But if anyone is really worried, take Giz's suggestion and use four 1N4148 diodes in a full wave bridge. This will not only make the circuit no longer polarity sensitive, but the bridge provides inductive snubbing (although at -1.4 volts rather than -0.7). 
Post 4 made on Monday June 22, 2009 at 14:12
Senior Member
June 2004
This could all be done with IR as well. You could leave the PC in sleep / standby mode and use an IR wireless USB keyboard. Send any keyboard stroke IR code to wake it up and use the sleep/standby ir code from the keyboard to "shut it down" so to speak.

Some wireless keyboards have programmable shortcuts to launch programs, which only requires a single IR command too.

Make sure you turn off auto standby in the power options.

You can pick up IR keyboards for $5-10 on eBay.
Post 5 made on Tuesday June 23, 2009 at 02:09
Ernie Bornn-Gilman
Yes, That Ernie!
December 2001
Tried to get into avsforums to look at the circuit, can't.

So I haven't been able to look at it.


Does the power switch on the computer have 120 volts running through it? I hope not, but I've never heard or looked into it. If so, that's a bit of a dangerous circuit as you're extending 120 volts out into the chassis without disclaimers and warnings.

Unless, of course, those are on the drawings I can't look at yet.
A good answer is easier with a clear question giving the make and model of everything.
"The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
OP | Post 6 made on Tuesday June 23, 2009 at 08:59
Long Time Member
February 2008

Executive summary: The circuit only connects to low voltage points on the motherboard and presents no hazard to the installer.

Detailed Explanation:

This circuit connects in parallel with the low voltage front panel switch on your computer, not the mains power switch which is sometimes integral to the power supply in the back of the computer.

Nothing on a computer motherboard has 120V on it. The power supply in a PC takes 120 (or 240) and typically converts it to +5, +12, +5 standby, 3.3V , -5V and -12V. None of these are especially dangerous. The voltage on the power switch is low voltage and high impedance as well. The motherboard uses the +5V standby to provide a logical high voltage (i.e. 5 volts) to one side of the power switch header on the motherboard through a pull up resistor or transistor current source. The other header pin is ground. When the power switch is pressed, the high side of the line is shorted and a sensing circuit turns on the power supply through the supply's PS_ON control line. Thus, the highest voltage on the power switch pins on the motherboard is +5 volts and even that is through a limiting resistor or high impedance current source. You cannot hurt yourself touching +5 volts, so you do not have to worry about plugging in the header in my circuit in parallel with the power switch . The +12 volt fan connection is at a higher voltage and lower impedance but still is not very dangerous. It is current flow that harms you but you have to get the voltage high enough to force that current through the human body, 12V is too low to do that. (But if you ever touched your tongue to a 9V battery you got a tingle because the wetness and salinity of your saliva allows some current to flow and your tongue is very sensitive.)

See [Link:]

Nevertheless, I always recommend unplugging the power cord when working inside of any mains connected device (after watching my dad melt a notch in a stainless steel butter knife trying to get a piece of bread out of the toaster when I was ten years old).

Last edited by dabrams on June 24, 2009 23:01.

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