R.G. Keen's Blog

Remote Power Management, Part Two: A Button Pusher

Posted in Power Considerations, Remote Management by rgkeen on April 27, 2010

Having found a LAN-based controller that can turn on/off up to four AC power outlets, my quest for remote control resolved into needing a way to have an AC power line on/off be the equivalent of pressing the “reset” and “power on/off” buttons on an ATX computer.  Remember, the issue to be avoided is using only an AC power line for power off/on.  That’s necessary in some cases, but an extreme measure, and to be avoided if possible.

Here are a few ways to accomplish the task that came to mind:

  • Power a solenoid from a switched outlet, and have the solenoid plunger actually press the buttons. I discarded this as too clumsy and requiring too much mechanical fabrication. Likewise, other electro-mechanical approaches fall into the same category.
  • Power a relay with a 120Vac coil from the switched outlet, and connect the relay contacts in parallel with the front-panel switches. The power line outlet coming on causes the relay contacts to act like a front panel switch without being one. This one has some merit, and is conceptually easier to handle for non-electronically adept people.  It amounts to using a pre-manufactured solenoid plus extra switch contacts to do the first approach. I discarded it only when I thought about possible contact bounce and relay life issues.
  • Skip the fancy stuff and just use WOL. Nice enough, but my motherboard does not do this. A Whack-On-LAN card would work, probably. But I didn’t want to spend the time getting a possibly flakey card that is not otherwise used in the box to work reliably.
  • Power a small electronic circuit from the switched outlet, and have this circuit cause an electronic closure in parallel with the actual front-panel switches. Obviously this is inspired by the Whack-On-LAN device.  This is what I finally chose to do.

Having decided to design an AC-line to switch closure circuit, I sketched out my objectives. The final circuit has to short the contacts when the power line is on, release it when the power line is off. It has to do this cleanly, no funny wandering around between open and closed, and no bouncing open/closed/open/closed to confuse the power supply. Oh, yeah – it has to not electrocute me, burn down the house, or destroy the computer it’s controlling.

Update Notice:
The following is the kind of thinking that would go into designing a right-down-to-the-bare-metal device. I had a belated brainstorm that makes this kind of moot, as well as safer. If you’re going to do something like this read Part Three: A Safer Button Pusher first. Using a pre-existing wall wart relieves many safety concerns, as well as being quicker to actually get. Most Goodwill and used-computer stores have bins of wall warts at rock bottom prices.

This last is where all the complication is buried in this approach. Building a circuit which goes from the AC power line into a metal box that people can touch is a clearly dangerous situation. But there are rules for how to do this. Safety testing agencies have been specifying this for decades.  There are three Big Rules. They are:

  1. Provide physical spacing and testable 2500Vac to 5000Vac isolation between the AC line and metal that a person can touch.
  2. Make the circuit not start fires if any part fails in the worst possible way.
  3. Don’t generate any isolated voltage that is dangerous to people or property like the AC power line is.

We’re in luck on the first one. The electronics industry makes optical isolators which have an internal multi-kV isolation. They provide an output which is usable for the remote-shorted-switch operation, and are safety lab tested for the Big Rules. And they’re cheap. A suitable device is less than US$1.00 in almost all cases. I found one for US$0.30.  This is too good not to use.

The second task is to turn the isolator on and off. The input side of the isolators are universally an LED (Light Emitting Diode), which needs to be driven with about 5-10ma of current to make the output go on/shorted, and no current to make it turn off. I pondered designing a transistor circuit to do this, then an IC circuit (National Semi LM10), and finally found an IC which does the whole thing cleanly. This is a three pin IC which generates a clean, snap action on/off voltage in response to its power supply. The Microchip Technology TC54 comes in a TO-92 package and costs US$0.50. Perfect.

What remains is to make some kind of circuit to power up the TC54 when the AC power outlet goes on, and de-power it when the socket goes off.  The complication is that the socket provides 120Vac, and the TC54 wants a voltage in the range of 1 to 10 Vdc. I found this article by Robert Kollman and Brian King in back issues of EDN. It does the necessary job perfectly, and uses only light, small electronic components.  I’ve adapted it for the button pusher.

It is also possible to use a step-down transformer to make an isolated DC supply to power the TC54. In fact, in many ways this is a simpler approach, as the transformer itself encapsulates all the safety isolation considerations.

I’ll post both methods in an upcoming addition to this entry.

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