Thursday, August 20, 2009

haunt juice

In years past I've strung extension cords all over my lawn, stuck flood light holders all over the place, and filled power strips with wall warts and X10 controllers to power my Halloween props and lights. It got the job done (mostly), but it was a mess and potentially a fire hazard. Since all my existing props run on either 12 volts or 5 volts already, the decision to move to low voltage lighting is a simple one. I've already started making small spot lights, so it's time to focus on powering and controlling them.

My plan is to power the lights and a few small props with a modified computer power supply, and control it all with a couple of homemade "Kit74" type parallel port relay controllers and Brookshire's VSA software.

Here's my victim - a cheap generic ATX PC power supply. You can buy these new online for just a few dollars, or if you have an old junk PC laying around you could scavenge the power supply from it before you send it to the recyclers.

This one says it's rated at 300 watts. I suspect that's 300 watts ILS - if lightning strikes.

An ATX power supply puts out +3.3 volts DC, + and - 5 volts DC, and + and - 12 volts DC. The main connector has either 20 or 24 pins, and contains all the power leads as well as leads for status and turn-on. There are also power leads in all the smaller connectors on the supply.

The color of the wires denotes the voltage: yellow is +12 volts, red is +5 volts, orange is +3.3 volts, black is ground, blue is -12 volts, and white is -5 volts.
Grounding the green wire turns the supply on. The gray (or sometimes brown) wire is the PC's status lead. The only wires we're concerned with are the positive voltage leads, the ground, and the turn on lead.

PLEASE NOTE: ELECTRICITY IS DANGEROUS! Be extremely careful when working on this or any other project that uses electricity, particularly mains voltage! I'm not responsible for any harm caused by attempting this project! If you're gonna do this, do yourself a favor and short across one of the red leads and one of the black leads before you start cutting wires or open up the case of the power supply. And it should go without saying - make sure it's unplugged!

Here I've cut off all the connectors leaving as much length as possible outside the housing of the power supply, and cut away all but the green wire, four of the black wires, and three each of the orange, red, and yellow wires. (The number of leads is arbitrary - just make sure you have at least one of each.) I'll connect a switch between the green wire and any one of the black (ground) wires. I bought this switch from Radio Shack, (excuse me, I mean The Shack.) Not sure what part number it is, but any SPST switch rated for 120 volts should do.

Here I've drilled a hole in the top of the supply's housing and fed the green and black wires through to connect to the switch. Be careful to make sure you have room inside the supply's housing for the switch, and that the wires don't make contact with anything.

Oh, and if your switch is held in place by a nut on the back side, don't do what I did and forget to put the nut over the wires before you solder the wires on the switch. I guess I must have been in the mood to do it over.

For safety's sake, you really should have a constant load on your supply so that the capacitors drain when it's shut off without a load. A 10 ohm 10 watt sand cast resistor between one of the 5 volt (red) leads and a ground (black) lead will ensure that the caps drain and there won't be any unwanted shocks or surprises when you hook the supply up. I didn't have one on hand here, but I left a red and a black lead available so I can add one. (Just need to make another run to the love Shack.)

Here's my almost finished product waiting to be added to my yet-to-be-built controller board.

And just because I have to show it in action, here it is powering a 12 volt bulb.

Next time - the controller.

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