Mad Scientist Extension Cord




About: My name is Randy and I am a Community Manager in these here parts. In a previous life I had founded and run the Instructables Design Studio (RIP) @ Autodesk's Pier 9 Technology Center. I'm also the author ...

The Mad Scientist Extension Cord uses a monster movie style knife switch to turn power to the extension cord on and off. I like using it to turn my Jacob's Ladder on and off. This whole setup consists of a heavy duty 3PDT knife switch and 18 AWG 3 conductor fabric wire that is rated for up to 5A of current.

The high voltage might make this seem complicated, but it is a rather simple project. In fact, we are only wiring up half of the switch's connections, such that is wired more like a 3PST switch. Ultimately, it is just an extension cord with a switch in the middle. We are simply extending the three prongs from the wall socket and making or breaking each of three connections.

While this is simple, I'd once again like to point out that the electrical contact from the wall is slightly exposed, and there exists a real chance of electric shock if used wrong. You will need to be careful when handling this, and it is not good to leave plugged in anywhere you may have children, small animals or people who don't understand electricity. Since I have just described most places, it is not recommended you keep this plugged in when not actively demonstrating it. In fact, is likely in violation of most building codes in most places, so this is only a novelty item that you should always be careful when handling, and never leave plugged in or unattended. But it's great for your Halloween Haunted House display, or for impressing house guests.

To learn more about electricity and switches, check out the Electronics Class.

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Step 1: Materials

For the Mad Scientist Extension Cord you will need:

(x1) 3PDT 100A knife switch
(x1) Cord end plug
(x1) Cord end socket
(x1) 25' heavy duty fabric cord
(x2) Zip ties

Step 2: Remove the Cover

Remove the half of the cover which exposes most of the screw terminals for the center electrical contacts.

Step 3: Widen the Hole

We need to widen the center hole of the cover to allow the fabric cord to be cleanly inserted.

Since this is a very soft plastic, we can misuse a hand drill drill to cut horizontally and extend the hole.

Insert a 3/8" or 1/2" drill bit and at a medium to high speed gently cut inward to widen the hole.

Step 4: Remove the Insulation

Using a razor blade, box cutter, or craft knife, carefully remove 3" of the outer insulation from one end of a 8' section of fabric cable. Wrap the cut end of the insulation with tape (preferably the color of the cable) so it does not fray.

Repeat this process for a second cable.

Step 5: Wire It Up

Strip a little insulation off the end of each of the three inner wires.

Wrap each wire clockwise around a screw terminal, and fasten each one firmly in place.

Which color gets connected to which is unimportant at this juncture.

While you are at it, affix a zip tie to the end of the cable (around the tape) and trim away most of the tail. This zip tie will be used to prevent the cord from being able to fit back through the hole in the cover.

Step 6: Fully Uncover

Remove the second half of the cover.

Step 7: Remove the Center Screw

Remove the center screw from the other outer set of screw terminals. This will allow you to easily pass the cable through later.

Step 8: Finish the Wiring

Strip insulation of the three inner wires of the remaining cable.

Connect these wire to the center terminals such that wires of the same color are aligned with one another.

Attach a zip tie to this cable near the edge of the switch and cut off most of the tail. Again, this is meant to prevent the cable from being pulled free from the switch.

Step 9: Case Closed

Put the protective covers back on while ensuring the zip tie attach to each cable is on the inside of the covers.

Step 10: Wire the Socket

The last thing left to do is to attach the socket and plug to each end of the cable. The socket should go to the end connected to the center terminals. This is very important to get right because if you do this backwards, the center bars of the switch will always be hot, which increases the likelihood of electric shock dramatically.

Anyhow, disassemble the socket and pass the cable through its casing.

Strip away about an inch and a half of the wire's outer insulation, and then strip a half inch from the ends of the inner wire. Insert each of them into one of the screw terminals in the socket. The wire color is not important, so long as it ultimately matches the wiring on the plug.

Finally, wrap the end of the cut insulation with tape to keep it from eventually fraying.

Step 11: Wire the Plug

Repeat the wiring process for the plug, making sure you attach the inner color wires identically to the plug as you did the socket.

You can ensure you got it right by inserting the extension cord's plug into its own socket and seeing if the color wires line up.

Step 12: Close Up the Cases

Once the colored wires are all aligned, close both of their cases back up.

The plug and socket should be assembled and it should now be complete.

Step 13: Let 'er Rip

When it's done, plug something in and throw switch (kuh-chunk!) to turn it on. It works well for dramatic effect with either of the projects from the previous lesson. Don't forget not to leave it unattended.

Also, if you liked this project, don't forget to check out all of my other Instructables.

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    11 Discussions


    2 years ago

    Please be aware, for citizens of the United Kingdom, should you reproduce this design and someone is injured as a consequence, you will be held liable and may be prosecuted under the law.


    Some points to note;

    1. The originator is using the switch to break the current flowing to a heavily inductive load increasing the risk of exposure to localized arcing/flash-over.

    2. The switching mechanism is designed to be mounted on a fixed vertical platform (a switch board/panel or wall). As you can see from the animated gif the device jumps as the retaining friction is overcome, resulting in the hand of said individual coming perilously close to the exposed conductors.

    3. The switching enclosure is meant for a fixed cable loom, presumably single core, hence there is no strain relief on the cables. Mandatory in the UK on portable appliances.

    4. The device itself doesn't conform to any BS standards or have a CE mark and therefore has no place in the UK.

    5. The device breaks the earth connection at the same time as the Live and Neutral conductors. Unbelievable!

    6. Cables within the device run the very real risk of becoming trapped beneath the switching blade edge given they are not anchored further increasing risk of injury.

    7. Once located in the 'on' position, live conductors are exposed and un-shielded.


    Quite frankly this article is reckless.


    2 years ago

    Looks great :)
    You can coat the handle with some plasti dip to make it much safer.


    2 years ago

    Hey randofo, I've read the safety note but I don't think it justifies publishing a highly dangerous and possibly deadly project on instructables. Your own gif beautifully shows how easy it is to get very close to the conductors. Sure, one shouldn't do anything on the internet without a second thought, but I believe instructables shouldn't skimp on safety for the sake of simplicity for any it's own content.


    2 years ago

    I am amazed at how many dangerous things get put in this site... For starters it is not considering high voltage.. When talking about AC, then anything under 50V is considered extra low voltage, anything over 50V and up to 1000V is considered low voltage and anything above 1000V is called high voltage.

    In regards to the earth do you realise that the whole reason the earth pin is longer is so it is the first to make contact when it is plugged in and the last to lose contact when it is removed. By switching the earth with that switch it is making and breaking at the same time.. You should never switch the earth like you have in this example, you should have left it permanently wired and bypassed the switch.


    2 years ago

    Looks cool. It would be safest if the side that plugs into the wall is *not* the side that connects to the middle. I believe this is opposite to how you have it wired, but I cannot be sure. That way when the switch is up out of the block, the copper will not be hot.

    1 reply

    2 years ago

    Looks awesome!


    2 years ago

    To handle mains voltage, wire this to a low voltage transformer & use a L.V. relay to switch the mains- it should satisfy any code issue.


    2 years ago

    this is very dangerous , these type of switches should be only used with low voltage 12-24 volts


    2 years ago

    I have always worried about knife switches like these. And I have seen them in the typical Frankenstein movies and thought, wow, what a huge potential for a very bad and potentially lethal shock. What would happen if you made the three prongs from some non-conductive material to merely mimic some type metal and then use a momentary 3PDT push type switch that the knife handles switches underneath? Just an idea and a totally safe way to accomplish everything and still have that nostalgic look.


    2 years ago

    You forgot step 13A: Shout "MUHAHAHAHA" as you throw the switch. Nice job.