Introduction: Electromagnetic Mjolnir (From Thor's Hammer Prank)

EDIT: I've embedded the original video above. It went crazy viral almost a month ago, but if you haven't seen it then the rest of this Instructable may not make much sense, so It's now here for reference.

SAFETY: An electromagnet like this one is serious business! If you're demonstrating around children, make sure their hands and feet aren't anywhere near the magnet when placing it on the ground. If someone has a pacemaker, keep the electromagnet away from their chest area!

Parts List:

  • Costume Prop Mjolnir - Decent price, pretty accurate size, and personally I like the grainy patina look. If you use a different hammer make sure the head is hollow.
  • Arduino Pro Mini 5V - Don't forget, you'll need an appropriate FTDI cable to program this board! It also really helps if you have a regular Arduino Uno to prototype on first before you move everything to the Pro Mini.
  • Sparkfun Fingerprint Scanner - You probably don't need a lot of prints to be stored, so this is the cheaper of the two scanners carried by Sparkfun. Don't forget to get the matching JST connector! There are great Instructables using this Fingerprint Scanner, but they use a voltage divider to keep Rx and Tx at 3.3V. As far as I've seen this is unnecessary, no level shifting requried.
  • TTP223 Capacitive Touch Sensor - From eBay, it's really cheap! There's an equivalent from Adafruit as well if you're wary of eBay parts.
  • 4N35 Optocoupler - A ubiquitous optocoupler, got it from All Electronics. Used with a 1K resistor.
  • 3.7 V 150 mAH Lithium Battery - I used something like this to power the capacitive sensor, but you can avoid it if you don't accidentally connect the handle to the Arduino Ground like I did.
  • 4 AA Battery Holders - I used 4 separate single AA battery holders because I was really tight on space and had to find nooks and crannies to keep the batteries in. I put Dollar Tree AA batteries in them.
  • 4 12V 1.2AH SLA Batteries - I got these from All Electronics because they have a physical storefront near me. You can probably get cheaper from eBay, and really you should just spend a little more on 22.2V Lithium batteries. The 1.2 AH Lead-acid batteries don't really like putting out 1 whole amp continuously, and I'll probably be making that upgrade soon.
  • Crydom CMX60D10 60V 10A Solid State Relay - Again from All Electronics, you may be able to find it cheaper elsewhere.
  • 1n400X diodes - I had 1n4007's on hand from another project, but 1n4002's would be fine. This series of diodes are rated different voltages (50 - 1000) but what's more important for a flyback diode is current rating, and they're all rated at 1 amp. Use them in parallel for at least twice the amount of current your magnet will draw when on (in my case, 2 to handle 2 amps, but these parts are cheap so don't be afraid to overestimate).
  • 10 inch 3/4" Galvanized Steel Nipple - From Home Depot, along with corresponding flange and coupler. Best to go to a hardware store and see them in person, a 12 inch long pipe would be more screen accurate but will give more leverage making the hammer easier to "pry" off a metal surface.
  • 2 Drawer Pulls - I needed a way to mount the awkwardly shaped transformer to a board of wood that could take some pulling punishment. I decided on modifying metal drawer handles to act as braces through the gaps of the "E" shape. This is another thing you'd do best to find in person, preferably with a prepared transformer to test fit. This particular part worked for me, but you'll likely need to use something different.
  • Microwave Oven Transformer - You'll want the largest one possible that's assembled with an "E" piece and an "I" piece cheaply welded together. That way you'll be able to disassemble easily with a dremel cutting wheel or angle grinder without damaging the windings. This tutorial illustrates how to do that in detail. And this great article by K&J Magnetics explains why the "E" shape is so important for holding strength. You might even be able to just buy the "E" laminations online...
  • Scrap Wood - I was lucky and had access to scrap 13 ply baltic birch plywood, which is notoriously sturdy stuff. You'd probably be fine with a decent hardwood/plywood/mdf of comparable thickness (3/4").
  • Leather Tennis Grip - I got lucky with this too, there was a tennis store by me that happened to carry old timey leather wraps for tennis rackets. They weren't really on sale, they just had some on hand that they let me have for something like $4. There are leather like tennis grip wraps on the internet, but none of the colors or prices worked for me.
  • Glues, Epoxies, Washers, Screws, Bolts, Heat Shrink, Tape, Magnets, Wires, Etc. - These will all depend on how you build your own hammer. I mostly used super glue and hot glue for the batteries, since I wanted them to be semi-removable later if I decided to spring for Lithium batteries instead. I used a set of 4 neodymium magnets to help keep the lid of the hammer closed, they sort of work.

I used Conductak to connect the Capacitive Sensor to the handle. It's not commercially available yet, so you can use alligator clips or try soldering, though it can be difficult soldering a wire to a large piece of metal like a flange.

Step 1: Diagram

Picture of Diagram

Fritzing doesn't have all of the components, so there are some substitutions with equivalent wiring:

  • The 9V batteries represent the 12V SLA Batteries.
  • The Antennae represents the handle of the hammer.
  • The Flash Memory represents the Fingerprint Scanner.
  • The Solenoid represents the electromagnet.
  • The Solid State Relay pictured is for AC power rather than DC.
  • The Op-Amp breakout represents the Capacitive Sensor.
  • In reality there is also a wire connecting the ground of the Capacitive Sensor to the core of the electromagnet. This provides a path to literal earth when the hammer is positioned on the physical ground.

There are plenty of improvements that can be made! As long as you don't accidentally ground the handle like I did, you can nix the capacitive sensor's power supply and the optocoupler. You may even be able to just use the Arduino capsense library and do without the capacitive sensor altogether, but it may be finicky. On that note, if all you care about is controlling the hammer, you can get rid of the fingerprint scanner and the Arduino completely and just get a remote control unit, such as this one. All you'd have to do is connect the output of the receiver to the input of the Solid State Relay, and boom, remote controlled Mjolnir. No programming required!

EDIT: I forgot to include switches! You'll want some simple slide switches to turn the Arduino and the capacitive sensor on and off.

Step 2: Code

Don't forget to get the FPS library!

The code is just copy pasted below, the .ino file is also attached:

/*
FPS library created by Josh Hawley, July 23rd 2013 Licensed for non-commercial use, must include this license message basically, Feel free to hack away at it, but just give me credit for my work =) TLDR; Wil Wheaton's Law */

#include "FPS_GT511C3.h" #include "SoftwareSerial.h"

FPS_GT511C3 fps(4, 5);

int touch = 0; int capPin = 9; int flag = 0;

void setup() { Serial.begin(9600); // fps.UseSerialDebug = true; // so you can see the messages in the serial debug screen fps.Open(); pinMode(10, OUTPUT); digitalWrite(10, LOW); pinMode(capPin, INPUT_PULLUP); }

void loop() { touch = digitalRead(capPin); //Serial.println(touch); if ((touch == 0) && flag == 0) { digitalWrite(10, HIGH); fps.SetLED(true); if (fps.IsPressFinger()) { fps.CaptureFinger(false); int id = fps.Identify1_N(); if (id<200) { //Don't care which fingerprint matches, just as long as there is a match digitalWrite(10, LOW); fps.SetLED(false); flag = 1; } } } else { fps.SetLED(false); digitalWrite(10, LOW); } if ((touch == 1) && flag == 1) { //Reset the flag after the hammer has been lifted to return to normal behavior flag = 0; } }

Comments

techm8 (author)2016-03-20

Great Instructable Allen. I really like the concept you came up with. My teachers and classmates really got a kick out of this. We set up in the student center at my college. So much fun watching giant weightlifters try to lift it but to no avail. Looking forward to seeing more great projects from you.

Force VenomG (author)techm82017-11-20

what did you use in class to have it sit on for people to lift from that was heavy enough?

DerickF made it! (author)2015-12-09

hey Allen Pan just wanted to say great job on introducing this indestructible. I actually made this for my digital systems class as a project. The demonstration was successful. I am guessing mine is not as strong as yours though. I used the 4 12v sla batteries as you did and it stuck pretty well but was still able to pull it off with probably 120lbs of force. How much stronger do you think yours is compared to mine? and also when I demonstrated it some of them pulled it at of an angle just a little bit allowing them to pick it up. is this because mines not as strong you think? thanks!

Nice build, it looks great! The surface your hammer sits on is super important, if there's any rust or dirt it really affects performance, and the metal has to be pretty thick. My hammer's probably about as strong as yours on certain manhole covers. The best shots you see in the original video are when the hammer was sitting on a giant smooth steel sheet in the road, and it gripped with at least a couple hundred lbs. I've since upgraded to LiPo batteries and it makes a ton of difference! Those poor lead acid batteries really weren't meant for this kind of power output, and the LiPos perform much better and are lighter :)

can you send me a link of where you got the Lipo batteries please?

Awesome Instructable and very well put together guide. I'm very impressed. I've been contemplating building one of these as my next project. As I search for parts I am questioning whether upgrading to LiPo would be worth it for the price. Anything 22.2V I have found has been crazy expensive. What did you use?

I just got some cheap lipos off of ebay. It might be less expensive to get four 11.1V batteries, this application isn't super intense so you won't need the best LiPos for this.

Great Instructable, glad that you did this. I was thinking of doing 11.1v since 22.2v are so expensive. Any brand that you'd recommend?

mattyo2 (author)DerickF2016-06-26

how do you get the code to work, im a noob at code and cant get it to work :(

LukeM68 (author)DerickF2016-04-24

Hi Derick, great job on the build. I was wondering if you had a step by step guide on the wiring side of things? Like what to connect to where? I'm looking to make this for a charity fancy dress function at work and this would be the icing on the cake.

Thanks in advance.

JohnK259 (author)DerickF2016-03-24

Its all about the transformer, some could have more winding then others based on the model. You could always try to rewind it yourself or buy a new one.

chabib (author)DerickF2015-12-13

That is what I am thinking. The one in video is strong and no one can take it off. But the one you made some people can take it off. I think its because of coil and magnetic field. The magnetic field could be low. And the down surface of hummer should cover more area.

Try this method: Add a wide metal plate on down side of hummer, so it might cover more area.
place the magnatic coil to the end down side of hammer.
;)

Supersonic415 (author)2016-07-03

Could you just wear a magnetic ring and make a corasponding switch of some sort instead of all the arduinos and finger print scanners or remote. I need it as cheap and simple with as few parts as i possibly can. Im not near any stores and i dont like online shopping. Some one pls check out if this would work and get back to me pls

yes you can if you use a reed switch or something like that which is what I am working on

Stalker15 (author)2017-10-28

how can i add a fingerprint?

Stalker15 (author)2017-10-28

how can i add a fingerprint

Stalker15 (author)2017-10-28
how can I add a fingerprint?
Stalker15 made it! (author)2017-10-28

How I can add my fingerprint?

Stalker15 (author)2017-10-28

How I can add my fingerprint? Please help!

richleyance (author)2017-06-20

Does anyone else have a sensitivity issue with the touch sensor? Mine turns on when someone grabs the handle but it takes a split second too long to register, allowing people to pick up the hammer.

John FK (author)2017-05-05

Do i really need the code ?

Neiljams made it! (author)2016-11-29

????

ElishaunN (author)Neiljams2017-01-09

Well done. Looks perfect. I am on my way to finishing mine with a remote. I pumping 58 volts through it with a bunch of 3.7v lithium batteries. I have it working well, but it seams I can pry it off with enough force. I might shorten the handle and up the volts. What do you. Think?

Neiljams (author)2016-11-29

Thanks again Allen Pan for this amazing project. It was a real journey for me figuring it all out (read comments below for dramatic effect). I was able to pick up a pipe wrench with no ease of pulling it off. Totally awesome!

ElishaunN (author)2016-11-08

I'm making one with a remote, that I can activate and deactivate on command. I bought 12 3.7v batteries to use. however, when I connect just two of the batteries to the hammer, it heats up my negative coil to where it turns red hot. Am I missing something? Maybe a resistor where the power goes through the hammer and back into the batteries? Also, I am using the primary, (thicker wire), as opposed to the secondary (thinner wire), because I could not seam to get the thinner wire to work.

Neiljams (author)ElishaunN2016-11-13

Made the same mistake myself. The primary is thick and light producing only about an ohm of resistance which is way to much current draw, dont use it. Just very carefully remove the varnish from the ends of the thin secondary coil wire. Youll be able to get an ohmic reading and solder to them then. High heat can cause the thin wire to snap off, so be carefull about doing that as well. Hope this helped.

ElishaunN (author)Neiljams2016-11-16

I have an undamaged thin wire (secondary). I'm putting 15v through it, and I have a reading with a voltage meter, however, I have no magnatism?

Neiljams (author)ElishaunN2016-11-16

You need more voltage. Try using the 4x 12v lead acid from the instructable or 4x 11.1v lipos.

Neiljams (author)2016-11-13

I was doing it all wrong. When prototyping, it would be wise to set your meter on continuity and connect it to the load side of your relay before adding the 44v load on. Then grab the handle, if your meter starts to beep scan your thumbprint until it stops beeping. If it stops your in business. Ive gotta re-center the handle and then imma finish this bad boy.

Neiljams (author)2016-10-29

I tried the example code with GT511C3 libraries again and was able to enroll and verify my thumbprints. Also the mjolnirb.fzz code was verified and loaded into the Arduino mini pro. However digital pin 10 isn't staying on HIGH when I grab the handle and scan my thumb. I've tried many line changes and at one point was able to get the relay to stay on but wasn't able to turn it off. I'm so close to getting this made. Can someone please help!!!!

Neiljams (author)2016-10-16

Tried the code again today and again the FPS zip library isn't being added to the Arduino list. I keep getting a "no valid library exists" error. Not finding much help online either. I tried to get the library from another source and was able to find one the Arduino would add, however when I tried to verify the code I got an #include "application.h" error. How do I add the FPS library if its not recognizing it as valid? Please someone respond.

Neiljams (author)2016-10-15

Why does the conductak image show 9 wires connected to the arduino and the fritzing shows 8? The gnd wiring for the relay, fps and arduino look like they all cennect to the same arduino gnd pin in the fritz diagram as well. On a side note lipos really made it roomy. And im nixing the touch sensor power system, i used insulating varnish on the pipe opening and around the fps along with electrical tape for good measure. Crossing my fingers i get the wiring right.

mattyo2 (author)2016-06-26

hi i cant get the code to work. :( please help

This is amazing nice work thor the mighty

mattyo2 (author)2016-06-14

hey dude! my windings on my magnet are getting a little warm is this same on yours?

RS29 (author)2016-05-02

Hey really great instructable and invention overall... One thing though, I'm a beginner at electronics and would easily mess up the wiring for the arduino and the fingerprint sensor. How would one go about using either just a hidden switch or a remote control?

Neiljams (author)2016-04-24

Well, i tried but to no avail. How did u guys do the wiring?

Neiljams (author)2016-04-23

HI Allen Pan, I just wanted to express my gratitude for this project. Coolest ever. I remember watching the original video sometime back, so when i found out you posted an instructable I just had to make it. I ran into some issues though. My secondary wasnt working so I tried using the primary and it fried my wires and blew my relay. Im lost now on what to do and was hoping for a little help. I know others have made it, so im feeling like im not following the instructable correctly. Any help will be much appreciated. Unless im just not worthy of making thors hammer....lol

Hainsjojo (author)2016-04-22

How Long a 12v 7Ah battery would last for this project?

Lizcbuurernyo (author)2016-03-13

Hey Allen, My husband 30th birthday is coming up and I would love to use this hammer idea in his surprise party as in "only he is worthy" (avengers theme party). I am very tempted to build one myself but I honestly think I'll probably just mess it up and waste materials. Is there a chance of you building one for me, if so what would be the total cost of you putting one together for me and shipping it. (Also I like the idea of remote control over fingerprint since I want to surprise him and won't be able to get his fingerprints before the party)

Davbay1 (author)2016-02-29

Would a metal instead of plastic case be fine? Or would the magnet somehow screw with it?

dreamloger (author)2015-12-08

Great instructable!! Do you think a 600 lb door magnetic lock may work?

I tried off the shelf door locks first, but they don't fit in the hammer as nicely and only get their rated "grip" on the metal bars they come with. They're really low powered otherwise and won't hold onto manhole covers very well at all. It could work if you designed a stand for the hammer for demonstration purposes.

SwiperF (author)2015-12-05

On about @7:07 of the second video "Mjolnir - How to", you mentioned soldering wires to the parflange but the metal acts as a heat sink and cools down before you can solder wires to it.

To ground your wires, using an extra wire, you can loop one exposed end of the wire to the screws before screwing it on the parlflange (or flange) and use the other exposed end to connect the wire(s) you need to ground. So no need to solder to the parflange.

Thanks for the tip, wish I had thought of that sooner :)

HavocRC (author)2016-01-08

Try coating the fingerprint scanner in colored nail polish, so you can see if you missed any spots, or brush on electrical tape.

FauxBreath (author)2015-11-29

Hi! I am wanting to make this for a school project and I was wondering whether you could help? Even though I have worked with Arduino in the past, I am wanting to save money and some time and go for the remote control route instead.

First of all, does the output of the SSR have to be AC or DC? If so which is the best and why? I have googled it but no answers seem to give me what I'm looking for.

Lastly, you said to plug the output of the Receiver into the input of the SSR. I am looking to use 2 x 12v, 1.2AH batteries. Would the wiring be the same as the diagram given, or will it be different? Thanks a lot, and I hope to see more videos in the future!

ssss fire 52 (author)2015-11-20

The amount of people in that video that tried to lift it with their back and not their legs makes me cringe.

oldsmarracin (author)2015-11-20

dont know if someone has said this already but how bout the fingerprint scanners like found on laptops that you slide your finger across. its a little more hidden but other than that this is awesome

Leners (author)2015-11-18

Looking great! Would it be possible to weld the flange to the E-bracket? I don't trust the wood piece in between to hold back peoples unleashed power!

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Bio: Conductak is a conductive sticky tack. Use it to stick circuits anywhere!
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