# Designing a Santa alarm (honestly)

You read the title and that's what I am trying to do.

Synopsis: I have a young son who is determined to catch Santa (to what ends I don't know). He even dreamed up using some kind of pressure plate and connecting it to a light that will turn on when Santa steps on it to get cookies. I would love to help him build this contraption. We have been down at RadioShack repeatedly.

More practically, I am not an EE and have minimal experience with (and time for) bread boards.

So, I was hoping some people here could give some guidance.

I have been evaluating Little Bits but not sure which way to proceed. I am thinking a pressure sensor under a carpet near the cookies that somehow trips a relay and turns on a light or something else clever.

Naturally, I don't want this to succeed just yet in capturing Santa; consider it a lesson in drafting better requirements and a more complete design. Next year we can drive a new design for catching Santa.

So, while the ultimate goal is a bit childish, is it a serious enough question and maybe I can jump start my son's interest in engineering.

• Why not a string trip wire that pulls a limit switch, that closes the circuit on a battery and a buzzer. Then at night cut the string and leave a note from Santa that says " nice try kids name" – Some Hardware Guy Dec 3 '14 at 18:25
• I made an account here to just let you know that I'm nominating you for Dad of the Year. Well done good sir. Well done. – RubberDuck Dec 3 '14 at 20:23
• This is awesome. If I may ask, how old is your son? Whatever you do, can you post the results after Christmas? I hope to hear that the alarm goes off, your son comes running just in time to hear sleigh bells and feet clomping on the roof. :-) – Moby Disk Dec 3 '14 at 20:33
• whatever your design, It may be more fun to trip it and leave evidence that her was there but got away. like a piece of red velvet caught on a nail. It would blow his mind. – user59701 Dec 4 '14 at 17:48
• Welcome to Parenting.SE! – mgarciaisaia Dec 4 '14 at 18:12

## 7 Answers

A thick piece of cloth, some tin foil, card stock, glue and wire is all you need to make a simple pressure sensitive switch. Here's the pdf I made of this process years ago:

http://semiaxis.com/wp-content/uploads/2011/06/PressureSensitiveSwitch.pdf

And the kickstarter video is still up which might give you some additional insight:

http://www.kickstarter.com/projects/2070729777/pressure-sensing-switch-make-and-take

Connect a watch battery and a 3V buzzer as in the video and it will sound whenever stepped on.

Here's an image of the pdf:

• I am going to try this come the weekend! Geek + electronics + crafting? OMG! Sister will want in too! – Christian Bongiorno Dec 4 '14 at 17:44
• This is the idea I would go with. Much simpler and materials are easy to acquire. – krs013 Dec 5 '14 at 19:28
• Ok, I made this up. But my question is: Why cut both corners? I am going to probably want to run 60 FT of wire (20 gauge). I will test it tomorrow. I can probably just check for continuity with my multimeter. – Christian Bongiorno Dec 8 '14 at 7:15
• @ChristianBongiorno The main reason was to avoid people asking me which corner they should cut. You only need to cut one. The secondary reason is that some people realized the cut corner was only for the wire, and so made a small cut. This slowed them down in feeding the wire through, and further if they didn't empty air from the bag when closing it, it didn't work right away until the air bled from the bag. So there's no technical reason for it, just a user experience reason. – Adam Davis Dec 8 '14 at 12:04
• @AdamDavis Well, as I am a software engineer by trade cutting only one tiny whole for the wire seemed to be the purpose and that's all I did :). Now I need to make a latch, get some power to it and hook up an alarm. – Christian Bongiorno Dec 8 '14 at 17:03

Adorable.

Frankly, the Little Bits kit is way overkill and incredibly expensive. If your goal is to make a simple sensor that detects pressure and turns on a light bulb, that can be done using stuff you probably have around the house.

Here's a basic idea that might be "good enough" for this year. Next year may require some more sophistication as your son's imagination develops.

Instead of a force transducer pad or something equally expensive, use a metal plate that is propped up by a small spring. When the plate is stepped on, it compresses the spring and makes contact with another plate on the floor. In doing so, a circuit is completed and activates a light source.

The metal plates can be cardboard or plastic wrapped in aluminum foil. Or thin sheet metal. The spring can be an actual spring or any squishable material that deforms enough under the weight of a person (or one jolly fat guy). The whole assembly can be hidden under a rug with only a slight bump giving away its presence.

• Ou, Ou, (waves hand) And make one of the wires RED. Then Santa can cut the red wire and disable the alarm. :^) – George Herold Dec 3 '14 at 19:58
• Just in case to clarify, that can't be a normal mains powered light bulb. You can use the small kind, or better LEDs (don't forget the current limiting resistors). – tehwalris Dec 3 '14 at 20:49
• You can make the light stay on after the pad is released by using an electronic latch. A really simple latch can be cheaply built just by using a relay. The power to the relay coil is switched by either the sensor or the relay's own switch (i.e. the switches are in parallel), so when the sensor causes the relay to activate, the relay's own switch keeps power to the coil afterwards, locking the relay "on" until the power is removed manually. – Mark K Cowan Dec 4 '14 at 12:49
• The mark of good engineering is simplicity. This is therefore awesome engineering. I love it. – Crashworks Dec 5 '14 at 2:13
• This is why I use a teleporter. – Santa Claus Dec 8 '14 at 4:55

When catching Santa, you want more than just a quick flash of light - stepping on a switch is fine, but the light will only blink for a few seconds. Even if the light stays on for a while, the battery will run down. Instead of a switch, you want a "latch" - something that triggers once, and locks into the "on" position until it's reset. Thus, I present the Santa Trap:

Supplies:

• Wire
• Aluminum foil
• A small DC motor (6v)
• 2 to 4 AAA, AA, C, or D batteries (the more you use, the faster the motor spins)
• Tape
• String
• Heavy, rolling object (baseball, soccer ball, toy car)
• Light switch

1. Create a switch using the aluminum foil; the other responses to this question will help you there. Basically, place a sheet of cardboard with holes punched in it between two sheets of foil. Stepping on the "pressure plate" will cause the two sheets of foil to touch.

2. Tape the two batteries together, end to end.

3. Connect one end of the wire to one of the foils sheets, and tape it in place; connect the other end of the wire to the positive side of the battery (the side with the '+' sign; the 'bumped-up' side), and tape it in place.

4. Connect a second piece of wire between the other piece of foil, and one of the motor terminals, and tape it in place.

5. Connect a third piece of wire between the second motor terminals and the negative side of the battery (the flat side), and again tape the connections.

6. Tape the battery down, so it doesn't jump all over when it runs.

7. Now, test the circuit; pressing the foil together should make the motor spin. If it doesn't, go back and check your connections.

That is only half of this trap, however. It we stopped there, the motor would spin when Santa steps on the switch, but it would turn off as soon as he steps off again. And so, part two:

1. Tie the string around the heavy object. You may need to tape it on. Place the object on a table or shelf near a light switch. It should be balanced precariously - not hanging off far enough that it will fall immediately, but enough that light pressure would send it plummeting)

2. Tape the other end of the string to the motor. When the motor starts spinning, it will pull the object off the table/shelf. Add a lot of extra tape, so the motor is sure to wrap the string around the shaft.

3. Finally, tie the string to the light switch. A three-way light switch can be flipped such that "down" is "on"; otherwise, you'll need some sort of pulley to cause the string to pull "up" on the switch. A clothes hanger works well, if you have a place to hang it over the light.

When Santa steps on the plate, the motor will spin. The string will wind around the motor shaft, pulling on the heavy object and causing it to fall. The object will pull down on the string on the light switch, causing the switch to flip. Even after Santa leaves, you'll have proof that he visited - the light was flipped on!

As a bonus, here are two alternate designs:

• You can leave out the electrical part by simply using a tripwire; when Santa snags the tripwire, that is what pulls the heavy object down. This method saves on batteries, too.

• Instead of just turning on a light, the heavy object could also land on another switch (a camera, spotlight, or loud radio), or cause some longer Rube Goldberg contraption to start. Use your imagination!

• Rube Goldberg would love it :) – Jasmine Dec 5 '14 at 18:19
• MacGyver too. ;) – try-catch-finally Dec 6 '14 at 14:44
• I actually bought a $4 siren from Radio shack. Makes an unholy noise – Christian Bongiorno Dec 18 '14 at 22:27 The Raspberry Pi website is on your wavelength today: http://www.raspberrypi.org/resource/santa-detector/ Maybe adding in some processing power could be one of the "improvements" for the new year (I'm all for starting simple, and indeed keeping simple, but this is a good opportunity to learn some cool stuff) I assume you could substitute / augment the PIR with the pressure switch. Other input methods: tripwire, beam-break IR (or laser pointer for added "mission impossible" cool factor / demonstration of principle), light sensor under cookie, load sensor or temperature sensor under glass of milk, PiCamera + OpenCV to either detect motion or identify change-of-scene (disappearance of cookie). Not all of those would work or be reliable (or pass Elf & Safety Inspection), some would be subject to false alarm (especially if you have pets) but that's all good fun to experiment with & think about. • If your son likes the idea of building elaborate traps for Santa and is determined to "build a better mousetrap" after this year's "failure", I think the Pi might be a way to go. It shouldn't run you much more than$50 for all the parts for the above-mentioned project, and you'll have a small embedded system for use in future projects afterwards :) – Doktor J Dec 5 '14 at 15:11

Ok, to all those who contributed, thank you! I combined a few nuggets from you all and a simple 6-12 volt siren from Radio Shack with a battery pack and produced the traps (See pics). We placed the pressure sensor under a carpet near the table with the cookies and milkand taped the battery pack and siren under the table. The trap was set!

I worked with my neighbor to have him come in after they were asleep on Christmas night to set off the alarm. The kids were giddy with excitement!

Twas the night before Christmas and all through the house not a gizmo was whirring not even a computer mouse. The trap was taped under the table with care with hopes that our neighbor (a-hem, St. Nick) would soon be there.

The children were nestled all snug in their beds; While visions of tablets and iPods danced in their heads;

When out near the door there arose such a clatter, My wife sprang from our bed to see what was the matter. Away to the kiddos' room she flew like a flash, to wake them to show them that Santa had made a mad dash

But alas they did not wake, no matter the noise how great.

And so it was that Santa was not fated to be discovered yet on this christmas season :)

• Looking forward for this years Santa alarm 2.0, may be a spy cam that could at least capture Santa's pic before he escapes. – 3.1415926535897932384626433832 Feb 6 '15 at 1:13

Not a complete answer, but I just want to second George Herold's mention of snap circuits -- extremely easy for kids to manipulate, price is good, and the manuals are full of plenty of examples. I recall there being some alarm circuits in there as well. If your son is showing the inclination, I'd go ahead and splurge on the largest kit they sell. (I did so for my daughters and am glad I did -- years later they've both graduated into arduino and raspberry pi stuff, and still going.)

If you want to use a light instead of an audible alarm, and want the light to be a mains-voltage lamp, you're going to need some sort of relay. I recall makershed selling something that takes 5vdc input and has a male and female 120vac plug on it -- it basically looks like a very short extension cord, with the 5v input added. That will give you a safe and easy way to control anything at mains voltage.

As far as success or failure, it should be pretty easy to arrange for the alarm to trip and Santa to escape cleanly before anyone has time to react -- he is pretty quick after all. ;-)

• With a kid being guided by a novice, I suggest avoiding mains power altogether. – 200_success Dec 4 '14 at 6:52
• Pretty sure they've got at least 4 alarms that would work for Santa in the 300 kit. (And you can still get it from American Science Surplus for cheap!) – Peter Turner Dec 4 '14 at 22:01
• @200_success see makershed.com/products/powerswitch-tail-ii -- The "danger" of this for said novice is little more than plugging a lamp into an extension cord. As long as OP makes sure to teach his 7 year old to respect electricity and how to properly plug and unplug electronics, there is minimal danger of electrocution. – Doktor J Dec 5 '14 at 0:53
• @200_success Energizing the system from stepped-down mains power would be ok if you're using devices meant for the purpose. Just don't try to manipulate it other than with something like the ball & light switch approach in another post. – Loren Pechtel Dec 8 '14 at 0:13

Take a clothespin a piece of fishing line, some tin foil/tape and a piece of plastic. Coat the jaws of the clothespin in tinfoil, and attatch a wire to each leading to the battery, light, etc. Then place the piece of plastic between the jaws, keeping the fil separated. Then attatch the fishing line to the plastic and fasten the clothespin switch to one thing, and the other end of the fishing line to another. Then all you have to do is wait for Santa to pull the tripwire, allowing the foil electrodes to come into contact ith one another and completing the circuit. Have fun :D