Hi Everyone!
This instructable will be easy to create, fun to play with, safe, and extremely inexpensive to create!  I may post a laser trip-wire instructable in the future.  There are hundreds of ways to make one of these, but this one is EXTREMELY SIMPLE, and really costs less than a dollar or two to create.  Please have a look at my other instructables, and visit www.engineeringshock.com or www.electroniclessons.com !

1) Soldering iron (with solder)
2)  A DC Piezo buzzer or siren
3) A 10k Ohm resistor
4) An NPN transistor (I used a 2N2222, but a 2N4401, or really any NPN transistor will do.
5) Two nails (Optional)
6) Some extremely thin wire (I prefer extremely thing gauge magnet wire.
7) 4x AA or AAA batteries, and preferrably a holder

You actually have a lot of room for the imagination here.  You'll see what I mean as we progress.

Step 1: The Circuit

The circuit is extremely easy.  Let's go though it in detail!

1) If we place our 4x AA or AAA batteries in series, we're going to have a voltage of roughly 6v, which is perfect.  You can use a 9v battery, but you have to make sure that your buzzer is designed to withstand 9v.  If not, you may need a 5v LM7805 regulator.  In this case, place the batteries in series.

2) As you can see, we've got a 10k ohm resistor connected between the positive battery lead and the base of the NPN transistor, which is also connected to the first post.  Note that the second post is connected to the negative lead of the battery pack.  For those of you who don't know much about transistors, we are using it as a switch.  When power is applied to the base of an NPN transistor, the power at the collector will drain through to the emitter, which is connected to ground.  We're going to get back to this portion of the circuit in just a second.  Please note that I've included a bunch of images that give examples of how to use NPN transistors as switches.

3) We have the positive lead of the DC buzzer connected to the positive lead of the battery.  The negative lead of the DC buzzer is connected to the collector of the transistor.  Since the emitter of the transistor is connected to the ground line (negative lead of the battery), then power is applied to the base of the transistor, power is applied to the piezo and drains to ground from the collector to the emitter (Completing the circuit).  When there is no power tied to the base of the transistor, the buzzer circuit is not complete.  In other words, the buzzer will not sound.  When power is applied to the base of the transistor, the buzzer circuit is completed, and the buzzer turns on.

4) Let's have a look at the nail posts.  We have one nail post connected to the base of the transistor and the protective resistor.  The second nail post is connected to the ground line.  When we connect our magnet wire loop between the nail posts, this connects the base of the transistor to ground, which means that there is no power applied.  As soon as that magnet wire breaks, power from the positive lead of the battery pack is applied to the base of the transistor, and the buzzer will sound until you can either turn the power off, or reconnect the wire.

5) You can loop the wire pretty much as far as you want. As long as your magnet wire is intact, the buzzer will not activate. 

- Power is required for the circuit to work. We are using a 6v battery pack (4x AA or AAA batteries in series)
- Until power is applied to the base of the transistor, power at the collector (connected to the negative lead of the DC buzzer) will not connect thorugh the emitter of the transistor and connect with ground.
- The 10k Ohm resistor acts to protect the base of the transistor, as well as limit the current to the circuit.  Without it, there would be absoluately no load, and the battery pack would short circuit.   
- When the magnet wire between the posts is connected properly, the power at the base of the transistor is sunk to ground (0v)
- When the magnet wire is broken, the path to ground at the base is broken  When this happens, the base of the transistor receives power, and the DC buzzer circuit is completed.  the result?BEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEP!!!!!  

Step 2: Mounting

All we really need is the circuit.  You don't need to mount the circuit on to anything if you don't want to.  If you do want to mount the circuit, you can either do what I did, or use your imagination.  I used a thin piece of laminate board.  I used a glue gun to glue the battery pack and the circuit to the board.  If you are using nails for your mounts, make sure that you nail them in properly, and that you solder  or wire-wrap the circuit points properly.   You can also substitute the nails with a terminal block if you'd like. 

From here, you are free to take metres and metres of magnet wire, or any kind of wire and make a trap.  As soon as that wire is broken, the DC buzzer circuit completes, and you're going to hear it! 

Step 3: Tips and Safety


1)  Magnet wire is easily broken, and if the gauge is thin enough, no one will be able to see it.  However, magnet wire has a thin chemical insulation covering it.  The ends will need to be soldered to the posts.

2)  If you want, you can use a 2-pin terminal block as opposed to the posts.  I personally think that using posts makes things easier.

3) You do NOT have to use magnet wire.  I do suggest you use a thin wire that is easily broken, and that is hard to see.

4) Some transistors will have a lower current requirement for activating the base. This means that you can use a higher value resistor (50k Ohms+).  However, I suggest using less than 20k for the 2N4401, and less than 30k for the 2N2222.

5) While this circuit requires very little power when the DC buzzer is not activated, there are still ways to make this circuit more power efficient.  if you're interested, ask me.

6) If you don't have a battery pack, you can use a 9v battery.  However, you have to make sure that your DC buzzer is rated for 9v or less.  If it is not, and you still want to use a 9v battery, you have to implement an LM7805 5v regulator IC into the circuit.

7) You can easily hook this circuit up to a wall wart, but please only do so if you really know what you're doing!


1) Do NOT use this circuit to detect children.  You don't want to cause an accident. 

2) Do not burn yourself with either the hot glue gun or a soldering Iron.  If you are young, and have no experience, has an adult give you a hand.  Safety first =)

3)  Try to keep the trip wire at least four feet from the ground.  This way, you're not likely to hit anyone in the face or trip anybody with the wire.

Step 4: Many Thanks

I hope you enjoyed this instructable.  It is extremely easy and inexpensive to make.  When I have kids, I'm going to create something similar for my liquor cabinet, so that I can catch my kids (As teenagers) in the act =)

Check out my other instructables, and please visit www.electroniclessons.com and/or Register at www.engineeringshock.com

Thanks again!  Happy security to all!
Alert: Although some has solder thru' insulation, <u>most thin &quot;magnet wire&quot; has a tough insulating layer on it</u> (usually some kind of clear enamel), that will NOT conduct!&nbsp; <strong>It has to be scraped or burnt off</strong> before a reliable conductive joint can be made.<br> <br> I know this may sound trivial, but it actually caught me out some 50 years back when I wound&nbsp; a crystal set coil &amp; became frustrated with it's nogo performance. Thankfully an old hand put me right - no wonder the darn wire wouldn't solder!<br> <br> Naturally the above circuit then would react as if the wire was broken if the insulation was still present, but check anyway. &nbsp;&nbsp; Regards- Stan. ( ham ZL2APS)
Hello, I'm planning to do this project because it fits my needs in our house. I just want to ask if what is the size of that resistor you are using? 1/4 or 1/2 ?
I also had to splice some wire on the ground side. I didn't have enough. Thats the purple you can see.<br>Here is the magnet wire I am using. <br>I am trying to get this up and running for a friend of mine who's x boyfriend keeps slashing her tires late at night.<br>
Nice set-up? <br>I've actually used buzzers like that before. Try this (just for fun). When the buzzer should be active, flickit with your finger to see if you get a reaction. <br> <br>Another idea, change the value of the resistor at the base of the transistor to 500 Ohms to 1k Ohms. <br> <br>Lastly, the gauge of your wire may just be too small, and therefore can't work as a solid connection to ground. Try these ideas, and get back to me through ebay. <br> <br>Thanks <br>Pat
one more thing... If I wrap this 36 gauge wire back on itself 10 or so times and then connect all the wrap together strands to the two nail posts it STILL will not work. Could it be the wire I am using? The place I bought said it was magnet wire. Could you give me a specific company or brand name perhaps? I am thinking this wire is the issue.
I followed the instructions carefully and I am reasonably certain everything is in order. However, I attached 36 gauge magnet wire to the posts and I couldn't get it to work. I did get it to work with a thin piece of wire from some 14 gauge. If 36 gauge is not working what gauge will???<br>A few differences in mine are I used a 6v dc buzzer not a piezo buzzer. I also used a battery pack with an on off switch.<br>Any comment on what's up with mine not working please let me know
I love this circuit, i wish i could have found this earlier and made a sentry light with the trip wire.

About This Instructable




Bio: Hi there! My name is Patrick, and I am an electronics engineering technician who works full time as a lab tech, and part time as ... More »
More by EngineeringShock:The Viciously Simple Clap-ON Clap-OFF Circuit For Arduino The Multi-Program Laser Tripwire Set - Video User Manual! Creating A Resistor Based Keypad & Interface With Arduino! 
Add instructable to: