Laser Tripwire Nerf Sentry Bomb




About: Howdy! How ya doin'? I'm Michael, and I LOVE to build. My workshop is a mess, my room has scattered devices across the floor, and I am co-owner of a small Java game website. Tis' a maker's life for me.

Well, it all started when I accidentally bypassed the voltage regulator on my
Arduino (thus reducing an amazing bit of technology to a USB powered fire hazard).   I needed something to fill time while a new one shipped to me.  Then it hit me--the Nerf dart, I mean--and I realized what I wanted to build.  So, without further ado, I will show you how to build a laser tripwire sentry nerf bomb with a few parts form Radio-Shack and Jameco, and a pet-toy laser pointer. It's also worth pointing out that the tape on the canister you see in the picture has no apparent use. It did in a previus prototype, but you need not add it.. To briefly describe the end result, we will be building an elastic-loaded canister which can fire 4-5 Nerf darts at a person when that person walks through the laser beam. i call it the sentry bomb and it's great for Nerf games and startling you're freinds. Not to mention that with minor modifications you could turn it into a security device, minor modifications like replacing the launcher with an alarm. If you're new to electronics, then the circuit might be tricky for you, but once you understand it, its really quite simple. Have fun building!

To the parts list!

Step 1: Parts List

The primary problem with building things is that you need the correct parts to do so. Ah well, the best I can do is tell you were to buy them I guess.

LM393AN voltage comparotor or similar ----------- Jameco part # 212177 1.90$ for 10
cadmuim sulfide photoresistor ----------Jameco part 202411 1$ each
approximately 2.3 K ohm pot
0.3 K ohm resistor and 0.68 K ohm resistor  (actuly I used two to make the 0.68 K ohm one)
one LED
a solonoid ---------- Jameco part # 1919191 10$ each
a 2-pole 2-thingy switch. aka one with two separate switches in one and three posts per switch
a cat laser. I got mine at wallgreens for 5$
a soldering board----------Radio Shack, but I don't remember the price
pnp amplification transistor. It doesn't really matter which one I think. i just found one lying around.
solderless breadbord for prototyping----------Radio Shack, butI I don't remember the price
hot glue, solder, elastic, a rubber band, batteries, Nerf darts or similar, 2 small nails, two peices of K'nex or a dowel or somthing, cardboard and wire.

I think that's it.  Boy, it didn't seem like that many things when I was rooting though my box-o-junk for parts. Also this parts list is for one sentry bomb, and as we all know, the more the merrier! In case you don't know about Jameco, it's a really good online electronic supply store which can be found here.

Step 2: The Theory

The theory is quite simple. The voltage comparator determines which resistance(ohms) is greater, either the fixed point or the photoresistor (which corrospond to input a and b). A bright room will lower the resistance of the photoresistor a little, but a laser all but eliminates it entirely. When a person is in the way of the laser the resistance of the fixed point is lower than the photoresistor, causing the comparator to activate and connect its output to ground. If the switch is in the left position, the signal gets sent to the transistor, which powers the solonoid, which fires the weapon, If the switch is in the right position, the led tells you whether the laser is aligned for seting up or not. Below I have the circuit diagram, which should clear up the situation for those who know how to read it. 

Step 3: The Canister

Before we can build anything else, we need somthing to fire the Nerf darts. I originaly came up with this idea as an attachment Nerf shotgun, but it really works much better for this. First you need a VERY  strong cardboard tube that can hold 4 or 5 nerf darts. Did I forget to mention REALLY STRONG? If you don't have good enough cardboard, maybe you should use pvc? I don't know, but if it isn't strong enough it will collapse in on itself when you try to load it. Once you have your strong tube, you need to cut it to the lenth of a Nerf dart. Next you need to make the K'nex rod. I used a red K'nex peice but in case you don't have one and need to use a wooden dowel or somthing instead, it's 5 inches long plus 0.5 inches for the gray peice. Of course that K'nex won't do much good with nothing to propel it, so you need elastic. It should be long enough for you to just bearly pull the pin all the way out of the canister while it's attached. Make sure that if you let go it flies out with great speed, because this is the force that will launch all those Nerf darts. After you know it works, you should poke a hole in the elastic, stick the K'nex rod through, and hot glue it in place.

Step 4: Mounting It

Before we can move on to building the firing pin mechanism, we need to mount what we already have. Let the picture be your guide and use plenty of hot glue. It's also worth noting that the plank is 9 by 4.5 inches, and double layerd for strenth. While you're at it,  you should also mount the solonoid, as it will be used next step.

Step 5: The Firing Pin

This part was harder than the whole rest of the project combined. Lucky for you, I've alredy done the hard part. Here comes the complex part. Cut the popsicle stick into the shape you see in the picture, almost 2 inches long with a tooth at the end. No, not a real tooth.  Just cut it into the shape you see below and sand it so it won't stick. Once you have the popsicle stick cut and de-dusted, smother it in hot glue and put it in the opening of the solonoid. Then smother the entire area in loads of hot glue. It IS nescesary to use this much hot glue. I tried with less hot glue and it didn't work. Before the glue hardens, pound a small nail through the hole and though the popsicle stick. Only pound the nail a little way though as it needs to stick out for rubber band mounting, which comes later.

Step 6: Testing If It Can Fire

For it to fire, you will need a nail that sits just below the K'nex while it's loaded. That way when the solonoid retracts, the K'nex rod can't move down and instead pops off the firing pin. Make sure when you measure where you should put it that you have the solonoid extended almost a centemeter. Also use hot glue to reinforce the nail since it's only being stuffed thruough cardbaord. Once the nail is in place, pull the K'nex rod back and hook it over the firing pin. Then pull the firing pin down into the solonoid and see if it fires. If it dosn't, you have a problem. Otherwise go on to the next step.

Trouble with shooting :  Now, I don't know what your problem is, but this is one of the places where you might have one. Is the nail holding all right? Make sure it's stable and placed high enough for the K'nex rod to pop off when the firing pin retracts. If the firing pin completely retracts and the nail holds, then you need to move the nail up some. If the K'nex won't stay on your popsicle stick make sure the hook it flat, and deep enough. If it won't come off, sand down the tip of the hook to make it shorter. Does the cardboard disk easily slide though the canister? If not, cut it smaller. If the pin falls apart use more hot glue and make sure the nail is doing its job of holding the popsicle stick in the solonoid. If all else fails, just ask me for help in the comments section, as I would hate for someone to go un-shot do to poorly written instructions >=).

Step 7: So It Fires by Hand. Will It Fire by Electricity?

Load it again and connect the solonoid to a well-charged battery. The battery should be at least 9 volts. The pictured sentry bomb works fine with a nine volt, and only has that huge battery pack becaus the nine volts needed recharging. Polarity dosn't matter for the solonoid. Hopefully it will fire. If not (and it probably won't) you need a rubber band.

The rubber band: the thing about the rubber band is that the solonoid dosn't have that much power. It would be better if you used the recommended power level (24 volts) but unless you want to do that a rubber band is the best way. Wrap a rubber band around the solonoid so it pulls the pin into the solonoid. Now try loading it. It is quite possible it will fire itself without any help from the battery in which case you need to weaken the rubber band. Other wise try and fire it from the battery again. Hopefully it will work, but if not you need to strengthen the rubber band. Just keep trying different rubber bands stretched different amounts untill it works. You just gotta hate trial and error. Oh well. 

Step 8: Prototyping the Circuit

This circuit diagram provides complete instructions for building the circuit. One thing that may not be clear, however, is the functioning of the switch. On one side I have the center pin hooked up to the output from the comparator, the left pin hooked up to the transistor base, and the right pin hooked up to the LED. That way you can change whether it's in testing mode or firing mode. the other side of the switch has the centre connected to the positive battery terminal, with the left and right pins soldered together, thus constituting the positive bus. The reason for this is that when the switch is at either side, the bus has power. Only when the switch is in the middle (which officially it’s not supposed to be able to do) is the battery disconnected. Now you could use two switches, but then you would be contributing to our ultimate demise by using more plastic, and the death by global warming of all those cute polar bears would be on your hands. Just kidding, I just don’t like having more than one switch. When you're done putting it together make sure that when in testing mode the led turns off when the laser shines on the photoresistor. If it does the opposite, reverse inputs A and B to the comparator. Also check the sensitivity. If its not how you want it, adjust the pot with a screwdriver. Ideally it should turn off the led if more than just a tiny bit of the laser is on the sensor, because if the whole laser has the be on the center of the sensor, it will be hard to set up. Don’t worry, though, you can always adjust it later. Also
it's worth noting that if your photoresistor has a different resistance than mine r1 will need to be different. r1 should be above the resistance of the photoresistor when the laser is shining and below when it isn’t. Do not be concerned if you can't find the same transistor I used, as there is nothing special about it. It’s a pnp amplifier Darlington transistor with medium gain and low operating voltage. Just so long as you don’t melt it though, just about any transistor would do fine. make sure you get the pin out right. For most, it's collector base emitter, but you certainly shouldn’t count on that. If it doesn’t work make sure the LED is in right, and that all the parts work. I had a comparator quit on me for no apparent reason while making this, probably static but I can’t say for sure. Unlike transistors, most comparator chips have the same pin out so all you have to worry about is operating voltage.

Step 9: Soldering

I hope you know how to solder, because I can't teach you. There is probably a good instructable about it, but I learned how from Robot Building for Beginners and Intermediate Robot Building, both by David Cook. They are probably the best books I have ever read and I highly recommend them. You may notice the similarity between the laser circuit and Sandwich's line-following circuit. Anyway the one thing I can tell you is, DONT REVERSE THE INPUTS WHILE SOLDERING. I did that, and I had to de-solder both inputs. It’s not quite the end of the world but it's close enough so I should warn you against it. When you're done, hot glue it on to the plank where it is on mine, and make sure that the photoresistor points forward and isn’t covered by sticky-uppy wires. After all, the laser has to hit it directly for best results.

Step 10: Congratulations!!!!

Now load it, stick Nerf darts in the tube, put it in testing mode,and  aim the laser (this takes a minute).  Now switch to firing mode. (If, when you do this, it fires immediately, turn down sensitivity or practice turning the switch with a steady hand). Ask a family member to come into the room you booby trapped. BOOM, Nerf pwnd! That’s the idea anyway; It's possible you failed and it didn’t work the first time. Well I can't to hard on you, mine didn’t work the first time either. It turned out the rubber band needed to be adjusted, the batteries needed recharging, and the sensitivity was too high. Also I replaced the cardboard disk on the K’nex rod with stronger cardboard. If none of these are your problem I suggest you re-read this Instructable and make sure you did everything right. If you still can't figure it out, you can post your problem in the comments section and I'll help you with it.
I hope you had fun building my laser tripwire Nerf bomb and that you will have even more fun using it. Don’t hesitate to ask questions as I should answer promptly unless (and this is unlikely) I’m on vacation. Special thanks to my dog Professor for helping me though my frustrations with transistors.  I'd also like to thank my mom for editting, as my skills with letters do not compare to my skilles with circuits.



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    13 Discussions


    8 years ago on Introduction

    Almost 80 people have read this sombody tweeted about it and sombody else has a link to it on facebookk and yet nobody has commented or rated it. If anybody likes this instructable, now is you're exclusive one time only chance to influence my next project, which will be posted on instructables. I would like to build an electronic device that is either ussfull or a nerf compatible wepon of some sort and any suggestions would be apreiciated. Also if anyone builds this tell me so I can brag to my freinds >=)

    8 replies

    Reply 8 years ago on Introduction

    Give it a little while. It's been up for three days. No need to go promoting yourself everywhere just yet. Although that is pretty cool.


    Reply 8 years ago on Introduction

    No kidding, he was complaining about how this wasn't featured on a different instructable.


    Reply 8 years ago on Introduction

    Ditto, and it isn't very good either.I already have 5 methods that would work better, excluding a couple i had a while ago.


    Reply 8 years ago on Introduction

    I dident put the twitter about it up, sombody else did, no idea who. As for the facebook reference, that was actuly my mom so that dosent say much for the size of my audiance. anyway you guys may have missed you're exclusive chance to influance my next project becaus im brainstorming on cat toy/ dog toy/ alarmclock that would wait a while and than go ziping around very fast untill somone cought it. it would have an onboard treat for dogs and an onboard buzzer for an alarm, as for cats they don't need any more motivation than movment to go berserk and attack somthing. my cat hides under the couch and amushes peoples feet. im thinking that wouldent be to hard...speed would be an issue but i've done obsticle detection. now i just need another arduino and a few gearheads. I can't wait!


    Reply 8 years ago on Introduction

    the main perpose of saying that was so you would find this. I wasn't actually upset and I hope I didn't seem so. I do find it funny that a fart joke got featured but I didn't expect this instructable to be featured either so that's just fine =). I am quite glad I did so, as it seems to have attracted a lot of visitors, and I'm guessing anyone who found me though a tech instructable is interested in tech instructables so the people who I get that way might actuly build this. also can anyone explain the order of comments? they arent showing up in chronological order... it is most confusing. anyway i need to get back to work on my next project so goodnight! isent it cool how the things we build for ourselves can be veiwd by people all over the world? I've gotten a lot of ideas from othe instructables to. even better than wikipeidia


    Reply 8 years ago on Introduction

    This is a very good build. Great idea, well executed. At a guess, I'd say your barrier to being featured would be either that it hasn't been seen by an editor yet, or they are concerned about your text. I just gave this a cursory glance, and I spotted a number of typos, run-on sentences, and other errors that make your steps tricky to read. I know Instructables isn't English class, but it pays to take the time to go back through and check for errors, add some paragraph breaks, and generally make it more readable. Your best bet, by the way, is to do this with a friend. You'd be amazed at what a second set of eyes picks up. Oh, and in my experience (not vast, to be sure) I don't generally see my first rating until I've passed 200 views, and usually I'll get a few ratings before my first comment. Unless I got something wrong. Then the comments hit fast! :-)


    8 years ago on Introduction

    thanks! its nice to have some real feedback for once. as for the writing though, i am actuly terrible at grammer spelling and the like, although im fairly good at delivering content. my mom on the other hand is a profesional writer, and she is the one who looked over this and corrected all the mistakes. next time though i will write the whole thing in microsoft office word before i put it on instructables. I'll certainly look over it again though, as i said i would hate for someone to go un-shot becaus of bad writing on my part! =) as for hit count i was expecting an initial surge while it was on the recent list and than a rapid slowdown after as few days. you are obviusly right though, thats not how it works at all.


    8 years ago on Introduction

    LOl you guess don't actuly read the whole comment when i leave a long one do you? well I am new to building things of this sort, my speciality is actuly java, I just got tired of having my creations stuck on the screen. If you have methodes which would work better I would very much like to hear them since its hard to learn everything from books. since you know about this stuff btw how could i make a semi-outomatic gun for an arduino project? i was thinking i would have a pnumatic resevuar which would lead to a long tube with the projectile in it through a solonoid operated valve. help is alway apreciated! =)