install a trip alarm along the path.
PART ONE: MAKING THE ALARM
What you need:
(1) Loud, clearly visible fire cracker. A bottle rocket is probably the best for the space it takes up.
(1) Wooden clothespin
(2) Metal thumbtacks, not coated with paint
(2) Conductive wires, about one foot/14cm long or longer as needed.
(1) 9volt battery
(1) Piece of nichrome wire or small steel wool strip (acquisition discussed below)
(1) Small sliver of wood that can fit in clothespin head
(1) Length of trip wire (fishing line, paracord, string, yarn, heavy sewing thread, etc.)
The idea behind the trip alarm is simple. You turn the clothespin into a switch that closes a circuit. When the circuit closes, electricity from the battery can run through a nichrome wire/steel wool which heats it up red hot. This wire ignites your bottle rocket, notifying you that your alarm has been compromised.
So first off, let's mess with the clothespin. You want to drive your tacks on the inside of both halves so that they connect when the clothespin shuts. When driving the tacks, wrap a couple loops of copper wiring around them so that they will be stuck in place, effectively creating a switch on a single circuit.
Now go ahead and place your small piece of wood into the clothespin to see how it holds the switch open. You want your piece of wood to not slip out too easily lest you create false alarms or cause an accident while deploying the device, yet you do not want it to hold so well as to be almost impossible to yank out. Carve with a knife as necessary. I personally employ chopstick segments that I have cut to length, notched for trip wire, and flattened a small portion to hold in the clothespins better.
If this is getting confusing, there will be a diagram below.
At this point, you should make sure your trip wire will attach firmly to your piece of wood. Carve a notch around the end and tie the trip wire. A high rating, non-reflective fishing line works wonders but anything will work. The tougher and the harder to see the better.
Now you have a switch and a means to close it when someone walks along a path you don't want them to. We need that switch to actually do something, so let's set it aside and work on the alarm portion of the circuit.
You need nichrome wire for this, ideally. Nichrome is a material that heats up as electricity runs through it. Steel wool fibers can also be used in a pinch but are more brittle and thus prone to failure. You can buy nichrome wiring in hobby shops or hardware stores. You're only going to be pumping 9 volts through it, so you want something small that will actually heat up. You can also salvage nichrome from junked electronics such as hair driers and toasters. These will give you more than enough nichrome for various projects.
From this nichrome you need to make an element. Create a long, narrow loop so that it can be shoved up into your firecracker, but make sure the loop does not touch itself anywhere but the very tip. Remember that you are dealing with electricity and electricity will follow the shortest path possible. You want it to run through your entire wire so that 100% of the element heats up.
Alternatively, you may also wrap the nichrome around the pre-existing time fuze if a suitable element cannot be formed. If you choose this method, make sure your loops are as close to the base of the firecracker as possible so that there is as short of a delay as you can get. You want your alarm to sound the instant someone trips your wire. Pull the fuze from the firecracker and be ready to install it later when you deploy.
This, of course, brings us to the firecracker itself. You want something that you are going to hear and possibly see, but it should also be compact and easy to deploy. I recommend bottle rockets as they can shoot into the air to tell you exactly where the person is if you decide to employ more than one, is loud, and offers a visual signal immediately. Another good choice would be a whistling firecracker that screams for a few seconds at extremely high volume. Smokers may be a good choice in certain scenarios.
DO NOT WIRE YOUR ELEMENT INTO YOUR FIRECRACKER YET. You will do this when you deploy your alarm in the field.
We're almost done. The only thing left is the 9v battery itself. Get something strong and reliable, not a cheap off-brand battery. Now we just wire the whole thing together.
Connect one wire of your clothespin to the positive terminal of the 9v battery. Connect the other wire of the clothespin switch to one side of your nichrome element. Connect more wiring to the second side of your nichrome element but do not connect this to your battery yet. If your clothespin switch is closed, this will cause the element to heat up and burn out immediately. Instead, ensure they are separated. You may choose to optionally install a 9v battery terminal here so that instead of having to wire everything together in the field, you simply need to plug the battery in and it will be ready.
Optionally, you may need to wire in a capacitor if you only have access to heavier gauge nichrome wiring. Install this along the battery-to-clothespin wire so that it is the first thing to charge. When the switch closes, this should send more juice into the nichrome, ensuring it heats up properly.
Test and make sure it will heat up an element. Don't wire it into the firecracker yet, just make sure your circuit works properly.
Congratulations. You now have a pyrotechnic alarm ready to be employed in the field as needed.
Here is a wiring diagram of the alarm to give you an idea of what this needs to look like, and what it will resemble when deployed:
(Enjoy the MSPaint goodness)
PART TWO: DEPLOYING THE ALARM
Now you need to set this up. First, make sure you're carrying everything in a manner that won't cause premature ignition while you are carrying it. I recommend carrying this in a pouch with separate compartments so nothing can move and connect. Isolate your firecracker and battery from everything else including each other, especially.
When you are ready to deploy the alarm, the first thing you want to do is secure your clothespin switch. Stake it into the ground through a hole in the switch somewhere, tie it to a tree, strap it to a heavy rock, etc. After you have done this, ensure your wooden wedge is secure in the clothespin. Run your trip wire out so it covers the path you want to guard, but don't run it so long that it loses tension and cannot pull the wooden wedge anymore. Secure the opposite end to anything rigid, or have premade stake driven into the ground.
Double check your wedge to make sure it's still in the clothespin and the contacts are not touching each other.
Now, run your electric wires leading from your switch off to where you want to place the firecracker. Install your nichrome element now and place your firecracker so that it cannot fall over or otherwise fail you. Make sure it's also not going to be obvious to whoever is walking along the pathway. They probably won't notice the tripwire, but the switch and firecracker are pretty obvious. Set them behind brush, logs, tall grass, rocks, concrete, etc.
Check the wedge again. Make sure you are on the opposite side of the alarm in the direction you wish to continue traveling so that you do not risk tripping your own alarm. Finally, the last step and ALWAYS the last step, connect the battery. You alarm is now armed and can ignite the firecracker.
You may wish to mark the location in a manner only you can detect so that if your alarm is not tripped, you can retrieve it later. Either way, leave the area and keep an eye and ear out for the firecracker you deployed.
To retrieve the alarm, return to the location and stay off your pathway. Find your battery and disconnect it first. Then, remove your nichrome element from the firecracker. Finally, tear down the remaining pieces of the alarm and place them back into your pouch for future deployment.
I highly encourage you to test your alarm on yourself. Set it up and walk through it. Make sure it works. Get comfortable with how it works, how to set it up quickly, and how to tear it down fast. This shouldn't take you very long in a real situation to deploy - maybe a minute, if that.
This guide has been written as thoroughly as possible to help beginners understand it. The concept originates from basic guerrilla booby-trap methods and was chosen because it creates a flexible platform for other uses (hint: you can place more than mere firecrackers on the end of your element) and it's simplicity. While it may seem complex in the face of employing a simple pull-string firecracker, this allows you to employ any firecracker you want and is more reliable than friction-based pyrotechnics.
Obvious cautions apply - you are leaving behind items which may be found and examined. Do not leave fingerprints and do not in any way profile yourself through the use of this alarm.
Good luck and enjoy.
