Elements of a Security System – Part 3, by J.M.

(Continued from Part 2.)

There’s another potential option for alerting that could be categorized as ‘mobile centralized’ – it’s possible to set up a radio transmitter connected to a centralized console that would transmit a pre-recorded voice alert (e.g. ‘Alert in zone 3 North’) to a radio that you carry around with you. Some of the sensors I’ll be discussing later have this capability built-in, but you could implement a similar function using a Raspberry PI computer to monitor the sensors, connected to a RTL-SDR software-defined radio to transmit pre-recorded audio alerts. Implementing this would be moderately complicated and is beyond the scope of this article, but I’ve started experimenting with it and I may submit another article with more details in the future. (Remember that ‘never-ending project’ my wife complains about?)

Wired vs. Wireless Remote Sensors

When planning and selecting electronic remote sensors that trigger an alert back at a central console, you’ll need to consider how to get the signal from the sensor location to the console. There are two approaches you can take – wired or wireless. Wired tends to be more reliable, especially when dealing with long distances, but it does require that you purchase, run and connect the wiring, which typically means digging trenches and burying it for outside sensors or running it through walls for interior sensors. Longer wires can also impact the behavior of the system, since the electrical resistance will increase as the wire length increases and can vary with temperature, and longer wires can also act like antennas and potentially pick up interference from other electrical sources. Wired sensors tend to be simpler to design and implement, since the sensor can act like a simple switch that’s either ‘on’ or ‘off’ – you don’t need to worry about powering a remote sensor.

Using wireless sensors eliminates the need to run wires and allows you easily re-position sensors, but can introduce additional complexity into the system. Many manufacturers’ wireless alarm devices have their own unique receiving console or base station and communications protocol, and you may not be able to integrate them into a single consolidated console. Wireless signals can also be subject to interference from other electrical and radio sources, and severe weather can reduce the effective range. If you do want to integrate a remote wireless switch-type sensor into a central console, one approach would be to use a wireless remote relay to receive the signal and activate a light/buzzer on your console. You’ll need to open up the transmitter that comes with the device and solder some wires across the switch connections. Here’s how you could set it up:

The circuit board on the left is from the inside of the transmitter remote for the wireless relay that I linked to above. I soldered the two green wires to the ‘circuit closed’ terminals for the activation button, which then run to the screws in the clothespin. The piece of plastic between the screws keeps the circuit open and prevents the wireless signal from being sent. If the string on the piece of plastic is pulled out (like a tripwire being tripped), the circuit is closed and a signal is sent to the remote relay on the right. The two blue wires coming out of the relay on the receiver are connected to the common (COM) and Normally Open (NO) terminals on the relay and are used to activate a light/buzzer on a console (the red and black wires are 12V power). You’ll probably want to put this into some kind of camouflaged waterproof container before deploying it outside.

I’ve tested this unit around my property out to about 100’ in the woods and through several stick frame walls and it seems to do the job pretty well in most weather conditions. Note that once the alarm is tripped the remote will continue to send a signal until you replace the plastic between the screws in the clothespin, so you may run your transmitter battery down if you don’t get out to it for a while.

Keep in mind that wireless sensors require power to transmit, so you’ll need to stock and rotate batteries on a regular basis. The transmitter in the example I linked to above uses 12V A23 batteries, which as far as I know aren’t available in a rechargeable format, but you could easily rig some sort of other 12V battery to power it (like an 8 AA battery holder, or you could solar charge it). Be aware that batteries used in outdoor sensors will be subject to potentially significant temperature swings, which may reduce their useful life.

Tripwires

Tripwires are the Swiss army knife of sensors – they can be used a lot of different ways and work with a lot of different alerts and alarms. The most common type of tripwire that people think about is a wire stretched between a couple of trees across a trail – someone walking down the trail pulls the wire, causing some kind of alarm to go off. However, tripwires can also be used in other ways – you can attach them to doors so the alarm goes off if the door’s opened, you could stretch a really long one across an open field, and you could even use them underwater along a shoreline to detect someone trying to get ashore (make sure the alarm part is either 100% waterproof or located out of the water, though). When planning and implementing a tripwire sensor there are two aspects you need to consider – the sensing wire itself and the type of alarm or alert it’s connected to.

The type and positioning of the sensing wire is critical to the sensor working. Walk around the area you’re trying to cover and think about where and how a person might step or move through it and what the surrounding environment looks like (color, texture, plant life, etc.). For horizontal tripwires you don’t need to limit yourself to a single segment – you can use small screw eyes to run a multi-segment tripwire between multiple trees or posts that connects back to a single alarm. Screw eyes will work more smoothly than passing the wire directly over wood or tree bark, which may cause it to snag and prevent it from being pulled. You should also consider if you want the person tripping the wire to know they’ve been detected – tying a strong line across a path to an alarm solidly nailed to a tree will probably cause someone to trip and realize they’ve been detected, even if they can’t hear or see an alarm going off. You can use weaker line that will snap after a certain point, or connect one end using a bent piece of wire that will unbend and release the wire when enough force is applied. Here are some additional thoughts and considerations regarding setting up tripwire lines:

• For detecting humans, the ideal height of a tripwire is 6”-12” inches off of the ground.
• However, a tripwire 6” above the ground probably won’t work to well if it’s covered with 2’ of snow, so may you may need to adjust your tripwires as conditions change.
• Avoid running the line perfectly level – slanting the line can allow water drops and dew to run off of it, reducing its visibility.
• I’ve had tripwires set up in the woods around my house for many months, and even with deer, skunks, groundhogs, turkeys and tons of other wildlife in the area I’ve never had a false alarm. Animals seem to be able to avoid it (maybe by smell?). Of course, your mileage may vary.
• While plain old clear monofilament fishing line might seem like a good choice for a tripwire, it’s can actually be pretty easy to see under some lighting and background conditions. Depending on your location and conditions, you can get green or brown monofilament that might blend in better, but after years of testing I found that different shades of braided fishing line tend to work best in my area, since it’s less glossy. The downside of braided line is that water drops don’t run off it as easily. The OD green Vietnam War era tripwire also works well, but it’s harder to work with and more expensive per foot. You can also smear bits of dirt or plants on a line to break up its appearance.
• Attach the tripwire to the alarm using some kind of detachable loop connected to the alarm (I use fishing swivels). That way if you have to shorten the tripwire line when moving it to another location that requires a longer tripwire you don’t have to re-string the whole alarm. It also simplifies disconnecting and re-connecting the tripwire if you’re going to be working in the area.
• Check your tripwires regularly to make sure leaves or other debris haven’t gotten stuck on it or piled up around it, making it more obvious. A small branch with one end seemingly magically suspended in the air in the middle of a path can be a dead giveaway.
• Try to run the tripwire where it’s less visible – through some grass or weeds, on the other side of a branch that’s fallen across the trail, near a spider web, etc.
• Unlike the movies, tripwires don’t have to be tightly stretched across a trail to be effective. They’ll still work with little slack in the line and a slight bow may make it look more natural. The human brain tends to interpret perfectly straight lines as something not typically seen in nature.
• You aren’t limited to using trees – you can install stakes or posts pretty much anywhere to connect your tripwires. Of course you’ll want to make sure they aren’t obvious.
• Avoid marking the locations of your tripwires. If you use a trail or path regularly and always step over the tripwire, there will be a stretch of undisturbed material under the tripwire, making its location more obvious.
• You can use tripwires indoors as well as outside. For example, you can run a tripwire below windows to alert you if someone comes through them.

Having a tripwire doesn’t really accomplish anything (except maybe tripping someone) unless it’s connected to some type of alert or alarm. The good news is that there are a lot of options for alarms that you can connect to tripwires. As discussed earlier, there are two types of alerting – local and remote. Local alerts are those that go off near the tripwire itself. Some possible options for local alerts include:

• • A simple bear bell or a can with rocks in it that makes noise when the tripwire is moved.
  • A personal alarm that generates a super-loud alarm – just attach the tripwire to the pull pin and attach the alarm body to something solid.
  • A device that fires a blank cartridge when tripped. There are a couple of pre-made ones you can buy (.22 caliber and 12 gauge), or you can make your own out of surplus grenade fuses. Note that if you use one that takes 12 gauge shells you have a lot of other options beside blanks – flares, flash bangs, etc.
  • Mouse and rat traps offer a lot of different options for setting off alarms, and you can even fire shotshell primers or .22 caliber blanks.
  • An aerial flare fired from flare gun.
  • A smoke grenade.
  • For things that are ignited, you aren’t limited to mechanical firing pins – you can use a simple clothespin switch connected to a battery and an electric match to ignite almost anything (FYI – electric matches are almost the same diameter as a 209 shotshell primer, so you can use them in place of a primer to set off reloaded shotgun blanks).
  • A trip-acivated Cyalume light stick. [JWR Adds: There are also infrared chemlight sticks available.]
  • A clothespin switch connected to a battery to turn on a flashing light (you’ll need to open up the light and solder some wires in place of the on/off switch).
  • You can activate an air horn by partially burying it upright (assuming the button is on top) and leaning a heavy rock or log over it that’s supported by a stick connected to the tripwire. When the stick is pulled out the weight falls on the button, activating the horn.

One (hopefully) obvious warning is that if your alarm generates heat or fire, be very careful that you’re not going to start a brush fire when it goes off. You should also be careful if you’re going to use aerial flares – you need to ensure that they have an unobstructed path upwards and won’t hit any branches or other obstructions after they fire. You also need to ensure that someone tripping the wire with a lot of force doesn’t pull the flare over, causing it to shoot out sideways – use a piece of stiff wire to connect one end that will unbend after the flare’s been tripped but before it can be pulled askew.

(To be continued tomorrow, in Part 4.)