Our last visit to the topic of perimeter security took a look at a really nifty tripwire system I found on Ebay. It busts a .22 caliber nailgun blank, and you can hear it for miles on a quiet night in the country. It is extremely low-tech and electricity free, and it works. For this article I am following up with a couple of wireless options, one expensive and one pretty cheap, and both also work really well. Nobody wants to rely on batteries, but any tripwire system is going to have several basic drawbacks:
- The intruder knows he has been detected, and he also knows that someone is trying to detect him. – It is pretty easy to just back out and return with reinforcements, because for sure he just found out that there is something to steal here.
- The trip wire is visible. – My initial tests showed that monofilament fishing line as a trip line was just not the right choice, because it reflects light. One of the commenters on the last article noted that military trip wire is green wire, and that is for sure better, but it still can be seen.
- Once one device has been popped, you have no further information about which way the intruder is traveling. – The dude isn’t going to trip another one now that he knows what to look for.
- Murphy’s Law – If something can happen to your trip line, it will happen. The cap could get wet, which is the worse case scenario because the intruder now knows about you but you don’t know about him.
Detecting an intruder without a trip line can be done one of two ways. The first is obvious. Any home security system these days employs motion detectors, and most of have at least one motion activated light outside. Search Amazon or Ebay for “wireless home security” and you’ll find systems priced at under $100 that will cover your whole house, 100% battery operated. The other method is via a “geophone,” using seismic detection. This is by far the most useful method, because the sensors are easy to hide, and a geophone can detect movement on both sides of a barrier, without having to put a sensor outside the barrier. Compete geophone systems are also rare and expensive.
It is very difficult for me to recommend something that isn’t available and it is with trepidation that I show you the military seismic intrusion detector called the TRC-3 or TRC-3A. Right now there is one guy on Ebay who has the whole set and if you look back through the old ads for it, he thought he was going to get $1,100 for it and now he is down to $800 and accepting offers. But one recently popped on Ebay for $246, and one went in October for the same price. They are around, and if you know what to look for your local Army surplus store may have one sitting there. Gun shows are also a good source for things like these. Mine was bought about ten years ago for $150, on Ebay, and I also bought a similar military unit that was wired, but that was lost in a move.
The TRC-3 uses sealed seismic probes that are each connected to a base radio unit. When it detects a seismic event, it broadcasts a tone. Each transmitter in the 4 unit set has it’s own tone, so you can use the 4 units in overlapping paths to detect direction of travel. On the most sensitive setting the TRC-3 can detect a man walking up to about 50 yards away from the probe, so you can cover some real estate with the four probe units.
Mine didn’t come with the military receiver, and you will find that most out there don’t. Mine came with a little multi-band radio that includes the “air band,” which is used by aircraft to speak to the tower. Most “survival radios” these days don’t include air band, so to replace mine I had to buy one of the radios specifically made for air band, and it is hard to find one under $80. The frequency transmit range of the TRC-3 is 129-136mhz. Mine set broadcasts on 134.5mhz.
The cheaper option, if your set doesn’t come with a receiver, is to buy a Baofeng UV-5R handheld dual band Ham radio for about $35, and get the programming cable for it. Download Chirp software and then you can manually put the receive frequency of the radio below the 136mhz that comes from the factory.
The battery life of the dual 9 volts in the TRC-3 is up to 900 hours, with regular batteries. Modern lithium rechargeables will most likely last twice that. This is plenty of time to rotate two sets through a solar battery charger, which makes this system really realistic. The other thing I find really attractive is that there is no special receiver. You can leave a radio on at home base, and keep one in your pocket on guard duty connected to an ear phone. Any radio tuned to 134.5mhz within about 200 yards picks up the pulses on mine. The manual says it has up to a mile range, but that is probably with nothing in between the transmitter and receiver.
The only downside I see to this system (besides cost) is that in an urban setting of any kind it is completely useless. Traffic within a mile will trip the transmit tone, and my neighbor’s well pump tripped it on the lowest sensitivity more than 100 yards away. The manual says that even low flying planes can trip the tone.
If you decide to buy one a TRC-3, make sure you test it first. They are officially waterproof down to 3 meters, but mine has some cracks in the plastic case that are just from age. The circuit itself is fairly simple though, and almost every one that I have seen for sale has included a guarantee that the system works. Also note that the two 9v batteries are wired in parallel, so the transmitters will run on one 9v, but they don’t transmit as far.
Wireless Motion Detection
I chose the Chamberlain Wireless Motion Alert you see here in the pictures for a number of reasons:
- It’s cheap cheap. – The base unit and one sensor are available on Ebay and Amazon for $50 – $80. Additional sensors are $30 each, up to 7 per base unit.
- This is a very specific system. – There are no zones or window sensors. You can add up to 7 motion detectors to the system, and that’s it.
- The base unit is dual powered and small. – I can run the base unit from the wall wart or put 4 AA batteries into it and carry it in my pocket.
- The base unit can run silent. – There is no earphone jack, but you can turn off the audible tone and just use the LED light.
- No warning to the intruder. – The default on the Chamberlain sensors is a green LED that points forward, toward the threat, but the units themselves are just circuit boards screwed into a plastic inclosure. It is easy to cover the front facing LED with tape or liquid electrical tape.
- It is indoor/outdoor. – Each sensor comes with a hood that clips over it, protecting it from rain and snow.
You also can technically get more than one base unit to use with your sensors. The base unit registers the sensor by pushing a button on each at the same time, so more than one base can be activated by the same sensor. That gives you the same functionality as the TRC-3, but you have to buy the proprietary base units.
If I have a complaint about this type of device is that they are somewhat conspicuous. You have to point them at an open space, and that means you can’t hide them behind something, or in a thicket or something. The best I have been able to do is bury the sensor in a bush, then cut a detection window in front of it. If you aren’t looking for it I suppose it is not obvious, but in a survival situation with active hostiles I wouldn’t bet on them not seeing the sensor before it detects them. If you have more than one sensor, at least you can cross paths with them and get a reading before the sensor is detected. They also don’t have the different tones per sensor luxury feature of the TRC-3. When the alarm sounds there is no way to tell which sensor is being activated.
The Chamberlain instructions claim that the 4 AA batteries in each sensor will last up to two years. The base unit batteries will last about a week. Two sets in a solar charger will do just fine to keep your receiver alive. The sensors are very sensitive, and you get some false positives if there are any close branches that sway in the wind. The receiver works up to a mile away from the sensors, so these are perfect to watch your camp while you are out hunting or scouting. For the money, I don’t think you can beat this very simple system.