Letter Re: Photocell and Seismic Intrusion Detection Systems

Could you tell us more about a seismic intrusion detection system? Until your recent comments on this being necessary for the security of a hidden retreat, I had never even heard of such a thing. There must be more novices like me who are soaking up like a sponge everything you write, and would be very interested in knowing more. Thank you, – Joe.

JWR Replies:  I cannot over-emphasize the need for a proper intrusion detection system for a retreat. The simplest are the photocell “driveway alarms” which are commonly used on farms and ranches in the west. Most folks buy them just to have advance waning on when the UPS truck is approaching with a delivery. But they would also have some utility in a slow-slide scenario.  If looters are stupid enough to come right up your driveway in the middle of the night, such a system will tip you off and give you enough warning to man a defense. Unfortunately most of these are dependent on 117 VAC power. You can often find these on eBay. Just be sure to get sturdy “Commercial” style system if you want it to last. (The $20 cheapo made in China systems are not designed to last.) You can expect to pay $50 to $150 for one of the good reliable ones.

Far more sophisticated systems have been used my the U.S. military since the 1960s. These used buried seismic probes to detect approaching vehicles, the footfalls of approaching troops, and even the vibrations from low flying helicopters. These are battery operated, and designed for tactical field use in all weather. The early type are hard-wired (typically with commo wire.)  The later ones are wireless, but require more batteries, since a small radio transmitter is mated to each seismic sensor. Once you get used to using one of these, you can learn to easily differentiate between the footfalls of a man and a deer. I’m not kidding.

The “old reliable” is the hard-wired AN/PSR-1A.  It was still used by USMC active duty units up until a few years ago. In fact, a few might still be lingering around USMC reserve units.  They use six D-cell batteries, or can easily be adapted to any other 9 VDC source. They use 1950s technology (EMP proof) and are a bit heavy for man-pack use. The 1950s-style headphone supplied with these are a joke, but very simple to replace with a modern pair of headphones. Just make sure that the new ones have correctly matching impedance. Otherwise, I have no complaints about these units. They work fine for a fixed-site retreat. OBTW, SurvivalBlog readers Kitiara and John at the Forevervain Blog mentioned that they recently obtained one of these sets through eBay.  Good choice!

OBTW, if you are an electronics wizard, Al Glanze at STANO Components ( http://www.stano.night-vision.com/html/specials.html ) has several hundred spare PSR-1A seismic probes available. They are very rugged.  If you were to mate some of these with a modern chassis (the PSR-1A circuit diagram is pretty simple) with a DSP chip that could trigger an audible alarm, you could build yourself a fantastic retreat security system.

One of the best recent-production U.S. military systems is the AN/TRC-3A Wireless Seismic Intrusion Detection Set. This model will work well for both fixed site retreats and mobile (patrolling bivouac site) use. These are often in stock with a number of vendors including Ready Made Resources (one of our advertisers) and Fair Radio Sales. Both of these companies are very reputable. They can also be found on eBay, buy beware that eBay sellers are notorious for selling nonfunctional used electronic gear.

A seismic intrusion detection set will be a tremendous labor saver in the the event of TEOTWAWKI-type collapse. While they are not a proper substitute for a 24/7-manned LP/OP, having one of these sets could mean the difference between life and death if you are operating a survival retreat that is short-handed.  When prioritizing your purchases, a good quality (full mil spec) seismic intrusion detection set should be near the top of your list. Don’t skimp on this expense, or you will surely regret it later!