Unless you’re new to survivaldom then you already know about the “Dakota Alert” MURS products and their uses for home and retreat security, both before and after the SHTF. This article focuses specifically on one of the Dakota Alert products, the “MURS Alert Transmitter”, and how it can be modified to expand its functionality and usefulness for both security and as a force multiplier.
For the benefit of anyone that may be hearing about the Dakota Alert MURS Alert Transmitter for the first time, here’s the short description from the Owner’s Manual (http://www.dakotaalert.com/manuals/Murs-Alert.pdf):
“The MURS Alert transmitter is a passive infrared (PIR) detector that can be used to detect people, vehicles or large animals at selected locations. The PIR sensor will detect movement of objects up to 80 feet in front of the transmitter box. When a person, vehicle or large animal is detected, the MURS Alert will transmit a signal.”
A slightly more detailed description is that the MURS Alert Transmitter (hereafter just “MAT”) is a battery operated motion sensor and Multi-Use Radio Service (MURS) transmitter combined in a weather resistant enclosure with an external BNC connector for attaching the included telescoping antenna. The two primary functional components, the motion sensor and the transmitter, are each on their own circuit board within the enclosure. When motion is detected by the PIR sensor on the motion sensor circuit board, power is supplied to the transmitter board long enough for it to send one of four pre-programmed messages (“Alert Zone 1” through “Alert Zone 4”), on one of the five MURS frequencies, using one of 38 CTCSS tones – all of which are user selectable.
Here’s a picture of the standard MAT without modifications and showing the major assemblies.
The Dakota Alert system is readily expandable, you can start with a single MAT and any VHF receiver capable of tuning to the MURS frequencies, and grow the system by adding additional MATs and radios as needed. There’s no limit on the number of MATs that can be in use in the same system and at the same time. They are highly reliable units, and make excellent perimeter monitors and intrusion detectors indoors or out. We use one as a driveway monitor, another at our gate, two inside outbuildings, and two more at locations on our perimeter. We’ve also used them for detecting approaching game and monitoring baited areas while deer hunting. For use in a SHTF situation they can be easily integrated into a combined communication / security plan, allowing your group to monitor activity or intrusions into your area, and then coordinate your response using the same radio equipment if desired.
This post isn’t intended to be a complete overview of the MATs or how to deploy them, and is probably starting to sound like an advertisement for Dakota Alert (I have no stake in the company or any company that sells their products), but for all their benefits they do have their limitations, and some room for improvement. The first of which is…
The MATs are powered by six “AA” alkaline batteries, wired in series for 9v, which are installed in the battery holder that fits within the enclosure. Being able to replace the alkaline batteries with rechargeable NiMH batteries would be a nice feature, but isn’t really an option as the 1.2v from each NiMH battery only results in a total of 7.2V which is very close to the threshold for the “low battery” alert on the MAT. This alkaline battery requirement makes the MATs more expensive to operate, and less than ideal for use a long-term grid down situation where disposable alkaline batteries may be hard to replace. One detail that isn’t mentioned in the MAT Owners Manual however, learned in a discussion with one of the distributors, was that it could regulate and operate on a standard 12V battery voltage (12.8V) as well. Being unable to find any information to confirm this, I decided to take his word for it. I removed the “AA” battery holder and attached a 12V lawn/garden (Group 26) battery to the MAT, and it worked normally with 12.6V showing at the MAT. Deciding to push my luck, I attached a charger to the battery and watched as the voltage climbed to 14.8V (bulk charging voltage) with the MAT still functioning normally. Knowing that it will operate on voltages from about 7.2V (low battery) up to 14.8V (at least) gives us a number of other power options.
One option, and the one we now use in our entry gate monitor, was to replace the 6 battery holder included with the MAT with two 4 battery holders wired in series instead. This allows us to use 8 “AA” NiMH rechargeable batteries (9.6v total) to power the MAT and still fit inside the enclosure. This makes them much less expensive to operate in the long run, and is better suited to our grid down power plan. Battery holders of various shapes and sizes are inexpensive and available from any electronics supply dealer. Another advantage to using NiMH batteries is if you’re also using the Dakota Alert hand held transceivers, which use (six) “AA” NiMH batteries. This allows you to power all the components of a complete, easily transportable and quickly deployable, perimeter security system using the same rechargeable batteries – and it all fits in a shoebox.
Powering the MAT off a 12V battery can also be a convenient option in permanent/fixed installations where you have the space for an external battery and the means to keep it charged. We use automotive and lawn/garden batteries to power the MATs inside our remote outbuildings (as intrusion detectors), and added 5 watt solar panels to keep the batteries charged. The MATs draw so little power that the battery never discharges by any significant amount so deep-cycle batteries aren’t necessary. If you’re wondering about whether the higher voltage might cause overheating or any long term harm, I can only say that we’ve had two MATs running this way for 5 months so far with no problems. Another benefit, no more low battery alerts to wake us up at o’dark-thirty anymore.
Modifying the MAT to run off either the larger NiMH battery pack, an external 12V battery, a single 9v battery, or anything in between is simple. Just remove the snap-on plastic cover over the PIR sensor and locate the terminal strip with the two terminals marked INPUT (+) and INPUT (-). Loosen the screws and remove the wires from the original battery pack, replace them with those from your new power source, and re-tighten the screws. For safety, a 1 amp fuse on the positive wire is highly recommended if using a 12V battery. Also be sure to use wire rated for outdoor use where appropriate, 16 gauge landscape wire is a good choice and can be used indoors or out. Another advantage we’ve found when using 12V to power the MAT is that it seems to provide better reliability in high humidity conditions. While we rarely get any false alarms from our MATs, high humidity and condensation have caused them not to alarm when they should have on occasion, and the 12V power source seems to help in these wet conditions.
One last observation when using 12V to power the MAT is that transmitter power output appears to increase as well. My test equipment isn’t accurate enough to provide any numbers I trust, but it seemed worth noting.
Now you may be wondering why anyone would want to take a portable, lightweight, “AA” battery powered MAT and make undocumented, warranty-voiding modifications to use a heavy, bulky, 12V car battery instead? Well, one reason might be if…
You want to add a relay to your MAT
For any unfamiliar with relays, they are an electrically operated switch. One use for a relay is to allow a (typically small) control voltage to operate a (usually much larger amperage) device. There are many types of relays (http://en.wikipedia.org/wiki/Relay), but a common and particularly useful one for use with the MAT running off 12V power is a standard automotive Single Pole Double Throw (SPDT) relay (http://www.the12volt.com/relays/relays.asp). They are available at any automotive store for a few dollars each. They are a 1″ cube which easily fits within the MAT enclosure, with 5 blade type connectors labeled and configured as follows:
85 = relay coil (-)
86 = relay coil (+)
30 = common contact
87 = normally open contact
87a = normally closed contact
Connectors 85 and 86 are the coil which controls the switch position (open or closed) for the 87 and 87a contacts. The “Normally Open” and “Normally Closed” designations indicate the connectivity between the common contact (30) and the 87/ 87a contacts when the relay coil is NOT energized. So with no voltage running through the coil, contacts 30 and 87a are electrically connected (closed). If voltage is applied to the relay coil, then the 30 to 87a electrical connection is broken (opened), and 30 and 87 contacts are connected (closed) instead. The relay coil and the switch contacts are isolated from each other internally, so you can use 12VDC to power the coil while running 120VAC through the switch contacts for example (but check the specifications for your relay first of course).
Now as mentioned earlier, when motion is detected the motion sensor circuit board sends power to the MURS transmitter circuit board (via the TRANS +/- terminals) long enough for it to transmit the “Alert Zone 1” message three times. But for how we might use this with a relay it might be more helpful to just say: When motion is detected, TRANS (+) goes high, to almost the INPUT (+) voltage level, for 7 seconds before dropping back to zero volts.
So one way we might use a relay with the MAT is to have it supply power to another device when motion is detected. If we wire the TRANS (+) and TRANS (-) terminals on the MAT to connectors 86 and 85 on the relay, then when motion is detected the coil is energized and the 30 to 87 contacts close for 7 seconds. If over 7 seconds of power is required then a latching relay could be used (http://www.azettler.com/electromechanical-relay/az2501/). A typical automotive SPDT relay can handle 30 amps or more of current on the 30, 87, and 87a contacts, but for safety always ensure your relay is rated for 12VDC for the coil, and is designed to handle the voltage and current of the device(s) to be powered by the switch contacts.
A simple relay application, and one we use inside our remote outbuildings as an intrusion/burglar alarm, is wiring the coil on the relay to the TRANS (+) and (-) on the MAT as described above, and a car horn separately wired with it’s positive power wire from our 12V battery through connectors 30 and 87 on the relay. Everything is mounted high and out of reach, except for a hidden power switch between the battery and the MAT so we can disarm them when needed. When motion is detected the MAT transmits the “Alert Zone 1” message, while at the same time the 30 to 87 contacts close and the car horn sounds from the peak of the outbuilding for 7 seconds.
In case the wiring description wasn’t clear, to wire a car horn (or any other 12V device) to receive power when motion is detected:
1) Connect a wire from TRANS (-) on the MAT to relay contact 85, and another wire from TRANS (+) to relay contact 86.
2) Wire the 12V battery (+) terminal to relay contact 30, and another length of wire from relay contact 87 to the horn (+) terminal. Then wire the horn (-) directly back to the battery (-).
The wires used for the relay coil (85 and 86) can be small, 22 AWG is fine if the relay is mounted inside the MAT enclosure as very little current passes through them and the wire runs are short. The wire size needed for the 30/87/87a contacts will almost certainly need to be larger, adequate to handle both the amperage for the device(s) being powered and to avoid an unacceptable voltage drop over a possibly longer wire run. There is a chart here (http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm) with recommendations on what wire gauge is recommended based on the amperage of the device and the length of the wire run.
Other devices besides a car horn that might be useful to activate when motion is detected in a grid up/ROL world might include interior/exterior lighting, a DVR and video camera, or a phone dialer to call a pre-programmed number. In a grid down/WROL situation however you might consider other, more creative, ways to put these motion activated relays to use. Infrared illuminators, actuators to push a button or pull a lever, electric matches/squibs to ignite pyrotechnics, or a winch to raise and tighten tanglefoot, move heavy objects, or open/close gates or doors. You’re only limited by your imagination on what would work best as force multipliers for protecting your home or retreat. That said, these instructions and the hardware suggested are not immune to electrical, mechanical, or human failure – any device set as a trap to cause direct harm to an intruder is not only illegal, but just as likely to harm a member of your own group as it would a member of an opposing force. Nothing here should be construed as advocating mantraps that employ deadly force.
We’ve added relays and wired the coil to TRANS (+) and (-) on all our MAT’s, even those that don’t use them for anything, since it doesn’t interfere with their normal operation and makes it fast and easy to add a motion activated device later if desired.
Another limitation of the standard MAT is that the PIR sensor’s field of view is pretty tightly focused. Protecting the area around a home, or a campsite, would require many MATs, and could still leave sections unguarded. It would be easy to crawl under the view of one mounted 4′ high for example, and vegetation might limit it’s view if it was mounted at or near ground level. One option to guard a larger area using only a single MAT is..
Add a tripwire activated alert
If you need to monitor for intrusion from more than one direction or even 360 degrees around a small area using only a single MAT, then a simple solution is to use a tripwire to activate the MAT transmitter. The tripwire works in conjunction with the normal PIR motion sensor detection operation, so either PIR detected motion or “tripping” the tripwire activates the transmitter. To create a simple tripwire alarm, drill a whole in each side of a clothespin, run a screw through each hole, and add a couple nuts to hold the wires. Wire the positive lead from the battery to one side of the clothespin switch, and a second wire from the other side to the TRANS (+) terminal on the MAT. Attach the negative wire from the battery directly to TRANS (-) on the MAT. I used a second battery pack to power the transmitter for the tripwire alert, although it’s not required.
Just string out the tripwire in the area to be guarded, tie one end to a secure object, and attach the other end to a small non-conductive piece of plastic, then place the plastic between the screwheads on the clothespin. When the tripwire is pulled the piece of plastic is pulled out, the screw heads touch, and power is (continuously) supplied to the MURS transmitter board.
The next problem comes if you need to monitor a distant area or wander too far away, beyond the range of the MAT…
Replacing the standard antenna, or mounting it higher for greater range
Nothing complicated here, just buy or make a cable to attach to the female BNC connector on the MAT and the antenna of your choice (even the telescoping one) on the other end. The difference between the standard telescoping antenna with the tip at about 6′ high and a 5/8 wave MURS FireStik antenna at 17′ equates to about an extra mile of range in my neck of the woods. Antenna height is the most important factor to get increased range on VHF frequencies. We have a spare MAT we keep in our BOB with 8′ of RG-8 coax (plus a relay, clothespin switch, and fishing line) tucked inside the MAT enclosure, so we can mount the telescoping antenna higher at our backup BOL if needed.
Another modification you might consider is if you’re using over 4 MATs in a single system…
“Alert Zone 13”, or how to get more than 4 zones
While there’s no limit to the number of MATs you can have in use at one time, the four zone limit starts to become a problem once you add the fifth MAT to the system, and only gets worse with each one added after that. If you have a large perimeter to monitor, or just want to monitor more than four individual locations, once you’re forced to program two or more MATs to use the same zone number it’s difficult (to impossible) to determine for certain which of the MATs sharing that zone number actually detected motion. When our system grew beyond four MATs, we designated one zone number for our entire perimeter and the remaining three for the other locations within it. This worked fairly well, we knew when there was activity at our perimeter – but not at which specific location nor even the general direction. We could have used more zones for the perimeter MATs, but then wouldn’t have had enough zones left to use for our outbuildings and driveway/gate monitors. Using a different MURS channel or CTCSS tone and setting your receiver to monitor both is an option we tried, but isn’t much help unless you’re looking at the receiver (to ID which channel is active) while the alert is being received. The four zone limit is possibly the worst limitation in an otherwise great product.
What we determined is that it’s possible to modify the number of repetitions of the alert message each MAT transmits, and that’s one method of being able to uniquely identify more than four different MATs, which then effectively gives you more zones.
On the motion sensor circuit board, just to the right of the PIR sensor itself, is a dip switch marked SW2. SW2 has two slide switches marked “1” and “2” that control how long power is supplied to the transmitter board when motion is detected. In the factory default setting both slide switches are “ON” which provides 7 secs of power to transmitter board, long enough for the familiar “Alert Zone 1” message to transmit 3 times. By changing the 2 slide switch positions you can adjust the time to either 1, 3, 5, or 7 seconds, and that will determine how many times the message repeats.
Switch 1 Switch 2 Time Result
OFF OFF 1 sec “Alert Zone” once, no zone number
OFF ON 3 sec “Alert Zone 1” once
ON OFF 5 sec “Alert Zone 1” twice
ON ON 7 sec “Alert Zone 1” thrice
The zone number above, “1”, is just used as the example. The same holds true for any zone number you set the MAT for – so with 3 possible settings for each of the 4 zone numbers, plus the unnumbered 1 second “Alert Zone” message, this allows up to 13 unique zones to be designated in the same system using the same frequency/CTCSS settings. It’s far from a perfect solution, but with some careful attention to both the zone number and the number of times the message repeats, we can now determine where on the perimeter the activity is occurring and still have enough remaining zones to uniquely identify the other MATs in and around our outbuildings.
And lastly, a few suggestions on…
Tools, testing, and troubleshooting
A multimeter, a set of tweakers (micro screwdrivers), and a very small set of needlenose pliers (or a hemostat) is highly recommended for making modifications and troubleshooting problems with your MATs. Should a MAT fail beyond the warranty period, first confirm good connectivity/continuity on the wiring between the circuit boards and to the antenna with the power disconnected. Then reconnect power and check for 0 volts between the TRANS (+) and (-) when the motion sensor isn’t detecting motion, and for the INPUT voltage level there for 7 seconds after it should be detecting motion.
Some tests that might help isolate the failure:
If TRANS (+) and (-) isn’t showing the input voltage level when motion is detected, then remove the wires from the TRANS (+) and (-) terminals and set off the motion sensor again. If you’re still not seeing the input voltage level there
during the 7 second transmit period, then your motion sensor board is probably bad. You might then try supplying 9-12V directly to TRANS (+) and (-) while monitoring a receiver for the transmitted signal to confirm the transmitter board works.
If there is the input voltage level between TRANS (+) and (-) with motion detected, but ONLY when the TRANS (+) and (-) wires have been removed, your transmitter board is likely shorted/bad.
If TRANS (+) and (-) shows the input voltage level when motion is detected and with all wires connected, but the MAT isn’t transmitting as it should, then the transmitter board is probably bad.
Even if one board fails the other is probably still good, so it may still have some use. The motion activated relay doesn’t require the transmitter board to function, and the tripwire alarm would still work even if the motion sensor board was bad for example. Plus, you’ll have some spare parts in case another MAT fails.
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