Defective said:
1. Sure...why not? One of those cheap solar battery maintainers mounted on the roof.
2. I'd do it. Just add a relay into the circuit that powers the noise maker. Then run power to the brakes. Should slow them down pretty good.
Actually if you have electric brakes you can use the brake battery to run the alarm system with a solar battery maintainer to assist in keeping it charged. You need to wire the NC contacts that power the alarm's audio alert across the breakaway switch so the brakes are applied when the alarm is tripped.
Whether you use the breakaway brake battery or a separate battery you need to "size" the solar charger to keep the battery up while it is constantly being drained by the relay coil current. To reduce the solar panel requirement you want to select a relay that a) can handle the current through its contacts to apply the brakes AND b) run the noise maker but have a low coil current at 12 VDC.
Here is a little help working it out:
1. multiply the coil current (probably rated in milliamps) times 24 hours to get milliamp hours yo will use per day.
2. You will need to put in about twice the amp hours into the battery as you want to take out (they aren't the best energy storage devices.)
3. A solar panel aimed south and tilted about 15 degrees lower toward the horizon than your latitude is a good year round average aiming. For a mobile installation where you may not want to keep fussing with the panel, just put it flat on top of the trailer and it will be fairly good but you'll need a larger panel.
4. Panels are often rated in Watts. To get amps at 12 volts, divide Watts by 12. This is how much the panel will put out in full sun, aimed properly, and not overheated. A fair rule of thumb is to assume your total daily output is about equivalent to 5 hours of bright sun. In a typical day with the panel aimed south 15 degrees below your latitude the total power collected is about the same as 5 hours of direct sun. The approximate range of latitude in the US (lower 48) is about 30 to 45 degrees. In winter the US gets less sun than summer so aiming the panel a little lower toward the horizon optimizes solar gain for the winter when you would otherwise get less.
5. OK, so 5 hours of sun has to provide at least twice the power used in 24 to have a decent chance of breaking even. (Remember the battery gives back about half of what you put in.)
First you need to know the relay coil current. Oh, by the way, so called 12 volt lead acid batteries are more typically used at 12.6 to 14.2 volts (12 volts is just a handy reference name) after you know how many amp hours (or milliamp hours) your rrelay uses in 24 hours you need the solar panel to make at least twice that in 5 hours.
For an illustrative example: If your relay pulled 100 milliamps that would be 2400 mAh or 2.4 amp hours. You need twice that (4.8 AH) in 5 hours so we divide 4.8 by 5 and get approximately 1 amp. We have to double that because of the way we get short changed by the battery. So the solar panel needs to supply 2 amps in brings sun to break even with your current draw averaged out over a 24 hour period. 2 amps is about a 25 to 30 Watt panel. Panels may run about $10 a Watt depending on your ability to ferret out a bargain.
You need to decide if you want to spend over $200 to avoid an extension cord to power a $7 Harbor Freight battery maintainer. The Radio Shack relay I used has a coil current of 130 milliamps or 0.13 amps which would require 30% more solar power than the example I gave above.
So you can see that finding a relay with very low coil current is a GOOD IDEA to save money on the rest of the design. For a solar powered system (the design I gave for a simple alarm was not optimized for solar power) you want to find a relay with very low coil current and use it to energize the heavier relay with the high current contacts.
Frequently the relays with really low coil current do not have high current contacts so you want to use two relays in the solar powered version of the alarm system. The coil of the low current relay is wired into the circuit in place of the original higher powered relay. the normally open (closed when energized) contacts of the low powered relay are used to apply power to the high powered relay. The contacts of the high powered relay switch the siren and brakes.
Digikey.com has several relays suitable for this application. You can find them with coil currents under 10 milliamp which would cut the solar panel requirement of the original relay I used (130 ma) to less than 10 ma with a corresponding reduction in solar power requirement to only 10% of the example I gave (100 ma coil.) Then a 2.5 to 3 watt panel would do the job.
There is some self discharge in batteries. They just lose energy over time. At some point if you reduced the power requirement of the alarm circuit low enough the self discharge of the battery, which is not being reduced, would become the major concern in the charging circuit.
Another issue: If you over size the solar panel too much then you will need a charge controller to avoid overcharging the battery which could shorten its life considerably.
After I had a trailer stolen I thought up lots of clever anti-theft devices (nail the barn door shut after the horse is gone!) Why not a large capsule of dayglow red paint in a pipe (think zip gun) with an explosive charge to propel it. The pipe is mounted facing forward so it would definitely hit the rear of the thief's vehicle in the lisc plate area and higher. If the bad guy(s) was (were) moving the trailer by hand it might paint whoever was near the coupler.
I thought of a time delay circuit powered by the brake controller batt (could use a separate batt) to set off all sort of pyrotechnics which should make the bad guys more than a little self conscious and apprehensive. There is no limit to the number or degree of aggressiveness of these sort of things. To be legal you need to stay non-lethal. One of my ideas was to use a magnet on the rim of a trailer wheel and a sense coil to compute speed. When the trailer exceeded 5-6 MPH (clearly the bad guys would be in the vehicle not outside exposed personally to my device) the equivalent of a couple electrically fired shotguns would be fired so as to take out the rear tires of the tow vehicle. This along with fluorescent paint fired onto the tow vehicle might make an impression on a witness.
Pat
