What gate would you use to activate an alarm when the security is breached?
david a asked:
You are to design an alarm system for a house. The sensors are magnetic switches that produce a high when open and a low output when closed
Create a video blog…instantly.
You are to design an alarm system for a house. The sensors are magnetic switches that produce a high when open and a low output when closed
Create a video blog…instantly.
May 10th, 2009 at 8:52 am
security system sensor
There are hundreds of circuits in magazines, in books and on the net. Chose one that suits you.
Use reed switches for the perimeter and PIRs for the inside. Whole books have been written on this subject, and some are quite cheap. I suggest you buy one.
(I trust you will be usng an end-of-line resistor to making disabling the unit 5 minutes more difficult).
I have worked on and off with electronic security systems since the early 1970′s (using transistors back then). Some were very good and some could be disabled in a few minutes (using some VERY creative ideas – only a half would be electronic in nature). What I have picked up over the years would literally fill a book (in fact I could probably write a book on tamper circuits alone), without touching dozens of system types and wiring methods. And if you think you know what high electronic security is, you wanna see what some banks, and esp the military use. In my opinion it is way over the top, but it is pretty close to unbeatable (close – nothing is totally secure IMNSHO).
PIRs are good but can give false triggering (hence the 3 count option on most). Another unit, about $80 uses a PIR and radar unit built into one. If it senses movement and the PIR senses the IR and movement, it trips. It is about $80 but a superb idea. Whil not having used one, I would opt for that in any future installation I may do.
Also, there are professional outlets for this gear, don’t assume Jaycar, Radio Shack or Dick Smith sell the best stuff. Its nowhere near the really good stuff (although Jaycar sell a reputable system for a fair price).
Start with an introductory book and see all the ways you can do things, and take it from there. Start simple, then let it grow. Don’t start out with 32 bit interrogation units which poll everything on a single 3 core cable. It’s wonderful, but unless your surname is Tiffany, don’t go there.
(Then use 6 core cable with 3 tamper wires in it – that can be discouraging for a crook, especially if the tamper sensor wires have signals on them – there are a couple of really nasty things you can do to beat burglars with such a system).
Also, don’t forget wireless. I know little about it and whether it has any fundamental flaws, but laying cable is NOT fun. If it were me I would seek out the benefits of wireless (and the pitfalls).
There is so much to say about this – get a book – there are some very good ones but most assume a reasonable knowledge of electronics. Start basic!
Remember, most burglars aren’t carrying scopes, logic analysers, spectrum analysers, function generators… They are 20 year olds on crack or meth, and for them th height of “sophistocated” is using a tree branch to break a window (yes, there are several types of good glass beak sensors – most don’t need mounting on the window so can cover a shop frontage.
Read, read, read, read, read, and follow simple circuits until you understand them. Then seek the weakneses – pretend you are the burglar – how would you bpass the wires carrying the signals to the master unit or sensor? When you work that out, go the next step …
Most systems don’t get disabled, except where really large amounts of value are concerned.
May 11th, 2009 at 6:44 am
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Using positive logic or negative logic? Just messing with you.
You need a change in output anytime ONE of the inputs changes. So when security is breached, one of the swtiches opens and produces a HI output. So an OR gate output goes HI when any input goes HI. Unlike an AND gate which requires ALL inputs to go high before the output goes HI.