I will shortly be starting to install the old fashioned foil strip alarm for some porch windows, it will work off a 9 volt battery. So basically the foil stays live until the circuit is broken. (broken window = broken circuit) What type of `switch` do I require? that will trigger an alarm if the circuit is broken.
What sort of alarm is it going to be connected to?
You could use the normally closed contacts of a relay, where the coil is driven through the foil to hold the relay open, but it won't last long being powered by a 9V battery, the alarm will be constantly going off due to flat battery rather than broken windows ...
Not sure probably some sort of electronic buzzer.
Possibly a solid state relay?
Nobody will take much notice of a buzzer. You need something like a sound bomb, which makes it physically uncomfortable to remain in the building:
It will need 12V though.
You can use a relay, but that will run down the battery very quickly.
You need a circuit to change the normally-closed circuit of the foil loop to a normally-open switch to operate a buzzer. Eg
Owain
I would go for a very basic microcontroller and a bistable "latching" relay like the ones used in modern central heating thermostats.
The micro could check the foil continuity. If it sees it "closed" then it just sleeps and sets itself to awaken some seconds later for another look. If it sees it transition to "open" it then pulses the relay to switch the alarm. You could get probably get the power consumption down to using a few uA for a just a few microseconds every few secs.
The simplest is a darlington pair of transistors plus resistor from base to +ve. The foil shorts base to emitter, load goes from +ve to collector.
NT
Normally a relay of the latching kind was used in the el cheapo systems. The coil was energised on low current via the strip, all in series, and the relay contacts were changeover but only the unenergised side was connecting the circuit to the alarm. This way you would not need to make it latching of course. However the foil I used to find was quite high resistance and in the end if a lot of these windows were in a system one needed to really have some l kind of w semiconductor in the circuit with the foil acting as half the bias on the base so it either conducted or stopped conducting when the fi oil broke.
Are there not still available boxes that did all this for you?
Brian
Yes that is right, but maybe a battery backed mains system, so its trickle charged when running and the battery runs it when the power is of, at least for a while. If you use my transistor idea, the current can be very low, but the other end where the relay is, you would need something beefy to run the alarm itself. In the case I described the transistor is biased off by the foil from base to ground. The relay is in the emitter circuit, and comes on when the base goes high and the relay is energised. The battery should last for ages on the sensor end and it will of course not last that lond once the relay is switched on, but hopefully the nasty person will be long gone by then. Brian
Why? Do your porch windows regularly get broken?
Maplin probably sell garage alarms that could be subverted to do this sort of thing with a NC circuit that sounds an alarm when broken. It has to run on very low current in standby to be any use.
But the foil tape itself as a bluff might well be good enough (though unsightly). It works on my parent's garage windows.
Foil is a bit 1970s.
Try an acoustic break detector:
or toughened glass. Of ir its for security with no concern whatever for looks, laminated glass.
NT
Also fairly visible, which may be the point. I have used the foil without connecting it to anything just for the deterrent effect.
It is very 1970's but then the garage is 1950's brick built with a pit.
It may still work as a bluff at least wrt to casual thieves.
Or wired glass - that is what we have in the VH doors.
that has no security value.
NT
but it might collect blood from any burglar, so theer's be some DNA on record
/> Or wired glass - that is what we have in the VH doors.
that has no security value. /q
Neither does your favoured toughened glass.
Jim K
Thanks for all the replies guys, it seems I have underestimated what I thought was quite a simple task, I may ditch this idea and just go for some cheap thingies that go off when they detect vibration.
For info the area is not prone to break ins but the porch is in a darkened area of the garden and IF a break in were attempted I suspect this would be the place, dark and concealed. I have already (almost) got my led flood up and running in this area thanks to the guys on here for help with the wiring.
Why not just fit iron bars
NT
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