Electronic the guard, in my opinion, will be useful for tracking intrusions into a small section of a private territory. At the same time, it is necessary to have a security guard for whom such a scheme will be useful. If someone or something crawls into the territory / room guarded by such a thing and at the same time breaks the thinnest wire, then we will find out about this and it will not necessarily be loud so as not to frighten the attacker.
Sensor
A thin copper wire is easily located on the primary winding of any low-power transformer. It is so thin that it’s even smaller than a human hair and it’s nowhere to break it. It is also possible to use a fishing line / thread, then you need to do something like this in the picture so that the contact breaks when pulling. This can be more effective in terms of making it easier to pull a thicker thread without tearing it. Almost any can be long, which is very pleasing in terms of covering large areas.
Sometimes the wire will be even better than the laser sensor - this is true on the streets in all kinds of bushes, grasses, in general thickets, where the beam will be fawn and there are a lot of false positives from plants swaying from the wind.
Work scheme
The circuit is an electronic key; it is triggered when a security loop is broken. If the sensor wire is intact, then a minus passes through it from the power source to the base of the NPN transistor T3 (kt315) and it is in a closed state, since it needs a plus to open, and a negative voltage does not allow it to open. If the loop breaks, then the negative voltage no longer goes to the T3 base, and the positive voltage that goes through the high-resistance resistor R1 with a value of 2 MΩ opens the transistor T3 and the current begins to flow through its FE, which saturates the next semiconductor T2 and also through its FE current goes and opens the last transistor of medium power kt815 - now also through its conclusions the collector-emitter current flows from the battery plus to the audible alarm and it starts to work, because the minus is already connected to it. I used the KT940A transistor instead of KT915, it has the same pinout, we put it on the radiator.
Signal
If we place a loud warning siren, then the “criminal”, who has penetrated into private territory, will go nuts immediately, will get scared and will run out of sight or not)) ...
There is such a thought: to take not an alarm, but, for example, fireworks, firecrackers or something else from pyrotechnics. Imagine just what the effect will be from a powerful explosion or bright lights in the sky.I recommend taking the Polish P2000 mega-pirates, with small sizes they have very incredible power and a loud pop. The wick igniter is made of thin nichrome / tungsten wire. Do not forget that the chemical elements and the wick itself can become damp - this must be taken into account.
I think that it’s effective to put such a thing without a guard in a country house where you do not often visit, but do not want to see unwanted dirty tricks.
You can also take the LED indicator, then the indication will be silent, there are also disadvantages.
Board file
Nutrition
The recommended voltage for power supply is 9-12 V, but it works adequately with lower values. Since the security board and batteries / accumulators will most likely be on the street, care must be taken to insulate the chemical voltage source, especially Lithium-Ion / Polymer (Li-Ion / Po) batteries are especially frightened.
The big advantage of this system is the very small standby current, which is only 3-4 μA, it is so little that a normal power source will work for a very long period.
Ready device (photo, video)
