My *nix world

Fake alarm system to trick burglars

I have an apartment in a building where all the tenants have their storage room  in the basement. I use it to store various old things there, just like anyone else. These days someone has cut one of those two locks that used to secure that door and which should keep out the "aliens" away from my goods. No harm done yet but....who know what can happen tomorrow? I want to keep the burglars away but I don't want to pay for a security system more than I payed for my odds and ends.

My plan is to build a small security system that includes a pyroelectric IR motion sensor, a wireless transmitter and receiver (smth. like this) because I want to be notified via email or something, a buzzer (that hopefully will scare the burglar), a set of CR 2032 batteries (and optionally an Arduino Uno board). It should not cost more than five bucks, though. It's pretty hard to achieve this target with only five bucks, isn't it? Yes and no.

Right now I'm involved in few other projects so I don't have enough time to start and finish this project in like one hour. So I have to find an alternative solution until then, right?. I thought that I can mentally fake the burglars out by putting a "home security system sign" or something alike. There is no guarantee they will not test out whether it is valid of not but the idea is to deter them so at least it worth trying it.

So I am going to make a fake alarm system to trick burglars. At the design time it looks like this one:

flasher-circuitbreadboard

For the time being I'm trying to mimic my car alarm so I flashing led that is exposed just enough to be seen should do the trick, right? Also, it's better than nothing, I guess. I know, it is the most naive approach but we can do it just for fun, right?

The picture above shows the working circuit. The required parts for this project are:

  • 4 pcs. CR 2032 3V battery arranged in a pile (in series; also +,-,+,-,... and so on)
  • an resistor (its resistance could vary from 1 - 4 kΩ; the greater resistance the smaller frequency will have); you could also use a variable resistor instead
  • an NPN transistor (a 2N2222 will do the job; check also for its equivalents if you don't have exactly this one)
  • a light emitting diode (aka LED)  : 5mm red, 3V, 18mA current draw will be just fine; furthermore the red color means alarm so the it should fit, right?
  • a (electrolytic) capacitor of 1 mF
  • a prototype paper PCB where we'll solder all these components together
  • a soldering gun and soldering tin too, but if you want just to try this project for fun then you could use a solderless breadboard instead.

Such a system will cost maximum 1 USD so it's worth it. The circuit schematics looks like this:

flasher-circuit

If you have LTspice program and you want to simulate the above circuit then you can download the schematics from here: flasher-circuit.

Once you soldered all these components together your "alarm" should look like this:

By the way: that black thing that covers the circuit is nothing more than electrical tape. I use it just to mask the components because "the device" is placed somewhere where only the LED should be visible and no other component. From side you can see the pile of batteries at the back (that big rounded thing behind the board).

I really hope that it will keep the burglars away (LOL) at least until I will have enough time to make the real thing.

How it works

I want the LED to blink like 1 time each 3-4 seconds. For that we use a resistor (R1) and a capacitor (C1) (the so called resistor-capacitor circuit) create kind of a timer logic that will allow the current to flow through the LED within a regulated interval. Let me clarify this:

  • capacitance is charge per volt, i.e. C=\frac{q}{V}
  • current is change of charge per change in time, i.e. I=\frac{q}{t} \Rightarrow q=I\cdot t
  • according to Ohm's law I=\frac{U}{R}\Rightarrow \frac{I}{U}=\frac{1}{R}

From these three equations we can conclude that C=\frac{q}{V}=\frac{I}{U}\cdot t=\frac{t}{R} \Rightarrow t=C\cdot R also the time need to fully charge the capacitor grows with its capacitance and the resistance in the circuit. If we take a capacitor of n μF and a resistor of m Ω, the greater is n and/or m the longer it will take to charge the capacitor.

Let's do the math:

  • the capacitor has 10-3F
  • the resistor has 3.3 x 103 Ω
  • t=C x R=10-3F x 3.3 x 103 Ω = 3.3 sec

What's happening is that the resistor R1 will resist to the current flow according to the Ohm's law (i.e. I=U/R). So with an input of about 12V and a resistor of about 3.3 kΩ we allow only I=\frac{U}{R}=\frac{12V}{3300 ohm}\approx 3.64mA charges per time unit to flow through resistor. As the current wants to travel from the positive to negative (in real life it's exactly opposite; also +/- thing is rather  a convention) it can choose only two paths:

  • either flows through the capacitor (C1) and then to the negative end
  • either flows through the NPN transistor (Q1) and then to the negative end

The capacitor (C1) is like an emptied balloon. It will charge itself (will "eat electrons") until it is filled at its maximum capacity then it will discharge. Because the input current rate is low (≈3.64mA) it will take some time until this "tank" (the capacitor) will be fully charged. We use this time gap as our timer. So whenever the capacitor charges itself there will be no flow of electrons through the LED (or if it is it's so small that the LED will not light anyway). When the capacitor is fully charged it cannot take more electrons so the current will try to find other path to flow, e.g. through the NPN transistor (Q1). As the current goes through the transistor it finally reaches the LED which lights up for few milliseconds. In this time the capacitor discharges just enough to be hungry again for electrons. So it starts again eating electrons until its stomach become fully charged of these tiny particles. Meanwhile there will not be enough current (flow of electrons) to light up the LED so it will look like it's not lighting. So the process repeats itself over and over again as long we provide a source of voltage.

By the way:

  • the circuit is very similar with this one, so all credits should go to the original author
  • if you don't want to use a 12V battery but a 1.5V battery you should alter little bit the circuit in order to add Joule thief circuitry; a starting tutorial could be this one
  • if you want to calculate the optimal resistance for LED then check out this page
  • don't forget to check out this page if you prefer a 2 flashing LED circuit instead
  • for future reference How to make a simple Arduino alarm system

I must say that I did it most for fun so don't take it too serious, although I really believe that, in general, small thing can change the world (like e=mc2) or at least, paradoxically, make big difference.

Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.

 
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Fake alarm system to trick burglars

Eugen Mihailescu

Founder/programmer/one-man-show at Cubique Software
Always looking to learn more about *nix world, about the fundamental concepts of math, physics, electronics. I am also passionate about programming, database and systems administration. 16+ yrs experience in software development, designing enterprise systems, IT support and troubleshooting.
Fake alarm system to trick burglars

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