Sonntag, 10. Juli 2011

electricity with Mekkor

Hi people who actually read this,

since i've got a little knowledge about electricity i'll give you here and there some instructions to build some cool things by your self :D

If you dont have any clue about electricity then PLEASE FOR THE LOVE OF GOD AND FOR YOUR OWN SECURITY try to learn and understand voltage, current and resistance before you try to work with them. In this little guide you cant really hurt yourself but its always better to know what you are doing instead of following the instructions of a random stranger from the internet.

Today we are going to build a burglar alarm for your door/window. I know you could buy a finished one but to build it is more fun.

So, what you need:

1x magnetic contactor (a contactor which closes the circuit when the magnetic contactor is interrupted)
1x relay (take a 5v relais) (1-2$)
a few LEDs (red ones and if you want to build a sign out of them then get enough) (1-2$ if you dont want to buy a few hundred)
1x capacitor (1 millifarad) (this is only if you want to let your sing blink) (maybe 20 cent)
1x resistor 100Ohm (only if you want to let your signal blink) (a few cents)
1x singal sound ( i took a 5V 80dB buzzer which is really noisy and will wake you up for sure)(1-2$)
1x power supply (we take a 5V power supply for our buzzer. Take an old mobile phone charger. i had one at home so i bought everything else just after this phone charger)

so, this blueprint is the one without the blinking

and now guess what blueprint this one is.... right, its the one with the blinking

Edit: this is the right blueprint for a blinking burglar alarm

I think i should tell you some tips for your LED sign:
working with LEDs is like working with a hammer. Its easy if you use it the right way. LEDs are poled which means you have to look how you put them in your circuit. Red LEDs need around 0,7 Volt to work. So lets say you got 10 red LEDs. That means we need 10x0.7Volt. But our voltage supply only got 5V. So we cant put them straight in a line. This image may help you because i dont really know how i should put that in words without writting a book about it.

the "trick" is to put some parallel to others.
You got an amount of LEDs that doenst get exactly 5V no matter in what combination with parallel and straight you group them? Use a resistor! "Which resistor" you ask? Add for every LED you use 2mA and then look what voltage is left. Use this formula to find it out. R (resistance) = U (Voltage) / I (current)
2V are left. You got 7 LEDs.
R = 2V / (7 x 2mA)
R= 2V / 0,014A ( | Note: (for all stupid people: 0,014A = 7 x 2mA)
R = 142 OHM

how the whole thing i just build works, you ask? Well its really easy.
If someone opens the door, window, whereever you put your magnetic contactor on, the contact in the mangetic contactor will no longer interrupt the circuit so the current can flow. Then the relay will get enough voltage and the contacts will contact so your choosen LED sign will light up and your buzzer will wake you up. Then you grab whatever you have for your selfdefense and fight for your live ^^

And now an explanation how the blinking thing works:

You see the condensator? Good. When the magnetic contact is closed the condensator has almost no resistance for the current. Then it charges itself after this formula:
t (time till it is theoretically fully charged (in the praxis a condensator is never 100% charged but after t it is 99,6% charged so that doesnt matter much)) = 5 x R (resitance) x C (capacity of the condensator)
So you can decide how fast or slow your burglar alarm should blink.
In my case it looks like that.
I've got my condensator (1mF) and the resistor in that case is the buzzer (ca. 150 Ohm).
That means t = 5 x 0,001F x 150 Ohm = 0,75 sec
That means your condensator is charged after 0,75sec and will break the circuit so no current can flow because when the condensator is charged its resistance is almost infinite and no current can glow. after that the condensator will discharge itself over the diode till  its resistance is low enough so the relaise gets enough voltage to work and everything starts again.
It will actually blink in shorter intervals because of the relay the condensator wont discharge completely.                                                                                                                         

I dont think that i should mention any self-security-tips when working with voltage and current. We are working with 5v so in the worst case you will hang onto 5mA and you can take 10 times more till it gets really dangerous.

I was to lazy and played a lot of LoL (there will be a article about that game) to build my burglar alarm but i'll do it in the next days and upload a pic.


  1. very nice post dude!!!! refresh bombed... returning the favor would be nice ;)

  2. reminds my of the cool illustrated tutorials and extras from the Mickey Mouse Magazine :D. Your description seems to be professional and complete.