Security Alarm

In this project I'm going to make a security alarm. It's very great for beginners in electrical engineering. It's simple, but uses lots of different components, so one can learn basics of circuit analysis. You'll be able to understand the function of transistors, relay, capasitors and basic ICs.

PASSIVE ELEMENTS:

Resistors - 4x10k, 4x470, 2x1M, 100k

Capasitors - 2x68uF, 2x10uF, 0.01uF

Reed switch NO

Button

Switch

2xLED

1n4001 (diod)

ACTIVE ELEMENTS;

2x2n2222 (transistor)

2xNE555 (timer)

In this step, I'm going to explain how the circuit works.

Firstly, current goes to the fisrt switch. The switch is used to change the way circuit works. In its initial position, it allows you to check wether reed switch is closed or not (the idea is that you place one part of switch on a door and the other part right nest to it, so when door is closed, the switch is closed too). When switch is closed, the current goes through the LED, which is possible because reed switch connects its cathod with the ground. Otherwise, reed switch doesn't provide the current to go to the ground, so the LED is not turned on. That's how a user can understand, wheter door is closed or not.

Then, when the user is sure, that door is closed, he turns the switch and security alarm starts working. When the switch is closed (meaning dooe is closed), nothing happens, but when switch is opened, the current goes on transistor Q1 base, so the current now can go through this transistor. Now the current goes on the coil part of relat, which switches its outputs, through which the current goes. Then current exits the 5th output and goes to the capasitor. Firstly, while capasitor is not fully charged, the 2nd output of NE555 gets enough voltage to give low level on the 3rd output. Then, after capasitor has fully charged, the potential on its plate from the other side is 0, so the second output gets negative level, so the 3rd output gives positive voltage. The capasitor won't be discharged fully again, so the voltage on the 3rd output will be always positive. This activates speaker, which produces sound. I also wanted to clarify, what R9 and C4 do. They prevent timer to generate impules, when it turns on. In the first moments, when timer turns on, while C4 isn't charged, the current goes on the 4th output, which makes level on the 3rd output low. When C4 is charged, system works normally again.

Okay, we now understand what the main part of the circuit does, but what happens on the upper part? It's function is simple - to allow the user go throug the door without triggering the alarm. Before leaving, the user needs to press the button. When it was pressed, the level on the output 2 of NE555 is low. The cycle begins. The output 3 gives positive voltage on the base of transistor Q2. This transistor will ground the base of transistor Q1, so it stops alarm from working. The base of transistor Q2 will be getting positive voltage until capasitor C1 is fully discharged. That's aproximately 1 minute. The second LED indicates whether time has passed or not. When time has passed, it turns off (I've showed it on the video in the final step).

It was my first project, so I made a big mistake here. I've soldered both NE555 without IC holders. Firsly, they worked fine, but after some time they've become working significantly worse. Don't forget to buy and use IC holders!

Here you can see functioning of the alarm. Hope, everything was clear in my explanations.