Please help design a circuit diagram for an alarm?

let say i have a bicycle chain,i chained my bike n lock it and suddenly without me knowing someone cuts the chain to take my bike away.the function is to make my chain with alarm sound out loud,when the chain is cut.so please help me design the circuit for this

Answer:
The problem is that when your chain is locked then you have one closed circuit through the lock itself, so even if someone cuts the chain, you still have current flowing through the lock and the alarm will NOT go off.
What you can do is thread a thin, inconspicuous, insulated wire through the chain, and connect your "alarm system" to both ends of the wire. But you will have to hide - or make inaccessible - the battery and the alarm system otherwise anyone with a little knowledge of electrics can disable it by disconnecting the "famous red wire" from the battery.
Then I would buy a Smoke Detector from a Home Depot or B&Q or "Crappy Tire" (wherever you live), take the smoke sensor out and connect your piece of wire to the circuitry in such a way that it starts howling once the wire is cut. You need an on/off switch to make the circuit "hot", and you may also have to add a transistor to reverse the signal at the input to the smoke detector. Yes, a bit of electronics knowledge is required here..
Or you can buy one
http://www.bikemania.biz/attivo_bicycle_...
or
http://www.allproducts.com/manufacture6/...
Power Supply - Battery maybe
Alarm - compatible with battery.
NO/NC Relay Contact.

Connect the wires from the chain in such a way as the chain is cut, there is an open circuit, connect it to the relay NC , connect the alarm to the NO side.

So when the chain is cut the relay picks up and closes the circut on alarm side and the alarm goes on

I m lil tired right now otherwise i wud have explained it really well.
You can probably buy something cheaper and better but if you want the fun of making it then design something with a transistor that when switched on will operate an alarm. You then connect a potential divider (two resistors in series) with the mid point to the transistor base and each end to +ive and -ive. Your chain loop needs to break the circuit of one of the resistors so the transistor becomes biased on.

The resistors need to be sized to give sufficient voltage to turn the transistor on but not take too much current as this will quickly run your batteries down otherwise. I wouldn't recommend using a relay as it will soon run your batteries down anyway.

Search the Internet for a basic burglar alarm circuit to get a diagram and component list. It is a very simple circuit.

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