student wants alarm advice

[fordmyblimps]

i am a GCSE student at DHSB in Plymouth, and i have decided to do and alarm system for my bedroom, it has to be silent, easy to use by all, has to be able to switch off easily, using a 555 monostable circuit and a latching thyristor. it has to be activated by either pressure or by motion. also i need research on 4 different alarm systems that already exist. please cud u e-mail me some info on some products or a circuit diagram that i could possibly use

thanx in advance

Matt

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Moderator

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[ChrisRogers]

why dont you just do your research yourself? its what most of us had to do.

 

[andy]

you will get better help HERE

 

[felix]

The 555 has, over the years, been promoted as both a monostable and an astable oscillator but rarely as a bistable switch. I think this is how you should be using it. That latching thyristor (which most of them are) won't help you much. You want a simple transistor as a power buffer and leave the latching to the 555. Come to think of it, the 555 has an open collector transistor built in which might be all you need.

Hint 1: In some circles the 555 has been written off as a dead loss due to its habit of breaking into high frequency oscillation at the slightest provocation. This is grossly unfair. The data sheet quite clearly states that you should decouple the power supply pins close to the chip. I use a 47uF tantalum and a 0.1uF disk ceramic in parallel and have never had any trouble. It's a good idea to decouple pin 5 too.

Hint 2: The input sensors of alarm systems tend to be at the end of long wires. Smooth your inputs with a very long time constant (100 msec is not too much) or your circuit will go mad every time somebody switches a light on.