Urgent: Anyone knows how this Doorknob alarm circuit works?

[sodium kumar]

Hi, im new to edaboard and i seriously need anyone's help to explain how the circuit below works. Its a door knob alarm that has to be hanged on a metal door knob. It will produce alarm sound when anyone touches the door knob. This system is not applicable for metal door. I need to know how each component works and why its not applicable for metal doors...

http://www.escol.com.my/kits/ES-035.html

Help me please...thanks

 

[Nephazz]

Re: Urgent: Anyone knows how this Doorknob alarm circuit wor

This sensor works capacitive. If you hang a wire into the air it builds a capacitor to ground. If you touch it you change the capacity. The circuit meassures the cap and starts the beeper when it changes.

The door knob forms an oscillator together with L1, Q1 C2 and all the stuff left of them. When you touch it the oscillator gets off-tuned and turns Q2 on. This starts the rest of the circuit which is only there to make the speaker produce sound.

It doesn't work with metal doors because the change in capacity while one touches it would be too small to meassure. Even if you came up with a very sophisticated circuit the door could be grounded and there wouldn't be any capacity effect at all.