Doorknob Electromagnetic Alarm

M

marcos.placona

Senior Member
Hi guys for some time I've been thinking about doing something like this:

http://www.dealextreme.com/details.dx/sku.843

Just for fun, and to learn it's inner workings. I've came up with some ideas on how to do that, but didn't exactly get to a conclusion about how to close the circuit when the doorknob is touched. The idea here is to take any ordinary doorknob (conductive of course) and make it touch sensitive.

Do you guys have any idea on how to do such task? I believe it shouldn't be difficult at all, but as I know that some of the guys here are masters in coming up with some brilliant ideas, I decided it could be a good start for me.

Thanks,

Marcos Placona
 
T

Tom2000

Senior Member
Something like this touch switch might work. For connection to the Picaxe, you don't the the entire driver circuit. Start building with the MPF102 stage, check it with your voltmeter, and see if it's delivering enough signal to test with your Picaxe. Not enough? Build the next stage. Test, etc.

You might be able to build one of these with a 2N2222 (or, really, any good general purpose NPN transistor). Ground the emitter, feed the collector to +V through something like a 2.7k resistor (not critical). Measure between collector and ground with your voltmeter as you touch the open base terminal. Did you see a change?

There are also some Qprox capacitive touch sensor chips, but they are mostly available in tiny little SMD packages. There were some DIP-packaged chips that might still be in the system, or that might be in the process of being re-issued. If you can find a DIP Qprox chip, that would be the most reliable thing you could use.

But for something like this, I think it would be fun and educational to experiment.

Have fun!

Tom
 
D

Dippy

Moderator
Why not connect it to a dimmer switch. Then it could turn the lights out when you leave the house :)
 
D

Dippy

Moderator
Excellent. Yes , QProx has had many, many mentions on the Forum.
And I'm glad to see you don't bite your nails.
 
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