Door Alarm

[Flipsyde412]

I need help how to build a door alarm that is not to much expensive in parts,
can u give me a URL having those projects...thx a lot

 

[Papabravo]

Take a bucket and fill it full of water. Put it on top of a slightly ajar door. When the unsuspecting one walks through the door the loud expression of surprise will alert you. I guess you could build an amplifer to increase the volume of their screams.

 

[ljcox]

A simple door alarm would be:-
A micro switch attached to the door.
A buzzer or siren and battery.

When the door opens, the buzzer or siren sounds.

Piezo electric buzzers are cheap.

 

[Sceadwian]

Embed a simple magnet in the door itself, and use a magnetic reed switch on the doorframe, wire to buzzer. Shouldn't cost more than 10 bucks.

 

[bizzareodave]

Yea, get a magnetic reed switch on the door, wire it to a DC relay and then drive your siren, buzzer, annoying flashing lights etc. etc.

But I like Papabravo's idea also

Take a bucket and fill it full of water. Put it on top of a slightly ajar door. When the unsuspecting one walks through the door the loud expression of surprise will alert you. I guess you could build an amplifer to increase the volume of their screams.

 

[Sceadwian]

The contacts on most magnet reed switches goes up to at least 1amp, more than enough to drive a deafining buzzer. Seeing as how the switch is completly off when there's no magnet around the entire circuit can be driven using a 9 volt battery and will have a very long service life. Reed switches/relays are highly reliable.

 

[ljcox]

Reed switches are normally SPST so how do you arrange the magnet and reed so that the reed is open when the door is closed?

An alternative solution would be to use a SPDT reed (if you can buy one) so that the reed is "operated" when the door is closed and so the N/O contact is open. Thus when the door is opened, the reed "releases" and thus closes the buzzer circuit.

Another possible problem is that the reed may "stick" due to residual magnetism and therefore won't "release" when the door opens. If this proves to be a problem, another magnet (fixed in the door jam near the reed) could arranged to give a small amount of reverse flux when the door is open.

 

[bizzareodave]

Some reed switches now come in a N/O or N/C selectable configuration.

**broken link removed**

 

[ljcox]

Thanks, I did not know that. JayCar have several versions in their catalogue.