Door Alarm - Electronic Circuits Projects Diagrams Free

Flipsyde412 · 16th December 2006, 01:57 PM

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 · 16th December 2006, 03:06 PM

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 · 16th December 2006, 09:49 PM

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 · 17th December 2006, 03:52 AM

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 · 17th December 2006, 08:19 AM

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

Quote:

Originally Posted by 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.

Sceadwian · 17th December 2006, 10:59 PM

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 · 18th December 2006, 01:46 AM

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 · 18th December 2006, 07:59 AM

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

Like This One

ljcox · 18th December 2006, 08:48 PM

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

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

16th December 2006, 01:57 PM	(permalink)
Flipsyde412	Door Alarm 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

16th December 2006, 03:06 PM	(permalink)
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.

16th December 2006, 09:49 PM	(permalink)
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. __________________ Len

17th December 2006, 03:52 AM	(permalink)
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. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

17th December 2006, 10:59 PM	(permalink)
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. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

18th December 2006, 01:46 AM	(permalink)
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. __________________ Len

18th December 2006, 07:59 AM	(permalink)
bizzareodave	Some reed switches now come in a N/O or N/C selectable configuration. Like This One

18th December 2006, 08:48 PM	(permalink)
ljcox	Thanks, I did not know that. JayCar have several versions in their catalogue. __________________ Len