Create door opening wireless notifier? Reed switches?


I have a Vellerman K8055 USB experimental board that I have hooked up to one of my machines. Yesterday, I took our wireless doorbell apart and with a multimeter found the signal that sparks the 'ding-dong' sound when the remote bell is pressed. I wired that into the K8055 and then to cut a long story short, when the bell is pressed, it pops up on my TV and tells me that someone has pressed the bell (using MythTVOSD multicast if anyone is interested).

A link to the card: http://www.velleman.eu/products/view/?country=be&lang=en&id=351346

What I'd like to do is to extend this mechanism to my doors. When a door is opened in the house, then I need a signal to be sent to a receiver device also wired into the K8055 so that I can capture that event in my program.
I have thought about using magnetic reed switches on each door with hacked wireless doorbells (around 3-5 GBP on ebay from China).
One problem I have is that I can only find 'normally open' reed switches, meaning that when the door is closed so will the reed switch contacts - this in turn means that I will be constantly notified (should I be able to make this work) that the door is closed. If I could find 'normally closed' reed switches, I could have one of each on each door,
What is the cheapest/most elegant solution to this problem?
Is using hacked wireless doorbells the way to go, or is there a better way.

I'm a relative novice with electronics - I'm a dot net developer by day with experience of data capture cards, and have toyed with the Arduino, Lm386s, 555s, 4017s etc... some with instructions from this site, so whilst not a complete beginner, I am what I would call, "an enthusiastic amateur'.

I don't know if you've seen this one in stores where you live, but the attached link and picture are to a cheap battery-powered door alarm. In my country, the former U.S., that particular store, DollarTree(r), has locations almost everywhere.

The point here is that if you can find that gizmo, and find it cheap, circa 1USD per, then I think that item can be hacked to give you what you want.  I am pretty sure it contains a reed switch that closes when you take the magnet away, because that's the best way to design such a battery powered gizmo, so that the current from the battery is zero most of the time, while the door is closed and the switch is open.

By the way, if all you've got is the kind of reed switch that closes in presence of a magnetic field (open in zero magnetic field), those can be "inverted" by gluing a second magnet fixed near the reed switch in just the right place.  That is to say you have two magnets:  one fixed near the reed switch , one attached to the movable door or window.  These magnets are arranged so that when the door/window is closed, the fields of the two magnets cancel, giving zero field,and the reed switch opens.

You might be able to find a reed-switch and transmitter together in the same package, like for example:
at 7 USD per.  The trouble with that particular item, besides it being sold out of  the former U.S.,  is it is surplus and sold without the receiver.   It might take some significant effort to find, or build, the receiver that works with that thing.
dchurch24 (author)  Jack A Lopez5 years ago
Thank you (both).

I like this idea. Most of my - ongoing- home automation system is made from things found in dollar shops, dumpsters etc... I like it that way ;-)

My girlfriend said to me not long ago, "if we won the lottery we could just buy all this stuff" - that made me sad. Buying it wouldn't be anywhere near as much fun!

I have seen these on ebay: http://www.ebay.co.uk/itm/4-House-Room-Security-Home-Intruder-Burglar-Alarm-Alert-/220779760814?pt=UK_Burglar_Alarms&hash=item33677fccae
which look very similar.
I could fit one of these, then hack the doorbell apart and make it daisy chain up so that the receiver then gets the signal!
Use an inverting logic gate, ideally a schmitt inverting buffer like a 74HC14, keep the reed.