Building Alarm System w/ Battery Backup

[Nyx]

I'm having a few design issues with a project not directly related to robotics. I want to build a simple alarm system. I will be using reed switches for doors and windows. These will maintain an electrical circuit closed for as long as the doors and windows are closed. The circuit will open if a door/window is open, which will in turn trigger relay and then a circuit with a 555 timer to make an alarm ring for a few minutes. Now I'm thinking, I want to put the box with the alarm out of reach of bad guys, say, above a door. This way, if the door is opened and bad guys get in, they will have a hard time smashing the alarm/siren.

Siren:
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=330379877428&ssPageName=STRK:MEWNX:IT

Reed switches:
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=250545533325&ssPageName=STRK:MEWNX:IT

I would like to have a little box with a key switch accessible to arm/disarm the alarm, say, next to the door. The easy thing to do would be to have the key act as a switch that closes the circuit to arm the system. This isn't very bad guy proof, however, as cutting the wires to the switch box would stop the alarm. Hence I figured, the key should open the circuit to arm the system. This way, cutting/ripping the wires to the key box will not disarm it, they would have to be shorted together, which is impractical to do quickly.

However, there is another slight issue. This alarm needs some kind of power supply, and ideally, should have a battery backup inside of it, so that it will still operate when power failures occur, or keep ringing if the power cord is ripped off. I have thought that the simple thing to do would be to use 8xAA batteries as an emergency backup. If the main power goes out, a relay switches the power source to the batteries. The problem here is that these are not easily kept charged, they would have to be periodically replaced if there are power outages. Another issue is my key switch box. This box triggers a relay inside the alarm to keep the alarm shut off. These relays consume power, and thus, the alarm would deplete its battery power even when disarmed.

An alternative is to use rechargeable batteries, say, sealed lead acid. I happen to have an "automatic charger" (12V 400ma), which I *assume* could just be plugged in parallel with the battery, to keep it always charged. My issue here is again that the alarm will consume battery power whenever the main power fails, even when disarmed... And as you may know, lead acid batteries will die if completely discharged. I'm not really sure how to prevent this from happening.

Could I perhaps use a 12V relay hooked to the battery, such that the relay "powers itself"? This way, when the battery is too weak to keep the relay ON, it shuts off power to everything (including the relay). At what voltage is a lead acid battery considered "dangerously low" on charge?

[SmAsH]

So, what exactly do you need help with?

[Soeren]

Hi,

So, what exactly do you need help with?

Here's a tip... Look for question marks

Like in

Could I perhaps use a 12V relay hooked to the battery, such that the relay "powers itself"? This way, when the battery is too weak to keep the relay ON, it shuts off power to everything (including the relay). At what voltage is a lead acid battery considered "dangerously low" on charge?

[hopslink]

Anywhere you are considering using a relay in your circuit you can use a Field Effect Transistor (FET) instead. It will do the same job but use much (thousands of times) less power. As a bonus it should be cheaper and more reliable too.

A disconnect circuit can be built using a voltage reference and a potential divider network as inputs to a comparator driving a FET controlling input current. Many manufacturers produce comparators with an internal voltage reference like this one.

How reliable is your electrical supply? If your circuit is only drawing a few mA and you have a battery with 0.5AH capacity you should get over 100 of hours of standby power. If you do not commonly experience power outages longer than a few days then this may be a non-issue. 

For lead acid battery information go to The Battery University. Detailed information about the common battery types is here.

[Nyx]

A disconnect circuit can be built using a voltage reference and a potential divider network as inputs to a comparator driving a FET controlling input current. Many manufacturers produce comparators with an internal voltage reference like this one.

Do you have a diagram for a MOSFET disconnect circuit?