This is how to make what I call an "all in one" detector. It detects smoke, heat, and intrusion.
This is strictly a "for fun" project- don't actually use it as an actual burglar/fire alarm! (Needless to say)
I apologize in advance for my crappy camerawork...
Step 1: Get a Battery Powered Smoke Detector
You can buy 'em anywhere. Just make sure you get one that isn't sealed- you need to get access to the circuit board.
Step 2: Smoke Alarm Components 101
Here are the basic components of your typical smoke alarm. The test button is just a little plastic button that when you push it, it causes the metal test switch to connect with a jumper tab, causing an alarm.
The piezo horn is what makes all the noise.
The ion chamber has 1 microcurie of Americium-241. It is what detects the smoke.
The status LED normally blinks every minute. When in alarm, it blinks every second.
I hope you know what the battery terminals are for...
Step 3: Make a Hole for the Heat Sensor
Start off with maybe a 1/4 inch hole. You can make it bigger if need be later on.
Step 4: Attach the Alarm Wires
These wires will trigger an alarm if shorted. So, solder them to either side of the test switch mechanism as shown.
Step 5: Dress the Wires
Well, this part is really optional, but it is highly recommended to avoid damaging the wires and possibly causing false alarms.
Step 6: Get a Heat Sensor
This you probably won't have laying around. I got this heat sensor from an old hardwired 12 volt smoke detector. It came from my old security/fire control system, and of course I have since replaced it.
If you have one, thats great. If you don't, well, you might be able to find something similar on the internet.
Step 7: Connect the Heat Sensor
Attach the two wires to either side of the heat sensor. Leave room- you will need to solder two more wires onto it.
Step 8: Connect the Other Pair of Wires
Connect another pair of wires to the same terminals.
Step 9: Get Some Magnetic Window/door Reed Switches
You can find these at a local security system equipment dealer.
Step 10: Attach the Other Pair of Wires
Connect them to the normally open contacts on your reed switch. You may have to check with an ohmmeter which ones are normally open. If you pull the magnet away, it should show continuity.
Step 11: Give Yourself a Pat on the Back
Good work. This device should now work properly. If you want, you can install a longer cable going to the reed switch, so you can monitor your bedroom window, or whatever.
Another thing you can do to make it an arm/disarm system-
Wire a switch between the reed switch and the heat sensor. That way, when the switch is off, it will not sound no matter what the condition of the reed switch.