This IS a great forum… one of the reasons I am such a big Electric Imp fan.
My system is pretty simple, I just reused the magnetic door/windows sensors. There were wall mounted sound sensors, but they seemed fairly useless, so I didn’t hook them up. I also have heat sensors in the house that I need to add to the circuit, and those should be just like the door sensors.
So for mine, there really isn’t much of a schematic. The siren is triggered by a transistor relay circuit. 5V relay, 2N2222 transistor, 1k resistor and a diode across the relay coil. Configure a pin for DIGITAL_OUT and send it HIGH to flip the relay. The door sensor pins are just configured for DIGITAL_IN_PULLUP, which means they are normally pulled up to 3.3V. Since the sensors are normally closed, just connect one end to ground and the other to pin. When the door is opened, it will pull the pin to GND.
Here is the cool part about the E.Imp. You can also configure a “callback” on the pins, so when it is triggered, (pulled high or low) you can assign it a function to execute, so you don’t need to constantly check your pins with a loop function.
For really long runs to sensors, you may get some false triggers due to capacitance on the lines, but its pretty easy to check for false positive in your code.
I left the old board in place to provide 12V power for the siren, and charge the battery, though I don’t think it is, and I need to work on that… it doesn’t need to be in there. As soon as I have one of the Amber boards complete, I’m going to redo the system, though the Adafruit screw shield does work nicely.
As far as I know, there aren’t analog callbacks yet, but I think they are coming soon, so you could potentially set up a more sophisticated sensor system, like the one described in “Practical Arduino”.
If you sensors only use a single pin or you gang them together, you should have plenty of pins on the April. I have a few door and window sensors tied together into zones, and a few by themselves.
I’ll try to get some more info put together on my system, though like I said it is pretty simple. If you didn’t see this… here is the E.Imp code I use with my Pitchfork app.