Van alarm

Hi guys, I'm building a little alarm for my dad's van... I live in italy and recently a lot of vans have been stolen near my house, so we want to find a cheap and reliable way to protect it... These are my requirements: - I want a simple system, which doesn't need drilling the van structure or similar invasive operations. Something that won't affect the value of the vehicle (which is brand new) - the system needs to be a normally closed type, so that cutting the wires won't result in the deactivation of the system - I want to use an ultrasonic sensor coupled with a siren for notification of intrusions - I want very low power consumption

From here, I started designing the circuit of the system, with these things in mind: - the ultrasonic sensor (HCSR04) will be placed inside the van on the roof, pointing towards the driver-seat. When the sensor detects a variation in this distance (e.g. someone is sitting there), the alarm (unless when it's not engaged) will turn on. - I want the owner to have a 10-15s time span for the deactivation of the system from when he enters the vehicle. My idea of the deactivation system would be a spring loaded toggle switch (or a pushbutton) that needs to be continously pushed for, say, 5s, otherwise no deactivation occurs (this is a way to prevent the burglar from deactivating the system even if he's able to find the switch) - The system, when deactivated, goes to sleep and it engages again if the ultrasonic sensor detects the roof-seat distance (that means that no-one is inside) for more than, say, 5-10 mins. - the ultrasonic sensor is wired so that if one the 4 wires is cut (5V, echo, trigger, GND), the Arduino starts the siren. Since I'm new to burglar wiring, I designed a system using 6 wires and 2 resistors (that create a voltage divider). I'm open to suggestions, don't know if this is a good wiring scheme :) - the siren will be placed in the back of the van (which is not communicating with the cabin) so that it'll be difficult for a burglar to cut the power. This, however, poses a problem: I need to find a way to wire the siren to the arduino without drilling holes in the cabin panel... I was thinking of using rf or bluetooth communication, but this would mean using two arduinos, two power supplies and more hassles... So this weekend I'm gonna carefully inspect the van to check if there are already pre-drilled holes... - as far as the siren wiring is concerned, I want the siren to start screaming not only if the arduino tells it to, but also if the wire connecting the arduino to the siren is cut. How can I do this? Maybe using a transistor as a switch? The power supply for the siren will be placed in the back of the van so that it's hopefully untouchable... - finally I want to install a small buzzer that turns on during the 10-15s deactivation time... This will notify the user that he needs to turn the system off with the toggle switch before the siren starts screaming

With these thing in mind, I designed a mock-up circuit in Fritzing...

However, the siren is still missing the wiring... What do you suggest? Do you think this will be a good alarm system? Thanks for your help :)

Or the other option would be to hook up the arduino to the van's horn... Is it possible?

You seem to have pretty much everything covered. You would have to connect the siren to a normally closed relay which would be connected to the Arduino. If you tape all the wires from the Arduino together, the thief couldn't cut anything without cutting the power, so the relay would lose the signal telling it to stay open and it would make a noise. I think that's about right.

Thanks for the answer guys! I think I'm close to the final design... I put a FET for the siren... What do you think?