This project will require a few extras on top of the £20 Arduino circuit; a small speaker or two, motion sensors, and a voice recorder. All of this can be picked up second hand for under £30, meaning the entire project will cost under £50; a great bargain and a fun project.
The aim of this system is to act as a deterrent for any would-be-thieves who are considering attacking your house. This is best used in conjunction with an existing system, and offers no guarantees. Fortunately if history is anything to go by, thieves are opportunistic and will go for the weakest house, rather than one with some obvious defences.
Motion sensor lights are always great for home security, and can be added to with a simple Pi system. By programming the circuit to respond when the light is on, it's easy to identify any intruders by simply taking advantage of existing tech. This is where the voice recorder comes into play, though it may not be necessary. The first step is to record yourself identifying thieves. Record many of these, as if the same one is repeated it's easy to tell that the voice is fake. Three stages should be used for this; the question, the warning, and the action. On first trip of the motion sensor, a question should be asked like 'Who's there?' 'Anybody there?' or 'Who's that?'. This sounds simple, but this is often a great way of deterring B&E's.
If the motion sensor is set off a second time, a new message should be played in response. The warning can be simple like 'Get out of my garden' or 'I can see you' or more complicated 'I can see you sneaking around out there'. This is a huge red light for thieves, and most will have scattered by now.
If the motion sensor goes off more than this, one of the final messages can be played; 'I'm calling the police' or 'I'm letting the dog out, you're warned'. For Americans, this would probably be 'I'm getting the gun'. This action at the end is critical to the process, as it gives them a firm action to get out of there. Raspberry Pi can easily be set up to send a message with any stage of motion trip, texting a mobile phone or emailing authorities. It's also very easy to set sociable hours, where people may be on the property for legitimate purposes.
Another key way of defending your home from thieves is a tried and tested one; a dog barking. This is set up in exactly the same way as the external motion sensor, but is best when the speaker is indoors. Always go for a bigger dog bark rather than a smaller one; more thieves will be dissuaded by a German Shepard than a Chihuahua.
Of course, this system can be personalised in any way the user likes, that's the beauty of Raspberry Pi. Additions like indoor lights activated by the external motion sensor gives the illusion that someone is home, and the sound of multiple people indoors is more than enough to make a thief flee. Easier houses can be broken into, and it's not worth the risk when someone is home with a dog.