Can anyone tell me where I can purchase one of those alarm/buzzers that are attached to doors and make a sound when someone enters or leaves through it? Kind of like what you get in some shops. What are these devices generally known as?
no idea what they are called but you are likely to get one at Maplins. http://www.maplin.co.uk/
give them a call and I'm sure they could tell you what you need.
They are known as door chimes. If you have a burglar alarm fitted to the property, you could program the system to make a 'chime' when the door is opened opened. If you dont, maplins should be able to help.