This circuit is a simple wire loop alarm that can be used in doorways, hallways, or any other place the tripwire will be broken by intruders. The circuit has a built in siren, but it can be replaced by a relay to drive an external siren, commercial alarm, etc.
http://www.aaroncake.net/circuits/alarm1.asp
This circuit provides a delayed visual indication when a door bell switch is pressed. In addition, a DPDT switch can be moved from within the house which will light a lamp in the door bell switch. The lamp can illuminate the words Please Wait for anyone with walking difficulties.
http://www.zen22142.zen.co.uk/Circuits/Misc/deafdoorbell.htm
An electronic siren made from discrete components. A push button switch is used to create the sound, which rises in pitch when the switch is pressed, and falls in pitch when the switch is released.
http://www.zen22142.zen.co.uk/Circuits/Music/siren.htm
Beeps if you leave open the door over 20 seconds and uses 3V battery for operation.
http://www.redcircuits.com/Page15.htm
This circuit is intended for children fun, and can be installed on bicycles, battery powered cars and motorcycles, but also on models and various games and toys.
http://www.redcircuits.com/Page5.htm
The circuit aims to imitate a chirping canary made of electronic circuit components such as the Hartley Oscillator.Designing this circuit does not require much complicated connections. It can be powered by a 9 V battery source while the modification and adjustment being made to the circuit output may be controlled by the tank circuit utilizing the benefits of the Hartley oscillator.
http://www.zen22142.zen.co.uk/Circuits/Music/canary.htm
This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord.
http://www.redcircuits.com/Page50.htm
The purpose was to obtain more power than the siren circuit already available since many years,One-IC two-tones Siren and to avoid the use of ICs.
http://www.redcircuits.com/Page84.htm
A cheap and simple gate alarm made from a single CMOS Integrated Circuit.
http://www.mitedu.freeserve.co.uk/schematics.htm
This FM radio-controlled anti- theft alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park.
http://www.electronic-circuits-diagrams.com/alarmsimages/alarmsckt12.shtml