This simple circuit is the electronic version of the combination lock. Using the special purpose LS7220 digital lock IC, the circuit allows a 4 digit combination of your choice to activate a relay for a set period of time. This relay can then be used to trigger a lock solenoid, enable a starter button, open a motorized door, or many other tasks that require a momentary signal.
http://www.aaroncake.net/circuits/combolock.asp
The circuit is designed to detect audio signals on a remote area for security purposes.The LM386 has been designed to work with low voltages with the current being consumed from a power source or a battery is quite minimal.
http://www.zen22142.zen.co.uk/Circuits/Alarm/security_mon.htm
When you have popped in your fridge alarm, getting peckish in the middle of the night and forgetting to close the door along with escalating electricity bills will be a thing of the past. Italian inventor Flavio Dellepiane has designed a 3V battery powered fridge alarm that beeps if you leave the door open for more than 20 seconds.
http://www.electronicsweekly.com/blogs/gadget-freak/2007/08/some-fridges-are-always-alarme.html#more
This circuit has been designed to provide a clearly visible light, formed by 13 high efficiency flashing LEDs arranged in a pseudo rotating order. Due to low voltage, low drain battery operation and small size, the device is suitable for mounting on bicycles as a back light, or to put on by jogger or walkers.
http://www.redcircuits.com/Page24.htm
The circuit was designed to provide an alarm system that would transmit signals across the medium wave or the radio wave.A switch using the emitter follower pattern is consisted of a medium power BD131 transistor to act as the soft switch circuit to make sure that the power is developed steadily without the presence of any direct current blow, while the collector is attached to the permanent supply voltage.
http://www.zen22142.zen.co.uk/Circuits/Alarm/radiowavealm.htm
This is a simple single-zone burglar alarm circuit. Its features include automatic Exit and Entry delays. It is designed to be used with the usual types of normally-closed input devices such as - magnetic-reed contacts - micro switches - foil tape - and PIRs. It has an extremely small standby current - making it ideal for battery-powered operation.
http://www.zen22142.zen.co.uk/Circuits/Alarm/gsh.html
The idea behind this snore alarm, is just to rouse the snorer, not the entire household. To wake the sleeper, vibration is used, not an audible alert. The vibration is provided by a small motor housed in a small 35mm film case, which can be placed under the sleepers mattress or pillow. This circuit has a level control and peak display indicator, a variable trigger threshold and trigger indication.
http://www.zen22142.zen.co.uk/Circuits/Alarm/snore_alm.htm
The project uses a PIC 16C84 to monitor 4 NC alarm inputs that when triggered, activates an output relay. The first zone triggered is illuminated with a solid lit LED, while the second and subsequent zones are indicated with a flashing LED.
http://www.shucksmith.co.uk/burglaralarm