Small Sized Loop Alarm

Figure 1 

The circuit was constructed to provide an alarm circuit that is reliable, simple, small size, and low in power consumption.

• Light Emitting Diode (LED) – a semiconductor diode that is commonly a source of light when electric current pass through it
• 1N1418 – a small-signal silicon epitaxial planar diode used for fast switching applications with a reverse voltage of 100 V and forward current of 150 mA
• Zone – refers to an alarm control panel’s input from a protective circuit which divides the alarm system into separate independent areas of protection based on function which may include medical, fire, intrusion, or critical condition monitoring

There are two identical transistors used in this circuit which could be any of the NPN general purpose audio switches or amplifiers like the family of BC107/108/109 or similar family of 2N3904 that are particularly suitable for use in AF input and driver stages. Every alarm in the circuit is combined to the buzzer with a sound intensity of 82 dB. There are sensors present in the circuit that can be made of wire loops or close reed switches which operates due to the employed magnetic field. To prevent a signal from each sensor from triggering both LEDs, two 1N148 diodes were utilized.

A cut off time for the alarm is not set on the circuit since it is purposely built to be handy and can be easily manipulated. When the buzzer sounds, it does not turn off by itself unless done physically or returned the loop wire back to the jumpers. The behavior of the LED is the same with the buzzer as well.

During the use of long loop wires, unavoidable noise interference may occur and to prevent this, the two capacitors, having each a value of 100 nF, were placed in each zone. The circuit can be driven by a supply voltage of 9 V.

The main purpose of creating this alarm is for security reasons like protection of property or life and the circuit may be used in hallways, doorways, and in places where a trip wire can be setup for trespassers. It can also consist of a commercial alarm or a siren that is powered by a relay. Since the circuit was designed in a simple manner, it is easy to use, maintenance-free and affordable.

Source:www.zen22142.zen.co.uk/Circuits/Alarm/alarm.html