Portable Alarm for Personal Use

Figure 1 

Parts List

The circuit will be useful as personal alarm and anti-snatching bag unit since the design is very small and portable, which can also be used for windows and doors control.

• BC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selections
• BC327 – a PNP general purpose transistor in a TO-92 package, used for general purpose switching and amplifier applications which is suitable for AF driver stages and low power output stages of audio amplifiers due to its features of high current at 500 mA maximum and low voltage at 45 V maximum

The setup of using this circuit is placing a small magnet close to the reed switch SW1 while being connected to the hand or clothes of the person carrying the bag with the use of a small cord. Since the circuit is small, it can be placed in a small plastic box. The magnet will loose its contact with the reed switch once the bag is snatched suddenly. The circuit will start oscillating if SW1 opens and will produce a loud alarm sound from the loudspeaker. Alternatively, the device can be placed in a pocket while the cord is connected to the bag, to form a reverse method of connection.

The high efficiency oscillator is made up by wiring a complementary transistor-pair that will drive a small 8 Ohm loudspeaker directly. A very compact assembly is the result of 3 V battery supply B1 and low count of parts used.

Any type of loudspeaker can be used but the dimension is dictated by the box where it will be placed. Since the standby current being drawn is less than 20 uA, it is not necessary to add an ON/OFF switch. A current of approximately 100 mA is consumed by the circuit when the alarm is sounding. To make the circuit more attractive, a 3.5 mm mono jack can be used as replacement for the switch while a 3.55 mm mono jack plug with its internal leads shorted can be used in place of the magnet. The tiny cord will be connected to the jack plug. The voltage used to supply this circuit should not exceed 4.5 V since the transistor Q2 might get damaged. To be on the safe side, it is recommended to use a 3 V supply with two AA cells wired in series.

The personal alarms are one of the fastest growing segments in the self defense industry today because they are extremely loud and convenient to carry. It can come in the form of key chains, bright flashlights, levered hook, or wrist strap. It is used for scaring off an assailant by surprising them with high pitch alarm that may attract attention of others in the area. The opening of a window or door can provide a signaling method using this device which can be done by placing the box on the frame and the magnet on the movable part. In this way, the reed switch and the magnet will be very close when the window or door is closed.

Source:redcircuits.com/Page50.htm