Thwart the attempt of a Bag snatching during journey. Keep this Tiny Alarm in your bag and it will beep loudly when somebody attempts to snatch your bag. It can protect your Laptop bag, Mobile pouch, Luggage etc during the journey. It is essentially a “Vibration Alarm” using ultra sensitive Vibration sensor SW 18020 P. So even a mild shake can activate the alarm. Build it and its cost is just Rs.50.
You need the following
1. Vibration Sensor SW 18020 P
2. Transistor BC 547
3. 470 uF 25V Electrolytic capacitor
4. 100 uF 25V Electrolytic capacitor
5. 100 Ohms Resistor
6. Red LED 5mm
7. 3-12V Mini Buzzer
8. Battery Snap
9. 9V Battery
10. Perf board
The Vibration Sensor is the major component of the circuit. When powered, the 470 uF capacitor charges fully . When the Vibration sensor gets a vibration, its contact shorts and current passes through it to the base of T1. T1 then conducts to activate the buzzer and LED. Capacitor C1 keeps the base of T1 high for a short even if the vibration stops. This gives a beep of few seconds with a single vibration.
Assemble the circuit on the Perf board as compact as possible. The Circuit board should be small equal to the 9V battery size. You can also a 12V Mini Battery used in Car remote. Then the unit will be too small and can be hide anywhere. Add a Micro switch for the circuit.
About the working of Vibration Sensor
The Ultra sensitive Vibration Sensor SW 18020 P is now available from the Gaoxin. It can be used in different ways to sense mechanical vibrations to activate alarms and other surveillance systems and a variety of vibration detection projects.
The Vibration sensor has two electrical contacts which are not connected in idle condition. When an external force is applied due to movement or vibration , the Sensor’s contacts close. When the force is removed, the sensor terminal returns back to open contacts. The internal spring closes the contacts during vibration which triggers the circuit connected to it.
How it works?
The sensor is made up of a small spring mechanism, which makes the contactsON when a vibration force is applied above a certain threshold level. Two pins coming out of the sensor are insulated by a resistance of more than 10 M Ohm. When a Vibration force is applied on the switch, the spring inside the switch vibrates and makes a momentary short circuit between the two terminals. The terminals of the Vibration sensor has no polarity but one pin is thick. Use it to connect to Vcc through a resistor and connect the thin pin to the circuit to be triggered.
1. Maximum supply voltage is 12 V DC but it works from 3-12 V DC.
2. Maximum current required – Less than 5 mA
3. Open circuit resistance – Higher than 10 Mega Ohms
4. On resistance – Less than 5 Ohms
5. Life expectancy – More than 5 Lakh times
6. Response time – 2 ms