Here is a very interesting and useful project of a super sensitive vibration sensor switch circuit. The circuit can be used for variety of applications for example as a door knock sensor switch, shock sensor switch, touch sensor, pressure sensor etc. It can also be used for many other applications in
home or outside for example in automobiles for safeguard purposes. The circuit is using a PZT (peizo electric sensor) or peizo speaker to sense the vibration and sound. The peizo electric sensor is made up of peizo electric ceramic material pasted in round shape on a metal disk. These ceramic peizo sensors generates voltage from few milivolts to around 1 volt when detect a small amount of force or vibration on them. These same peizo electric sensors or peizo speaker are also used in peizo buzzers to generate buzzer sound.
The circuit is divided in to 5 stages. The first four are the amplification stages built with four transistors and the last fifth stage is built with the 555 timer IC. When the peizo sensor will detect any vibration/sound around it then it will generate a small electrical signal, which is amplified by the four transistors. The last transistor Q4 send the amplified signal to the trigger pin 2 of the 555 IC due to which the output at the pin3 of the IC goes high and activate the LED and the relay switch for a preset time period and after that time period the output of the IC become low and the LED and relay will become deactivated. This time period can be increase or decrease by increasing or decreasing the value of the 220uF electrolytic capacitor. The sensitivity of the circuit can be adjusted with 2M variable resistor. The operating voltage of the circuit is 9 to 12V DC. The circuit will also work on lower voltages, when operating with low voltage make sure to use low voltage relay also.