Smart Vibration Sensor

Here a simple Vibration sensor to protect door or window. It generates a loud beep when somebody tries to break the door or window. The alarm stops automatically after three minutes. The circuit uses a piezo element as the vibration sensor.

Piezo buzzer exploits the piezoelectric property of the piezo electric crystals. The piezoelectric effect may be direct piezoelectric effect in which the electric charge develops as a result of the mechanical stressor reverse or indirect piezoelectric effect (Converse piezoelectric effect) in which a mechanical force such as vibration develops due to the application of an electric field.

A typical example of direct piezoelectric effect is the generation of measurable amount of piezoelectricity when the Lead Zirconate Titanate crystals are deformed by mechanical or heat stress. The Lead Zirconate Titanate crystals also shows indirect piezoelectric effect by showing vibration when an electric potential is applied. This is the principle of sound generation in Piezo buzzer, Piezoelectric mic etc.

Here the direct piezoelectric effect is used to trigger the circuit. The piezo element is connected to the base of transistor T1 (The white piezo material in the centre of the element is positive while the metal rim is negative). When the piezo element vibrates, it generates around 1-2 volts that forward biases T1 followed by the triggering of T2. When T2 conducts, a short pulse will be given to the trigger pin 5 of IC1 through diode D1.

CD4538 (IC1) is the precision monostable multivibrator free from false triggering and more reliable than IC 555. Unlike 555, it triggers when it get a positive pulse in the trigger input. Timing resistor is connected between the positive rail and pin 2 and thetiming capacitor between the pins 2 and 1. Unlike 555, the output of CD 4538 remains high at power on and turns low when triggered by a low-to-hightransition pulse. Once triggered, it gives a low output depending on the values of timing resistor and timing capacitor. Here it is around 3 minutes.

Use 9 -12 volts DC for the circuit. Fix the piezo element on the door or window by applying glue on the smooth metal side. Connect the piezo element with the circuit using thin wire. Care should be taken while soldering the wire to the piezo element especially in the central part .The piezo material is coated as thin film in the central part. So first tin the tip of the wire and place the wire tip over the piezo material and gently apply solder once. Do not over heat or apply pressure to avoid peeling of the material. After soldering both the wires, paste a Cellotape over the soldering points so that, the wire will not detach.

Adjust VR1 to set the sensitivity of the circuit.