Electronics Door-Knock or Vibration Alarm
This is a simple circuit that activates an alarm when there is a knock on the door or there are any vibrations due to movement of heavy goods or furniture. The circuit uses readily available components. Circuit and working :- The circuit is built around quad-opamp LM324 (IC1), which is configured in amplifier mode. It uses the piezoelectric element of a piezo buzzer as the input sensor, two transistors BC547 (T1 through T2), a piezo buzzer and some other components for the alarm circuit. Fig. 1 shows the circuit diagram of the door-knock alarm. The reference voltage at pin 3 of IC1 is set by trimming potmeter VR1. The piezoelectric element plate is fixed at the centre of the door using cello tape. Apply a small quantity of adhesive on the edges between the plates. Wires from the piezo element are connected at CON2. These generate an input pulse when vibrations are caused by knocking on the door. The pulse is amplified by op-amp A1 of IC1. Remaining three op...