transistors, one vibration sensor, speaker and other discrete components. Working of the circuit is simple; the circuit is using a peizo as a vibration sensor. When someone knocks the door a small vibration is produced, this vibration is sensed by the peizo electric sensor which will convert these vibrations into electrical signals that are further amplified by the transistors Q1. The transistor Q2 is working as a switch, the amplified signals at the collector of the transistor Q1 is fed to the base of the Q2 due to which the transistor Q2 gets activated, when the transistor Q2 activated the 470uF capacitor become charged and Q3 also gets activated. When the transistor Q3 activated it provides power to the bird bell circuit that is built around the two 555 timer ICs. The Q3 will stay activated until the 470uF capacitor will discharged, this provides a simple time delay, when the 470uF capacitor completely discharged the Q3 gets deactivated and cuts the power going to the bell circuit.
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This door knock activated bird bell circuit can be used as a door bell to let you know if there is any visitor at your door. This bird bell becomes activated for a preset time period when the PZT or peizo sensor receives any vibration.
The circuit is using two 555 timer ICs, three
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Door Knock Activated Bird Bell Circuit Diagram