This is a project of a knock sensor circuit, it is a very interesting circuit and can be used to detect the door or table knocking and switch on a relay for a preset time period. You can connect any AC or DC appliance with the relay for example a bell.
The circuit is using a peizo electric sensor to detect
the vibrations generated by knocking. It converts these vibrations in to electric signals. The CA3140 IC and 2N3904 transistor amplifies these electrical signals and triggers the 555 timer IC due to which the relay connected at the output of the IC become activated for a preset time period. This preset time period can be increase by increase the value of capacitor Cx. The 10K variable resistor is used to control the sensitivity of the circuit. The operating voltage of the circuit is 9 to 12V DC.

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Knock Sensor Circuit