Knock Sensor Circuit using Piezoelectric

In this tutorial, we are making a project on the knock sensor circuit. This circuit is used to detect the door or table knocking and then activates the relay for a specific preset time. Any AC or DC appliance can be connected with the relay so that you can operate it with a circuit like a Bell, voice recording, etc.

To sense, the vibrations generated by knocking are sensed by a piezoelectric sensor PZT. These sensors are made up of a piezoelectric ceramic material pasted in a round shape on a metal disk. The ceramic piezo sensor generates voltage from a few milli-volts to 1 volt when detects a small amount of vibration or force on them.

Hardware Components

The following components are required to make a Knock Sensor Circuit

S.no	Component	Value	Quantity
1.	Piezo Sensor	–	1
2.	Transistor	2N4401	2
3.	LED	–	3
4.	Resistor	10K, 1K, 1M, 470Ω	1, 1, 2, 3
5.	Electrolytic Capacitor	2.2uF 35V	1, 1
6.	DC Supply	9-12V	1, 1

2N4401 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N4401

Knock Sensor Circuit

Working Explanation

The PZT sensor detects the vibrations and converts them into electrical signals which are then passed onto the circuit. To further amplify this signal we have used an op-amp IC CA3140 and a 2N3904 transistor. This amplified signal is now fed into another IC which is a 555 timer IC. On receiving the signal this IC will be triggered and give a high signal at its output pin 3 for a preset time period and activates the relay. The preset time period depends on the value of the capacitor Cx and can be increased or decreased by changing the value of the capacitor. A 10K variable is used to adjust the sensitivity of this circuit, this circuit can be operated at 9 to 12 volts DC. The relay should be of the same voltage as the operating voltage.

Applications and Uses

• Door Bells
• Burglar Alarms
• Door monitors, etc.