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  • Knock Twice Doorbell with Piezo Sensor

  • Created: Oct 01, 2014

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A doorbell is a device used for alerting people inside a house. Normally, it is placed near the entry door in a passageway. When a visitor presses a button the bell rings inside the building, notifying the presence of a visitor. In this circuit, the actual doorbell is replaced by a simple metal for tapping. The circuit only produces an output when the piezo detects two taps from the metal.

The circuit uses an HEF4011UB quadruple 2-input NAND gate. This unbuffered single stage version provides a direct implementation of the NAND function. The output impedance and output transition time depends on the input voltage and input rise and fall times applied.

The circuit takes only a few microamps and when a tap is detected by the piezo, the waveform from the transistor produces a HIGH on pin I2 and a HIGH on pin I1 which makes the output pin O1 goes LOW. This very quickly charges the 47nF and it is discharged via the 560kΩ to produce a brief pulse at pin O2. The 47nF is mainly to stop noise entering pin I3. Pin I4 is HIGH via the 2.7MegΩ and the LOW on pin I3 causes pin O2 to produce a HIGH pulse. The 47nF is discharged via the internal diodes on pin I8 and when it goes LOW, pin O4 goes HIGH and charges the 10nF via the 22kΩ and diode. This puts a HIGH on pin I6 for approximately 0.7 seconds and when a second tap is detected, pin I5 sees a HIGH and pin I7 goes LOW. This puts a LOW on pin O3 and a HIGH on pin I6. The LOW on pin O3 goes to pin I4. A HIGH and LOW on the second NAND gate produces a HIGH on pin O2 and the third NAND gate has a HIGH on both inputs. This makes pin I7 LOW and the 4.7µF starts to charge via the 2.7MegΩ resistor. After 5 seconds pin O3 sees a HIGH and pin O4 goes LOW. The 10nF is discharged via the 10MegΩ and when pin I6 sees a LOW, pin I7 goes HIGH. The output sits HIGH and goes LOW for about 7 seconds.



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