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Although convenient, most piezoelectric beeping devices that contain their own internal drivers, lack sound intensity. Most
units will only produce about 80db, when powered from a 9v battery. If more sound is required, the circuit shown below will generate an
ear-splitting 110db from 9 volts.
The circuit shown uses a single 74C14 (CD40106B) C-MOS hex Schmitt
inverter IC and must be used with a piezoelectric device that is equipped with a feedback terminal. The feedback terminal is attached to a
small central region on the wafer. When the beeper is driven at resonance, the feedback signal peaks. One inverter of the 74C14 is wired as an
astable oscillator.
The frequency is chosen to be about 5 times lower than the 3.2KHz resonant frequency of the piezoelectric device. Feedback
from the third pin of the beeper reinforces the correct drive frequency to insure maximum sound output. Four more inverter sections of the IC
are wired to form two separate drivers.
The output of one section is cross wired to the input of the second section. The differential drive signal that results
produces about 18 volts peak to peak, when measured across the beeper.
The last inverter section is wired as a second astable oscillator with a frequency of about 2Hz. It gates the main
oscillator on and off through a diode. If a continuous tone is needed, the modulation circuit can be deleted. |
