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 (Adobe PDF file)
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. |
