Most piezoelectric beeping devices with internal drivers lack sound intensity,
only producing about 80 dB when powered from a 9-V battery. If more sound is
required, this circuit will generate an ear-spliting 110 dB from 9 V (see
the figure).
The setup employs a single 74C14 (CD40106B) CMOS hex inverting Schmitt-trigger
IC, which must be used with a piezoelectric device with a feedback terminal.
The feedback terminal is attached to a central region on the piezoelectric 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 3.2-kHz resonant frequency of the
piezoelectric device. Feedback from the third pin of the beeper reinforces the
correct drive frequency to ensure maximum sound output.
Four other 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 V p-p when measured
across the beeper. The last inverter section is wired as a second astable oscillator
with a frequency of about 2 Hz. It gates the main oscillator on and off through
a diode. For a continuous tone, the modulation circuit can be deleted.
