I need a piezo sounder with variable frequency tone output. I used some passive sounders and drove them with square wave signal from the 8051 but the sound level is much lower the self-driven sounders. I can not hear the sound in noisy enviroment. Can you give me any recommendations?

Hallo Emre,

piezos come in two different flavors: Narrow-band and wide-band.

The narrow-band ones use a certain resonance of piezo crystal and amplify it heavily by the use of a certain resonator (Helmholtz resonator type). This resonator is formed by an additional small enclosure containing a little hole, put directly in front of piezo crystal. When designed properly this resonator will show a resonance at exact the resonance frequency of piezo crystal and will by this heavily amplify sound pressure of piezo. So, when the frequency of signal applied to piezo matches this resonance frequency an enormous sound pressure will result.
'Self-driven sounders', as you named them, do also use this principle and can sound extremely load.

Unfortunately, if frequency of signal applied to piezo does not match this resonance frequency, sound pressure drastically decreases! For your purpose, means for a variable frequency application, such a narrow-band piezo is totally unsuited!

What you need is a wide-band piezo, which is recommended for 'melody & sound' application. And if high sound pressure is an issue, then I would recommend you to use a bridge driver like that:




This bridge driver will apply a signal voltage of 10Vpp, if the 74HC14 is connected to Vcc = 5V. Without using a bridge driver, means if one terminal of piezo is connected to ground, a signal voltage of only 5Vpp can be applied.
Keep in mind, that equivalent circuit of piezo is a capacitance! So, whenever you apply a square wave with fast edges to a piezo, there should be some current limiting. In the schematic this current limiting is provided by the 150 Ohm resistors. A resistor in series with each output prevents one gate from 'hogging' the current.
Don't forget to apply good power supply decoupling across supply pins of 74HC14. 10µF/16V Tantal should suffice.

Kai