The AT-121 Piezo

Yes, I do still sell the AT-121 piezo sounder on the switch and parts page (this blog is called: A Crummy Commercial for a reason :)

http://wb9kzy.com/switch.htm

The AT-121 is 35 cents but they do look a little grungy and have short leads - I also still sell the larger CPE-6080 piezo at $2.50 each.

However one thing the AT-121 has that the CPE-6080 doesn't have is a third, blue wire - this is a feedback area of the piezo.  It's used with this type of simple circuit shown on the datasheet:

I realized that I never tried this circuit - when I've used the AT-121 in my own projects I always drove it via the red and black wires by a square wave from a PIC.  The blue wire was just clipped off.  But what are the advantages of the datasheet circuit ?  The main one that comes to mind is that it is simple but also since it operates at resonance, loud.  That volume is bordering on obnoxious at 9 volts but must be ear splitting at 28 volts.

So anyway I did try the circuit on a breadboard and it works fine.  The AT-121 can be keyed at the ground connection to the 1k ohm resistor.  Either a mechanical switch (key), a keying transistor OR a microcontroller pin could be used.  When the microcontroller output is grounded the piezo oscillator will turn on.  When the microcontroller pin is floated or brought high the piezo circuit will turn off.  But using a microcontroller would limit the supply voltage of the piezo circuit to the same power supply voltage as used with the microcontroller, so it might not be quite as loud.

Here is the waveform at the emitter of the PN2222 transistor (the black wire of the piezo):

Frequency 4.7 kHz, supply voltage 5 volts, supply current 3.8 ma.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm