I have an old alarm with the same style sounder. It runs on a 9v battery. There don’t seem to be enough electronic components in it to be producing a drive signal, so it may simply make a noise when supplied with 9v.

I suspect alarm sounders are resonant at a particular frequency for efficiency and output will be low unless the Arduino is tuned to it.   About 1500Hz would be my guess.  If I can find one in my junk box, I’ll experiment.

The Arduino tone() function is the easiest way to generate the frequency : should work with any IO Pin

As IO Pins are current limited, may be necessary to drive the sounder with a transistor connected to the 5V rail.  An external power supply would provide even more volts.

Dave

SOD’s circuit will work but not very well. The device is capacitive this circuit only charges the capcitance. There is no discharge path. Typically they are driven form a AC or differntial DC source.
They also need to be mounted in a resonant or impedance matched cavity to match the disk and driven at the resonant frequency.
The device pictured has a third connection this allows feedback to the drive circuit so it automatically operates at the resonant frequency.

see https://mspindy.com/technical-guides/transducers-self-drive-piezo/

Robert.

As a rough musical guide, 1500Hz is not a recognised note but somewhere around G-flat in the second octave above middle A.*

A cavity resonator is not necessary for the transducer to work, but without it the sound pressure level may be very low, especially if driven much away from the transducer’s resonant frequency. (A piezo-electric transducer like this usually has a very narrow transmitting response across its resonance.)

So the housing within the smoke-detector might have acted as a resonator to amplify the sound. You could carefully cut the cavity from the detector, or make a resonator of the same dimensions.

Though it needs an alternating voltage drive, it does not have to be a very pure tone, except perhaps for the listener’s sake. If the signal frequency can be adjusted this could be done to tune for maximum output empirically, short of any proper test method to determine the resonance.

….

*At work we had a test-rig driven from a PC, originally using a locally-written BASIC programme that included a “beep” to alert the operator when it completed its run. As far as I know, no-one ever twigged that a Certain Someone not otherwise noted for high IT abilities, had adjusted the “beep” from some random frequency to middle-A,  440Hz. Then by copying the command-line, had added two other acoustic prompts at other stages, at the A above and below that…

That final video link is excellent, Mike !

[no style, but lots of substance]

…thanks for sharing it.

MichaelG.

The plain disk versions require an external driver circuit, but for DIY projects, the same piezo type devices are sold with built in electronics, often for 5V DC drive. eg –

https://www.bitsboxuk.com/index.php?main_page=index&cPath=302_309

Hi, there is a project in the Magenta Electronics Ltd. book1 (Everyday Electronics) 1989, which used a piezo transducer as a microphone, to make an acoustic probe, (electronic “stethoscope”) Electronics Projects Book 1 (UNITED KINGDOM All Publications (worldradiohistory.com) The book can be found and downloaded from about half way down the left hand side, and the project is on pages 74 to 76,  but I don’t know if any of those from old smoke alarms would be suitable, but it might interest some people.

Regards Nick.

P. S. I made the Digital Capacitance Meter described in the book, back in 1992, and it still works extremely well.

Here’s another video showing how to use the 3-wire piezo.

https://www.youtube.com/watch?v=goUNElFt9Jg