DC buzzers

[kalel]

While reading an instructable (http://www.instructables.com/id/DIY-CONTINUITY-TESTER/) I figured that there must be DC buzzers/beepers. I was also able to find some by searching, and they seem to be mostly targeted at alarms.

Previously, I thought you needed your own oscillator to drive a piezo beeper. You probably still do in order to get a specific frequency.

How do these work, do they have a custom oscillator circuit inside of the case?

[Ian.M]

Leaving aside mechanical electromagnetic buzzers, and only considering Pizeo ones, the simplest uses a special three electrode pizeo element, a single transisistor and a few passives:

The three-terminal piezoelectric acoustic generator has a split electrode on the piezoelectric element. When the signal is applied between 2 and 1, the phase shifted signal will be induced between 3 and 1 so that it works as a piezoelectric transformer.

The phase shifted signal can be used as the feedback component in a simple oscillation circuit which operates automatically at the natural resonant frequency of the element.

A Piezo Transducer with a feedback tab makes the circuit oscillate without inductors and capacitors. The number of components is reduced and it is possible to generate from a sine wave to a trapezoidal wave.

The actual circuit can vary considerably: e.g it may take the base bias from the collector or it may use an inductor to increase the drive signal swing above the supply rail

See AVX-Kyocera: Piezoelectric Acoustic Generators

[Zero999]

The buzzer in the picture could just as easily be electromagnetic, as piezo. No specification or datasheet was provided.

An electromagnetic buzzer might use a mechanical interrupter as the osculating element. When current flows, the electromagnet turns on, opening a set of normally closed contacts, connected in series with it, causing the current to stop, the electromagnet to switch off and contacts to close again, then the electromagnet switches back on and the cycle repeats at an audio frequency. Alternatively, it could use an electronic oscillator, similar to the one used for the piezo, but the feedback coming from another winding, an arrangement known as a blocking oscillator.


http://www.sancoelectronics.com/mechanical-buzzer.asp

If you don't have a three wire piezo transducer, then the 74HC14 can be used to make an oscillator to drive it. The 74HC is only specified to work of 2V to 6V, so if you want to run at lower voltages (one or two AA cells) use the 74LV14, which will work down to 1V. If you want it to work above 6V, then use the CD40106, which will work up to 18V.

[Brumby]

How do these work, do they have a custom oscillator circuit inside of the case?

Some do and some don't.  The ones that do have a built-in oscillator just need power and off they go.  Generally, the frequency is pretty much fixed.

Previously, I thought you needed your own oscillator to drive a piezo beeper.

This is for the ones that don't have an internal oscillator.  What's inside of the case is just the transducer.  Sometimes there isn't even a case - and you just get the bare transducer.

With these, you can pick your frequency.  Just remember they won't give the same output level at every frequency.

[kalel]

How do these work, do they have a custom oscillator circuit inside of the case?

Some do and some don't.  The ones that do have a built-in oscillator just need power and off they go.  Generally, the frequency is pretty much fixed.

Previously, I thought you needed your own oscillator to drive a piezo beeper.

This is for the ones that don't have an internal oscillator.  What's inside of the case is just the transducer.  Sometimes there isn't even a case - and you just get the bare transducer.

With these, you can pick your frequency.  Just remember they won't give the same output level at every frequency.

Yes, there's a frequency response like with speakers. And I already know there are some that work in the high kilohertz range or higher. For example, piezo transducers for ultrasonic humidifiers. But most of these need a "high voltage" (as in more than power bank/USB adapter voltage) oscillating circuit to be ran efficiently. So an ultrasonic humidifier might not be a good beginner project (depends on the definition of beginner though, some beginners already understand most of the theory and just need practice).

[Audioguru]

A piezo transducer has a few resonances at which frequencies it has high efficiency and high loudness. A piezo beeper has its case designed to also resonate at the loudest resonance of the piezo. The oscillator in a piezo beeper automatically oscillates at the loudest resonant frequency.

If you drive a piezo transducer with your own oscillator then the frequency might produce low or high loudness and the frequency of your oscillator might change by temperature or battery voltage changes.

[Zero999]

A piezo transducer has a few resonances at which frequencies it has high efficiency and high loudness. A piezo beeper has its case designed to also resonate at the loudest resonance of the piezo. The oscillator in a piezo beeper automatically oscillates at the loudest resonant frequency.

If you drive a piezo transducer with your own oscillator then the frequency might produce low or high loudness and the frequency of your oscillator might change by temperature or battery voltage changes.

It is possible to make an oscillator which automatically drives a two wire piezo at its resonant frequency, but it isn't an easy thing to do. The easiest method is to use the circuit I posted earlier, with a trimmer, as part of the resistor and adjust it, until it sounds the loudest. As long as a decent capacitor is used, preferably film, rather than ceramic, the circuit I posted should be fairly stable.

[Damianos]

...
If you don't have a three wire piezo transducer, then the 74HC14 can be used to make an oscillator to drive it. The 74HC is only specified to work of 2V to 6V, so if you want to run at lower voltages (one or two AA cells) use the 74LV14, which will work down to 1V. If you want it to work above 6V, then use the CD40106, which will work up to 18V.

This circuit will produce zero output!
The last inverter has to be driven by the output of the previous one; this will double the voltage on the transducer.

[Zero999]

...
If you don't have a three wire piezo transducer, then the 74HC14 can be used to make an oscillator to drive it. The 74HC is only specified to work of 2V to 6V, so if you want to run at lower voltages (one or two AA cells) use the 74LV14, which will work down to 1V. If you want it to work above 6V, then use the CD40106, which will work up to 18V.

This circuit will produce zero output!
The last inverter has to be driven by the output of the previous one; this will double the voltage on the transducer.

Yes you're right. Another inverter is required to make the outputs complementary.