The buzzer is really old in terms of what the electronics industry has to offer today by way of noise makers, but having been around for decades, there are a lot of them “out there” in the field so a look at their operating principle is worth spending a little of our time.

A buzzer is built very much like an electromagnetic relay, but coil activation is controlled differently, as seen below.

In part 1 of the image above, the armature is being pulled to a resting position by a spring (not pictured here) at which position it has just arrived and before magnetic attraction from the coil has had its effect on armature position. The arm of that armature may even be curved a little bit like a deformed flat spring.

In part 2 of the image, the armature is being attracted toward the energized coil. The flat spring may get straightened out but the contacts are not necessarily separated yet, which means that the coil is still energized.

In part 3, the armature has moved toward the attracting coil and the contacts have opened up, thereby de-energizing the coil, which now is no longer an attractor. The armature then falls back to the part 1 position and the whole cycle repeats itself over and over again as shown below.

Buzzers have some significant advantages over more modern noise makers. They are mechanically rugged, they can function over very wide ranges of temperature, there is no radiation susceptibility, electrostatic discharges mean almost nothing, electromagnetic interference immunity is virtually infinite, and when properly coupled to the right acoustic environment, these things can be made loud enough to really command your attention.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

John, You correctly mention “electromagnetic interference immunity is virtually infinite,” but leave out perhaps the most important “negative” of the buzzer. They will generate a tremendous amount of EMI due to the contact sparks. However, this characteristic could also be turned into a useful entity. In the early days of wireless communication, buzzer-like devices were used to generate the RF in spark-gap transmitters. They would generate a lot of wide band energy, and then tune for the desired frequency. Clever way of turning these lemons into lemonade.

Not EMI susceptible, but a strong EMI emitter! I used buzzers as radio jammers. All you needed to add was some pieces of wire on both sides of the contacts to serve as an antenna.

You can wire a relay to act as a buzzer, only we normally call that a “chattering relay.” Old-fashioned large relays would make sense, a modern tiny relay is unlikely to have the mechanical characteristics to emit much noise, or at an audible frequency. It is not difficult to add some external components to limit the EMI emission as much as you want. Many years ago, I remember a three-phase relay which somehow got wired so it chattered, and the associated inductance and capacitance EMI circuit on the power input acted with the uncontrolled and variable contact make-and-break timing to do strange and wonderful things, creating such HV transients that arcing destroyed some of the circuitry. Definitely a technology to be aware of.

A long time ago, a colleague of mine wanted to check out the connections on a circuit board that he just finished assembling. He built a buzzer out of a small relay to use as a simple continuity tester. Unfortunately, when the relay coil opened on each click of the relay, it generated a very high voltage ( v = L di/dt). He destroyed every integrated circuit on the board. Not a good day in the lab.

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