Simplest Speaker Oscillator, Now Even Simpler

It never fails. Lay down some kind of superlative — fastest, cheapest, smallest — around this place and someone out there says, “Hold my beer” and gets to work. In this case, it’s another, even simpler audio oscillator, this time with just a loudspeaker and a battery.

Attentive readers will recall the previous title holder was indeed pretty simple, consisting only of the mic and speaker from an old landline telephone handset wired in series with a battery. Seeing this reminded [Hydrogen Time] of a lucky childhood accident while experimenting with a loudspeaker, which he recreates in the video below. The BOM for this one is even smaller than the previous one — just a small speaker and a battery, plus a small scrap of solid hookup wire. The wire is the key; rather than connecting directly to the speaker terminal, it connects to the speaker frame on one end while the other is carefully adjusted to just barely touch the flexible wire penetrating the speaker cone on its way to the voice coil.

When power is applied with the correct polarity, current flows through the wire into the voice coil, which moves the cone and breaks the circuit. The speaker’s diaphragm resets the cone, completing the circuit and repeating the whole process. The loudspeaker makes a little click with each cycle, leading to a very rough-sounding oscillator. [Hydrogen Time] doesn’t put a scope on it, but we suspect the waveform would be a ragged square wave whose frequency depends on the voltage, the spring constant of the diaphragm, and the spacing between the fixed wire and the voice coil lead.

Yes, we realize this is stretching the definition of an audio oscillator somewhat, but you’ve got to admit it’s simple. Can you get it even simpler?

21 thoughts on “Simplest Speaker Oscillator, Now Even Simpler

    1. Sorta, the car horn uses a magnetic interruptor, in which case an electromagnet, when energized, pulls a diaphragm, and also pulls open a series contact, which interrupts the current to the coil, which resets the diaphragm and contact, closing the circuit and starting the cycle over again, up to several thousand times per second. This is also how door buzzers and doorbells work, and also provided the pulsating current for Model T spark coils.

  1. This is starting to look more like the regular mechanical doorbel (Electromagnet + switch + noise making bent metal piece) that was already mentioned in the comment in the previous article.

  2. How to make it simpler – how do you qualify? Fewer parts? Fewer functional elements?

    There’s four functional elements here: the actuator (speaker), the energy source (battery), the controlling element (switch), and the connecting wires. We count the wires as one, but you may count each piece if you wish.

    There’s three distinct parts to the construction: battery, wiring, speaker.

    The way to simplify is to remove something. You can’t reduce the functional elements, but you can reduce the part count. How?

    You take an AA battery, wrap two pieces of paper on it to form two sliding cylinders half as long as the battery. You wrap magnet wire around both paper cylinders to form two coils. Now you glue one coil over one end of the battery and slide the other coil over the other end, so they meet in the middle. One coil is fixed, the other is free to slide along the battery. Connect the coil wires in series and anti-phase, such that when the circuit is completed with the battery, the two coils push apart. Now you bend the free end of the wire of the sliding coil so that it touches the end of the battery. Connect the fixed coil’s wire to the other end of the battery, and you have made the traditional buzzer circuit. The magnet wire is springy enough to pull the coil back into contact with the battery.

    Now you take a paper cone and glue it to the end of the moving coil, and you have made a loudspeaker, that is also the battery, and the switch, all in one part. Simple?

    1. Then, if you want to ditch the speaker entirely, I would go for a Rijke tube.

      It’s an open tube held vertically, with a metal mesh about a quarter way in from the bottom. The mesh is heated, e.g. with a candle and when the flame is removed, the tube starts oscillating and making a really loud blaring sound for a while.

      Replace the mesh with nichrome heating wire, and with just the right amount of power it should keep oscillating.

      1. Interesting. Thanks.
        If one were to make a reverse Rijke tube, that would be simpler than a normal Rijke tube, and so would win this Hackaday contest.

        “A “reverse” Rijke effect — namely, that a Rijke tube will also produce audio oscillations if hot air flows through a cold screen — was first observed by Rijke’s assistant Johannes Bosscha[3] and subsequently investigated by German physicist Peter Theophil Rieß“

      2. I didn’t need the mesh when I stuck a torch in and adjusted the depth until it sounded. It was probably much less efficient of course, but continuous for as long as I was able to hold the tube.

  3. Congratulations. You’ve just reinvented the buzzer. To simplify it further, get rid of the speaker horn and magnet, reducing it to a NC relay which breaks it’s own coil current when energized.

  4. Yeah just use a single dpdt relay

    One pole in series so its Normally closed and opens when u energize the coil,
    The other pole controls the load

    A capacitor across the relay controls the frequency as well as the amount of voltage and current….but isn’t needed for it to just oscillate

  5. For something that needs zero components excluding the power input and wires, I can think of one option. An arc isn’t a component, and it’s a negative resistance device. Normally you’d have carbon electrodes or a vacuum or whatever to make it not burn itself out, and you’d get it to resonate by giving it parallel LC to allow oscillations to build up at a given frequency. But if wire doesn’t count as a component, you can probably find an arrangement of wire with enough parasitic reactances to do the job, at least until it melted.
    See: Duddell’s “musical arc” and such.

    (Depending on your definitions, for one component excluding power and wires, there’s a tie between relays wired to buzz, bimetallic strip based cycling switches, horn systems that are a one-component version of this, and various other things. Relays are pretty easy, but the thermal blinker switches are a little simpler I suppose.)

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