No Active Components In This Mysterious Audio Oscillator

What’s the simplest audio frequency oscillator you can imagine? There’s the 555, of course, and we can think of a few designs using just two transistors or even a few with just one. But how about an oscillator with no active components? Now there’s a neat trick.

Replicating [Stelian]’s “simplest audio oscillator on the Internet” might take some doing on your part, since it relies on finding an old telephone. Like, really old — you’ll need one with the carbon granule cartridge in the handset, along with the speaker. Other than that, all you’ll need is a couple of 1.5-volt batteries, wiring everything in one big series loop, and placing the microphone and speaker right on top of each other. Apply power and you’re off to the races. [Stelian]’s specific setup yielded a 2.4-kHz tone that could be altered a bit by repositioning the speaker relative to the mic. On the oscilloscope, the waveform is a pretty heavily distorted sine wave.

It’s a bit of a mystery to [Stelian] as to how this works without something to provide at least a little gain. Perhaps the enclosure of the speaker or the mic has a paraboloid shape that amplifies the sound just enough to kick things off? Bah, who knows? Let the hand-waving begin!

39 thoughts on “No Active Components In This Mysterious Audio Oscillator

  1. A carbon capsule works as a rudimentary amplifier. Such devices were used in phone networks as repeaters back in the day.

    Point being that you put DC through the carbon granules and the compression of the grains increases or decreases the current passing through them by varying resistance. This creates AC which drives the speaker. The power required to move the grains can be less than the power passed through by the granules, which results in amplification. Add a suitable phase shift by the inertia of the speaker cone, and you’ve got a positive feedback system that results in oscillation.

    This is in principle no different than the traditional ringing doorbell with the solenoid hammer that strikes a switch, powering the solenoid on and off. The switch is only replaced with a continuously variable resistor and the striking hammer with air pressure from the speaker.

    1. great post. I’d also like to point out that marconi’s first radio receiver used a coherer as the radio detector and sounder to create audio (electric bell) . the coherer was a testube of iron filings that were continually agitated to stop them becoming solid.

      In theory this design could be modified into a receiver.

      1. The coherer works because the radio frequency signal causes the iron filings to stick together with micro-welds – it’s still a bit unclear why or how it works and there are several theories to it.

        The idea is that you have radio waves going into the antenna, and then you have a resonating “tank” circuit made of capacitors and coils which selects the frequency you’re listening to. The voltage and current in the tank circuit keeps growing in resonance with the radio signal coming from the antenna until it’s strong enough to cause the iron filings in the coherer to stick together – and that acts as a switch for the ringer.

        The theory is that the RF energy is enough to jump across the very thin insulating oxide (rust) layers on the iron grains, opening a conductive path. When that happens, the DC from the battery connected to the ringer starts following the same path, and it’s strong enough to cause the filings to weld together.

        The ringer has a solenoid (it’s the doorbell again), and when the solenoid hammer strikes the bell, it also shakes the vial with the iron filings and breaks them apart. That switches the solenoid off, the hammer retracts and the system resets. The RF signal in the tuning tank then builds up again until the coherer switches back on and the solenoid strikes again – keeping the ringer ringing for as long as there’s a radio wave coming.

      1. Of course. Varying direct current is AC – in this case it just ends up with a DC bias.

        Fortunately, the speaker doesn’t mind: the DC flowing through it simply offsets it from the neutral middle position. The AC riding on top of the DC then makes it vibrate as usual.

        1. I’m sorry, I’m just not on the same page as you terminologyically. A microphone is an amplifier? DC is AC? Pigs are monkeys? You zip back and forth between talking about the microphone and the speaker and that confuses this reader.

          Sorry, I feel like I’m being sold a bridge. A battery slowly running down would be AC (alternating current) because of being varying DC. AC riding on DC? Isn’t that a Hayes Code violation? So two streams of current in one circuit then? Please help me understand, thanks.

          1. If you have direct current flowing through a wire, but it’s varying in a cyclical fashion, then it’s DC and AC at the same time. The DC is the average value over time, and the AC is how much it ripples. If the DC average becomes zero, then you’re talking about pure AC, and vice versa if the AC ripple is zero then it’s pure DC.

            The carbon capsule microphone throttles the amount of DC current that gets passed through the speaker from the battery. It turns the sound waves into cyclical variations in current, or AC. The loudspeaker doesn’t do anything with the DC portion of the current, but does respond to the AC and makes sound, which goes back into the microphone and forms a closed feedback loop.

            The carbon capsule microphone acts as an amplifier because it takes the sound energy in and converts it into electrical energy in such a way that a small amount of air pressure variation controls a large amount of current in the wire. A little bit of sound power turns into a lot of electrical power – amplification.

            >A battery slowly running down would be AC

            Yes and no, since it is varying, but it’s not cyclical – so it doesn’t “alternate”.

    1. I agree. maybe the voice coil of the speaker induces something in the microphone which phase lags a bit, and varying the distance between speker and mic potentiates this. But i’m a total hack soooo..

      Sure sounds and acts like feedback though!


      1. If the wave arrives in the microphone 180 degrees lagging, it’s a negative feedback and it damps the oscillator. If the delay is a full 360 degrees or a complete wave cycle (or multiple), then it’s positive feedback and the oscillation grows until it hits some physical limit such as saturating the microphone. That saturation is what causes the distortions in the waveform – the signal clips because it can grow no more, plus some other stuff like resonances.

        At 2.4 kHz the wavelength is about 14 cm so the speaker and the microphone are far too close together to make that much delay. When they’re pressed up to one another there’s barely any. The distance doesn’t explain why it oscillates. It does however explain why it stops.

        When the speaker is wired “backwards” with respect to the microphone, it shifts the phase 180 degrees. The mass of the speaker coil and cone have inertia too, which adds the missing 180 degrees. It doesn’t need to be exact – anything plus or minus 90 degrees from a full cycle results in some positive feedback, so you can move the speaker back a bit and it’ll still work. Go too far, and it turns from positive to negative feedback, and the oscillator stops.

        The frequency of the oscillator is mainly a result of the inertia of the speaker: it starts oscillating at the frequency where the cone lags the input signal by about half a cycle. That means you can tune the frequency by adding weights or shaving off material from the speaker cone.

        Varying the distance has some effect too, but since the phase lag of the speaker becomes less at a lower frequency, the two effects mostly cancel and you end up at nearly the same frequency. You can use a much longer tube between the speaker and the microphone though, so you aren’t relying on the speaker’s phase lag so much. The tube is needed to contain the sound, otherwise it wouldn’t be loud enough for the mic.

  2. A much simpler one: take a relay with a NC contact, wire the NC contact in series with the coil and the power supply and it will start buzzing when supply is switched on. Just one relay, no other parts needed.
    As a side effect, the coil will generate very high voltage spikes due to back EMF, so be careful not to touch wires when in operation; this is how step up regulators were made in ancient times and has been used for example to supply high voltage to tubes in mobile radios.

  3. I think I know a simpler oscillator. It was a part of a children’s book from Disney. So you only need a 9v battery ,a small 8 ohms speaker from a cheap toy or radio, some wires and a piece of aluminum foil enough to cover the cone of the speaker, then you make a small stick using another piece of foil, the wiring is simple : from the B+ of the battery to one terminal of the speaker, the other terminal goes to the foil covering the cone, the negative terminal of the battery goes to the stick, now when you put the stick over the cover, vuala! you have a nice annoying sound.

      1. yeah ! indeed. Ok here I go again : so B+ ————-speaker———cover of Aluminum foil————> <——— the stick touching the cover at about 10° – 15° from the horizontal it makes a nice sound, you know the stick starts to bounce opening an closing the circuit.

          1. If I were grading no you didn’t because you don’t define terms. What is B+? I understand the positive terminal and the negative terminal but what is B+? Thanks.

          2. Lol, dude I’m struggling with my keyboard, give me a break :) my mind is so fast to think what I want to write but my fingers are introducing minutes of latency between words.

            btw yeah B+ is “pausitive” terminal and B- is “negation” terminal… If we only could have the edit button….

      1. It literally means “Look there!” and voici with a cedille means “Look here!” The problem is I don’t understand the “the B+ of the battery”.

      2. The correct way to misspell/missay it is viola!

        Idiots that don’t get the joke will correct you.
        Jokes on them.

        Also: In the USA it’s not pronounced Fraunce, it’s frAnce. Also also: quebecish, quebecee or quebecer.

    1. That’s just the simple relay buzzer (or the electric doorbell) again. The foil is the switch, so when the speaker cone moves the foil it breaks contact with your stick, which makes the foil return back and touch the wire again, repeating the cycle.

  4. When I was a kid we used to take the carbon mic and wire it to a big speaker and a 9 volt battery. Presto you got yourself a megaphone which was fairly loud although the mic would get hot if you used it for too long.

    1. Same here – back in the 1970s I made a megaphone (based on a similar Forrest Mims circuit) involving a carbon mic, a loudspeaker horn, and a lantern battery. A modification was to run everything through a big ol’ TO-2 power transistor. “Transistor Projects Vol. 2” if I recall correctly.

  5. not that this isn’t neat, but i confess i was hoping for an oscillator without any active components. i don’t know a good definition for ‘active’ but i reject the idea that a speaker is passive.

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