Tuning Fork Oscillator

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This is something I wanted to make for a long time. An oscillator with a tuning fork instead of a LC, RC, crystal or other resonator. I do not have (nor can I think of) a practical application for it. I build it just for fun.

I have failed a few times. The problem wasn't how to make the tuning fork resonate, a simple electromagnet as actuator does the job. The problem was how to detect the vibration for the feedback.

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Step 1: Photo Interrupter

I tried with HAL-sensors, coils and magnets. Always the influence of the magnetic field of the actuator was the problem. Recently I thought of photo interrupters, they at least aren't sensitive to magnetic fields. But I didn't know if the vibrations of the tuning for would be enough to measure with a photo interrupter. On Ebay I found a photo interrupter with a gap between the (IR) led and photo transistor wide enough to allow for a leg of the tuning fork.

Step 2:

It worked first time! With a little bending forward and backwards of the photo interruptor (see picture) , the leg of the tuning fork sits nicely halfway between the led and photo transistor. The vibrating leg produces about 500mV of signal. A dual opamp amplifies and schmitt-triggers the signal to a square wave. This is fed into a small signal npn transistor that in turn switches on and off the npn power transistor.

Step 3: Result

This is the signal coming out of the second opamp. As you see the frequency isn't what it is supposed to be, 440 Hz. The Chinese tuning fork is about 1.5 Hz too low. I could fix that by filing off some length of both legs but I don't think I ever will. (to lower the frequency, file a bit off where both legs meet)

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    30 Discussions

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    Muzammal Baig

    12 days ago

    You should remove the word "Chinese" as it seems racist and hateful. After all they provided the fork that your nation did not.

    4 replies
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    WilkoLMuzammal Baig

    Reply 11 days ago

    Germany provided the car that my nation didn't, now I'm not allowed to called it a German car? :-)

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    Muzammal BaigWilkoL

    Reply 8 days ago

    All I wanted to point out was that the phrase,
    "The Chinese tuning fork is about 1.5 Hz too low"
    makes it seem like its a mistake of the Chinese and that the Chinese products are not up to your mark.

    have you considered that all the components you rely on are also Chinese? i.e. the op amps the resistors and the transistors etc.

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    WilkoLMuzammal Baig

    Reply 8 days ago

    Well, then who's mistake was it? It was made in China and it resonates at 438.5 Hz, not at 440 Hz. Since when it saying that in some way racist?
    And I made it resonate on the right frequency anyway, by filing a bit off the tines.

    So now I have a Chinese tuning fork that resonates at the correct frequency. Am I allowed to say that?

    Try not to see racism in my story Muzzamal, it isn't there.

    BTW, while some of my components come from China, they come from all over the globe.

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    buttonkeyWilkoL

    Reply 3 days ago

    "try not to see racism in my article, it isn't there." Allow me to commend your eloquence!

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    xenoscientific

    14 days ago

    Note that the tuning fork may vibrate at a slightly different frequency in your project than it would if just struck. So it may (or may not) be as inaccurate as you noted. This is because any resonant device (i.e. a tuning fork) can be forced to oscillate at a range of frequencies near to its natural, unforced, oscillation frequency. Without going into the theory, to get the most unforced oscillation frequency you need to time the maximum magnetic impulse to the point in the oscillation of the fork where the deviation of the tines from their resting position is zero. Also, you can test the deviation from the forced to unforced oscillation in your project by simply (1) noting the oscillation frequency with the circuit running, (2) turning off the magnetic drive and then quickly measuring the frequency coming out of the photo-sensor before the tuning fork oscillation dies down. This later measurement may be difficult because it is time limited, but it is the unforced, natural vibration frequency of the fork.

    2 replies
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    WilkoLxenoscientific

    Reply 6 days ago

    Hello Xeno Scientific,
    That experiment had to wait. I had broken the tuning fork I was going to put in the tuning-fork-clock. The oscillator had to donate its tuning fork so I could continue work on the clock. But today Peter the Postman brought some new forks and the oscillator is complete and working again.

    And I can confirm that the oscillator does influence the resonant frequency. The new tuning fork oscillates on a 0.09 Hz higher frequency that it resonates when unforced. It is very simple to check this with the oscillator, the Q-factor of a tuning fork is so high that when I switch off the electromagnet it keeps resonating strong enough for half a minute to see the signal on an oscilloscope.

    So you are right (and wrong :-)) Yes there is some influence. Although for the original tuning fork it was in the wrong direction, resonating free, it would go even further away from the correct frequency of 440 Hz.

    And as I was experimenting anyway, I looked at the effect of warming up the fork. When I moved the oscillator in the sun, it went down in frequency about 0.3 Hz with an estimated rise in temperature of 15 Celsius (from 20 to 35 C)

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    xenoscientificWilkoL

    Reply 6 days ago

    Very cool that you collected this data! Might be able to compute the approximate Q of different tuning forks from this way. The temperature drift is in the direction I would expect from thermal expansion (my steel string guitar also does down in tune with increasing temp -- strings get longer so less tension).

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    In terms of practical uses: there was an electro-mechanical watch in the 1960-70’s called the Bulova Accutron, that used a tiny tuning fork as a time-keeping element. The frequency would’ve been something easily divided down to seconds... I think it was 3600Hz, but I’m not 100% sure. If you put it up to your ear, you could hear the tuning fork — the logo was even a tuning fork. These gave way to purely electronic watches that use quartz crystals for the timebase.

    Also, instead of using optical feedback like your circuit, the Accutron watch used the back-emf on the coil to tell when to give it a kick, otherwise the idea is pretty similar.

    Interesting project!

    3 replies
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    Leonardo2016richfiddler11

    Reply 14 days ago

    My Accutron Spaceview is 360Hz and the fork has two coils, one on each tine. One drives and the other senses. The electronics is one transistor, one resistor and one capacitor - the ultimate in simplicity, and all analogue.

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    richfiddler11Leonardo2016

    Reply 13 days ago

    Amazing what elegant circuits engineers can come up with with such basic components! Do you have a schematic and do you know what kind of motor it uses? The ultimate goal of the timebase circuit has to be to kick the motor at the right rate -- wonder if the motor is in series with the coil that drives the fork?

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    Leonardo2016richfiddler11

    Reply 13 days ago

    Hello richfiddler11, US patent US2971323 gives all the details - mechanics, schematic, waveforms.
    Not sure what you mean by 'motor'. effectively, the combination of the tuning fork and the drive/sense coils constitute a 'motor', I guess. This is the elegance of the Bulova - a tuning fork, made from Invar for temperature stablity, with two cups at the ends of the tines, encapsulating the coils and magnets, a battery and three components creates a remarkably accurate timepiece. The only complex parts are the 2.4mm diameter 360 tooth ratchet wheel and the incredibly small ruby pawls that drive it. Note that the patent says 300 teeth, I believe this is incorrect since the frequency is 360Hz. All this consumes 5-6 microamps and my battery lasts a year!
    I attach an image showing the Spaceview where you can see the much prettier fork than that shown in the patent and the three components are on the right - resistor on the top, with the transistor under it, and the capacitor at the bottom, under the tip of the second hand. Drive coils at the top and the fork extending down the centre to the bottom.

    accutron.jpg
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    WilkoLCarlo47

    Reply 14 days ago

    Thanks for the links.

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    soldeerridderWilkoL

    Reply 14 days ago

    You are not much when you are not Dutch!
    I also used a small file, but soon filed too much, after which I decreased the frequency a bit by adding some solder. These resonators were really small compared to your project. I tried to find an image on the internet without success, anyone?

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    smooth_jamie

    14 days ago

    Excellent project. Would love to see some videos of the scope trace when the tuning fork is struck by the solenoid. Perhaps you could use the oscillator to make a clock? That would be awesome.

    1 reply