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.
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)