Ultra accurate 440 hertz electronic tuning fork

Buying tuning forks on Amazon is often disappointing because they vary off the correct frequency sometimes by several hertz. Here we have instructions for making your own ultra accurate 440 hertz electronic tuning fork. Its accuracy is about 1 or 2 parts per million. This means that in about a half hour, it might be off by one cycle. It uses a temperature compensated crystal oscillator running at 11 megahertz. This is divided by 25,000 to give a 440 hertz square wave. Then two integration steps turns the square waves into an approximate sine wave. This is amplified to produce sound output from a small speaker. This is very useful for accurately calibrating tuning tools such as TuneLab or other musical instrument tuning tools, regardless of temperature (within reason). The circuit and box were designed by Viktors Berstis. Thanks also to Dan Gray for using the Eagle tool to design the circuit board and Taj Wurl-Koth for fine surface mount soldering. This design is in the public domain and free to use.

This is the first draft schematic:

Here is a cleaner version as output from Eagle:
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This is the raw circuit board with no parts:
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This is the circuit board as Eagle produced it:
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This is the circuit board with most parts soldered on it:
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This is the circuit board's other side with speaker, battery, switch and LED:
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The box and back can be 3D printed using files: top of box and bottom of box. These files were created using FreeCad files: top and bottom. The speaker is held away from the circuit board with 4 stand-offs and screws going through the bottom cover go through the circuit board into the stand-offs, holding the board against the back. Small holes are drilled in the four corners into the box top to affix the bottom to the top using 4 screws:

The Eagle files for the circuit board can be found here: schematic, board, library, Gerber files. Finally, the parts list is here: Parts List.

To illustrate the accuracy, I connected two units to an oscilloscope and made about a 39 minute time lapse video running a thousand times faster than real time. You can see that during this time, they were off by about 1 cycle out of (39 minutes x 60 secsperminute x 440hz = 1,029,600) cycles.