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Project Notes

#656 Crystal-Locked Schmitt Oscillator

Testing a basic crystal-locked Schmitt oscillator using a 74HC14.



I tested the basic Schmitt Oscillator a very long time ago in LEAP#022. I was prompted to try the crystal-locked variation after seeing CD40106 aka 74C14 covered in IMSAI Guy’s chip of the day:


Oscillators of this topology are variously known as Pierce Oscillators, Pierce-Gate Crystal Oscillators or CMOS Crystal Oscillators.

An alternative design using NOR Gates:


Circuit Design

The slowest crystal I have to hand is 4MHz, so I needed to add 20pF stabilization capacitors to maintain oscilation. The second inverter buffers the output and the 51kΩ feedback resistor ensure’s the oscillator is kicked into action.




Test Results

4.00019MHz according to my scope:


Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.