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Project Notes

#114 NOR Gate Oscillator

Test a low-speed square wave oscillator using NOR gates.

The Build

Here’s a quick video of the circuit in action:

NORGateOscillator demo


This circuit demonstrates a free-running oscillator based on two NOR gates - using 2/4 of a 74LS02. I got the idea from Beginning Digital Electronics Through Projects which features a NOR gate metronome. The circuit needed a few tweaks to get running with a different NOR IC and supply voltage.

It’s quite an interesting circuit, as you wouldn’t necessarily think that two NOR gates could form a reliable oscillator.

The basic operating principle (as far as I understand it so far):

  • when NOR(1) output is high, NOR(2) input is high and output low, and vice versa
  • the switching of the NOR(1) gate input is delayed by the RC network:
  • when NOR(2) output is low, C1 charges through the R4/R5 series from the NOR(1) high output
  • this raises the voltage on NOR(1) input to such point that the NOR(1) output switches low
  • then with NOR(2) output high, C1 discharges through R3 to the NOR(1) input to such point that the NOR(1) output switches high
  • rinse and repeat

The Arduino is not really a core part of the circuit. It only does the following:

  • provides +5V power supply (for convenience)
  • measures the input and output voltages for plotting

Here’s a sample trace recorded using the PlotNValues sketch. It has four traces, from top to bottom:

  • output of the first OR gate (which I belatedly realised was exactly the same node as the first trace, hence excluded from the schematics)
  • inputs to the first OR gate
  • output of the second OR gate (tapped as the main output)
  • inputs to the second OR gate

processing trace



The Schematic

The Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

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.