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Project Notes

#022 Square Wave - Schmitt Oscillator

Using an 74LS14 Hex Inverter with Schmitt Trigger Inputs to generate a square wave.

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

Schmitt Oscillator


One inverter unit of the 74LS14 Hex Inverter with Schmitt Trigger Inputs is used as an oscillator. The input and output levels are read with analog input pins, and the values is echoed to the Arduino serial port for plotting.

All unused outputs on the 74LS14 are left open circuit. Unused inputs are grounded, as is recommended to avoid unstable operation.

The frequency of the oscillation is calculated as:

f = 1 / (1.2 * R * C)
f = 1 / (1.2 * 1kΩ * 470µF)
f = 1.77 Hz

The oscillating output from the inverter is used to drive the base of a general-purpose NPN transistor, switching a LED on and off in turn. The transistor is used to isolate the 74LS14 from the load, as it is not designed to directly handle any significant current.

Here’s a sample trace recorded using LEAP#090 PlotNValues (a simple Processing sketch) to read the data collected by the SchmittOscillator.ino program. The lower trace is the input, and the upper trace is the output of the inverter:

processing trace

Schmitt inverters produce a non-symmetrical waveform. The duty cycle is about 33%, and levels not very stable. This is particular to the LS (low-power Schottky) version of the 7414. To oscillate at all, R values must be kept between 100Ω and 1kΩ, and C between 1nF and 1000µF.

Apparently 74HC14 has a duty cycle more like 50% and can handle higher values of R (but I don’t have one handy to test).

See Schmitt Waveform Generators for more, and Circuit Properties of LS and HC Digital Logic for comparison of LS and HC characteristics.

See the RingOscillator project for a related application of inverters in waveform generation.


The Breadboard

The Schematic

The Build

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.