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Project Notes

#039 Astable Opamp Oscillators

Use an Arduino to monitor the behaviour of an LM324 OpAmp astable oscillator and plot the results with Processing.


Demos the generation of a low-frequency square wave with an LM324 OpAmp.

The OpAmp unit is used as a comparator, with an RC circuit providing the oscillation.

The frequency is determined by the values of R4 and C1, and calculated using the formula:

frequency = 1/(2ln2 x RC)

The values selected for the circuit yield a period of about 652ms, or 1.53Hz:

frequency = 1/(1.386 x 4700 x 0.0001)
frequency = 1.53Hz

The square wave output is tapped at the base of the LED load. This ensures the “off” value gets fully pulled down to ground.

I’m also measuring the voltage at the inverted input. Interestingly, we get a reasonable triangle wave here, as its the middle of the RC network.

Here’s a sample trace of the oscillator output as measured by the Arduino. The lower trace is at the inverted input, the upper trace is the output.

processing trace


The Arduino only acts as a measurement device in this circuit.

LEAP#090 PlotNValues (a simple Processing sketch) reads the data from the serial port and plots the input and output value over time, with some coloration effects thrown in for good measure. In other words, we’re using Arduino and Processing as a basic oscilloscope! And it kind of works, mainly because the frequency is so low.

The Breadboard

The Schematic

The Build

NB: for simplicity, the circuit is powered from the Arduino 5V pin. That works fine, although it is equally possible to use a separate power supply (but ensuring there is a common ground connection).

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.