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Project Notes

#017 555 Timer - Monostable Demo

Use an Arduino to demonstrate the monostable mode of operation, and plot the results with Processing.

The Build

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

555 Timer - Monostable Demo


The monostable mode of operation is actually well described in the LM555 Datasheet.

When pin 2 is pulled low (the trigger), output pin 3 goes high and will remain high for a period of time determined by the values of R1 and C1.

Selecting R1, C1 Values

The circuit described uses R1=22k and C1=47uF which results in the output remaining high for just over a second when triggered. The formula for calculating the high period is:

t = 1.1 x R1 x C1

See the datasheet for your chip to confirm the formula. You can also find various websites that offer 555 timer calculators, but I decided to write my own Visual 555 Calculator so that I could play around with HTML5 animation of the resulting circuit;-)

Powering the project

The breadboard is powered directly from the Arduino 5V pin, although it could also be powered by an independent 5-9V supply.

Input/Output State Plots

The output voltage is tapped at the anode of the LED, read with an analog input pin and echoed to the Arduino serial port.

LEAP#090 PlotNValues (a simple Processing sketch) reads the data from the serial port and plots the 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.

Here’s a sample trace:

processing trace


The Breadboard

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.