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Project Notes

#183 Polarity Tester

Demonstrate a polarity-testing circuit using the CD4069 inverter.

The Build


A simple polarity testing circuit uses a series of inverters. The input signal is pumped into two parallel chains:

  • a single inverter
  • a series of two inverters

So, regardless of input polarity, one chain output will be high and the other low.

input Single-inverter Output Dual-inverter Output

The signals are tied together with two reverse-polarity LEDs in series. So one will always be alight when the other is not. The unlit LED is subject to a reverse voltage equivalent to the forward voltage of the other LED. Most LEDs will handle this without complaint.



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.