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Project Notes

#637 CD4069 Logic Probe

A simple logic probe using inverter gates.



This is a variation of the LEAP#636 CD4001 Logic Probe circuit but using inverter gates instead of NOR gates. It was suggested by Steve Schnepp as he noted that in the original circuit, most NOR gates were being used as inverters anyway.

This logic probe design can detect and indicate:

  • logic high (red LED on)
  • logic low (green LED on)
  • oscillating signal (yellow LED on)

About the CD4069

The CD4069UBC is a pack of 6 inverter circuits. Key features:

  • operating range (VDD): 3-15V
  • monolithic CMOS
  • low power TTL compatibility
  • all inputs protected by standard CMOS protection circuit; voltage at any pin −0.5V to VDD+0.5V

See the CD4069 datasheet for more.


The first inverter (IC1a) is used as a buffer of the input signal.

When LOW input:

  • first inverter output will be high
  • with high output, D2 (green) on, D1 (red) off

When HIGH input:

  • first inverter output will be low
  • with low output, D2 (green) off, D1 (red) on

The second inverter (IC1f) feeds a simple RC high-pass filter i.e. edge detector (C1,R4). This is buffered by IC1e and triggers an LED driven by IC1d; note these could be combined on one inverter. With an oscillating input, the edge detector will cause D3 (yellow) to flicker at the same frequency (limited by the RC time constant).



Breadboard test, with logic high input:


Breadboard test, with logic low input:


Breadboard test, with oscillating input (500Hz square wave from FY3200S signal generator):


Scope trace showing CH1 input (IC1 pin 1) and CH2 output (IC1 pin 8):


Circuit Simulation

Follow the link to see the simulation create by Steve Schnepp for this circuit concept:


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.