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Project Notes

#637 CD4069 Logic Probe

A simple logic probe using inverter gates.

Build

Notes

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.

Construction

The first inverter 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
  • a fixed high signal will stall the oscillator on the 5th and 6th inverter gates
  • R4 pulls the 5th inverter input high, thus final inverter output will also be high, and D3 (yellow) off

When HIGH input:

  • first inverter output will be low
  • with low output, D2 (green) off, D1 (red) on
  • a fixed low signal will stall the oscillator on the 2nd and 3rd NOR gates
  • R4 pulls the 5th inverter input high, thus final inverter output will also be high, and D3 (yellow) off

When input is oscillating:

  • D2 (green), D1 (red) will flicker based on instantaneous input state
  • oscillating input to the 5th inverter will enable the oscillator on 5th and 6th inverter gates
  • final inverter output will oscillate, causing D3 (yellow) to light up

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schematic

Breadboard test, with logic high input:

LogicProbe_bb_build_high

Breadboard test, with logic low input:

LogicProbe_bb_build_low

Breadboard test, with oscillating input (10kHz square wave from FY3200S signal generator):

LogicProbe_bb_build_osc

Circuit Simulation

Follow simulation

Credits and References

About LEAP#637 Digital LogicCMOS/TTL
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, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.