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Project Notes

#636 CD4001 Logic Probe

A simple logic probe using NOR gates.

Build

Notes

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 CD4001

The CD4001BC is a Quad 2-Input NOR Buffered B Series Gate. 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 CD4001 datasheet for more.

Construction

The first NOR gate is used as an inverting buffer of the input signal.

When LOW input:

  • first NOR gate output will be high
  • with high output, D2 (green) on, D1 (red) off
  • a fixed high signal will stall the oscillator on the 2nd and 3rd NOR gates
  • R4 pulls the 3rd NOR gate input high, thus final NOT gate output will also be high, thus D3 (yellow) off

When HIGH input:

  • first NOR gate 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 3rd NOR gate input high, thus final NOT gate output will also be high, thus D3 (yellow) off

When input is oscillating:

  • D2 (green), D1 (red) will flicker based on instantaneous input state
  • oscillating input to the 2nd NOR gate will enable the oscillator on 2nd and 3rd NOR gates
  • final NOR gate output will oscillate, causing D3 (yellow) to light up

Note:

  • R4, R5 values are not critical. High values such as 1MΩ are satisfactory
  • R5 is optional - it prevents oscillation when input is high Z/not connected.
  • the link/short between the anode of D1 and cathode of D2 is used to inhibit D1/D2 when there is no strong input signal high or low
    • this is optional; the circuit works without it.
    • removing it will reduce power consumption of the circuit but D1/D2 will tend to be on when there is no strong input signal

bb

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

Protoboard Build

I transferred the design to a piece of protoboard to capture the circuit for future use. A quick sketch of the layout:

protoboard_layout

Protoboard test, with logic high input:

protoboard_high

Protoboard test, with logic low input:

protoboard_low

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

protoboard_osc

Credits and References

About LEAP#636 Digital LogicCMOS/TTL

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

Project Source on GitHub Return to the LEAP Catalog
About LEAP

LEAP is my personal collection of electronics 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.

Projects are often inspired by things found wild on the net, or ideas from the many great electronics podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know or send a pull-request.

NOTE: For a while I included various scale modelling projects here too, but I've now split them off into a new repository: check out LittleModelArt if you are looking for these projects.

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