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Project Notes

#568 Beep Logic Probe

Building a logic probe with audio and LED feedback, based on a design from Elektor magazine January/February 2015.


Here’s a quick demo..



The “Beep Logic Probe” appeared in Elektor magazine January/February 2015 and is a nice little circuit for probing digital circuits. Main features:

  • “high” and “low” states generate a beep with high and low frequencies respectively
  • high Z/disconnected state is silent
  • a separate frequency counter triggers an audio tone for high frequency signals (I’ve used it to detected signals all the way up to 24MHz)
  • powered from the circuit under test with a wide voltage range (3-15V) since it uses 4000-series CMOS devices

I’ve made a few modifications to the original Elektor circuit:

  • the front-end is replaced by dual comparators using an LM358, so that the high-Z range is explicitly set between 1/3 and 2/3 of VCC
    • the three-stage voltage divider front-end in the original circuit did not work very well for me, as it would not unlatch low signals effectively -the transition voltages of the CD4040 NAND gate I’m using appeared to be on the high end of the specs, and thus defeated the “high Z” state of the circuit
  • although this circuit is primarily audio, I’ve added LED indicators just for fun:
    • red - low voltage indicator
    • green - high voltage indicator
    • orange - high frequency detection indicator
  • the original circuit tapped 3 selectable outputs from the CD4093 ripple counter. I’ve only tapped two, and used a third for the LED indicator.
  • I’m uing CD4049 inverting buffer instead of the original CD4050 non-inverting buffer. As this is used just for driving the audio output, the result is the same.

How it Works

See the Elektor article for a full description. The essentials:

  • low and high frequency audio indocators are generated with simple NAND-gate oscillators. This allows the neat trick of using the input signal as an enable input to the oscillators
  • higher frequency signals will soon drive the circuit beyond audible range. This is where the CD4093 ripple counter comes into play - it is capacitively coupled to the input and scales back the incoming signal. The frequency range switch is used to select between divide by 1024 or 2048.


First with a breadboard test:



Breadboard Build

A quick test with a function generator on the breadboard:


Altoids Can Construction

I transferred the circuit to a piece of protoboard using the following layout and then stuck it in an Altoids can..



A Quick Test

In the following demo, I’m probing a simple 555 astable oscillator, and then monitoring the output of a function generator all the way up to 24MHz


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.