Logic probe

I usually have logic analyzer and at least one low speed digital oscilloscope at hand, but simple logic probe is still invaluable for initial troubleshooting. My previous logic probe was built based on "7400 na 10 sposobów", article series with mini projects based on 7400 published in Polish "Elektronika Praktyczna" magazine in summer of 1995.

7400 logic probe

It is able to distinguish high impedance (no LED active), high/low digital states (red/green LEDs) and also signal short pulses (yellow LED, pulse length entended by univibrator).

7400 logic probe

This is still very useful tool, although my build is not too esthetic, there is some wear and corrosion and most importantly this probe requires 5V power supply which is more and more rare these days (typically probe is powered from tested board), so it was time for update.

Circuit from "Elektronika Praktyczna" would not work correctly with more modern CMOS-based gates. After looking for similar DIY probes I've tried this kind first:
Logic probe - not working
In my test this kind of probe was giving misleading readings though - if input was attached to some capacity (e.g. 1 meter of wire not connected anywhere else) both "H" and "L" LEDs were activated. Possible false reading with high impedance source was a deal breaker.

So, I've built something closer to my previous probe, but using more modern CMOS gates and able to operate with 3.3V power supply.

Logic probe schematic


Schematic: logic_probe.pdf.
Kicad 5 files: logic_probe_kicad.zip.
Bottom copper printout (for thermotransfer): logic_probe-B.Cu.pdf.
Logic probe assembly
logic probe
logic probe
Metal tip was made from some PCB standoff on improvised lathe (drill machine + file). It is held by four soldered wire (~1.0 mm diameter) loops.
logic probe
Extra pads on the other side of PCB are intended to hold ground/supply cable with soldered loop.
logic probe
Layer of transparent acrylic spray adds durability to PCB.
logic probe
Transparent shrink tube is used as inexpensive housing.
logic probe
logic probe
As described, one of the diodes was replaced with 0 Ohm resistor. Having all the components and multiple copper layer printout already I've made second piece to keep one at home and one at workplace.
logic probes

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