Unusual, but accurate low voltage probe (with schematic)

A strange electrical probe from eBay, that may be aimed primarily at vehicle use, but probably has other applications.  A side by side test with a Fluke meter showed an accuracy within 0.1V across the full range.

The voltage range stated on the packaging is 3 to 36V, but I'd recommend 35V tops, and preferably lower due to the use of a standard 3.3V regulator.
The circuitry is notable for the use of what I guess is tri-state multiplexing (charlieplexing) to drive the LED display, meaning that the three digits can be controlled by just six connections.

The red lead is negative and the black probe is positive...   So fortunately it does have polarity protection and an LED to indicate polarity.  Strangely, green means wrong polarity and red means correct polarity.

The only reason I can think of for the mystery diode between the input and output of the regulator is for the tiny current leakage via the sense resistor, that might reverse bias the voltage regulator if the probe was connected with the wrong polarity.


The eBay listing this probe came from had the following description:-
"12V Circuit Tester Probe 3-24v Electric Car Fuse Test Light Voltage Pen Detector"
I paid £8 for mine inclusive of shipping.

Note that some similar units do not have the same good accuracy.


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