This project came about through the recent trend in electronics towards lower operating voltages. If you look around at the latest chips being offered from semiconductor manufactures you will see that most are designed to operate on 3.3V or lower.

Having done a few recent designs with 3.3V circuits, I discovered that my old favourite test tool, the Logic Probe, wouldn’t operate below 5V.

I looked around my usual electronic suppliers but couldn’t find anything that would work on anything less than 5V. So I thought I’d build design and one.

The first requirement I had was to make it work over as wide an operating voltage as I could so that it could be used on the old legacy 5V systems and down to some of the latest processors at 2.8V, the second was low cost.

I took a look inside the existing probes I had, only to discover them to be full of analog components, some of which were now obsolete.

The quickest and easiest approach seemed to be to build something out of a small microcomputer, so I went on the hunt for anything that was small, cheap and worked on a wide supply voltage.

Where I ended up was at the Microchip website looking at our old friend, the PIC.

This photo shows the first prototype without the extra components added for higher voltage operation or input protection. Don't forget to keep the pen cap - it can save some nasty stabs! Also note the S1 access hole in the pen body.