Back in the November 1998 edition of SILICON CHIP, we described a very handy 3-LED Logic Probe.

The circuit is just as viable today as when it was published six years ago and literally thousands of kits have been sold. That’s no surprise: a logic probe is one of the "must have" test devices in any hobbyist’s, technician’s or even engineer’s test equipment armoury.

What’s more, it’s cheap – so it’s an ideal beginner’s or school project (not to mention one that will come in very handy over the years)!

One IC, three LEDs and a sprinkling of other components are all it takes to make a versatile Logic Probe. At left we are using the probe to check out the very first project ever published in SILICON CHIP, a 1GHz Digital Frequency Meter from November 1987. Yes, it still works perfectly!

So why re-invent perfectly good wheels and present it once again?

Simple: one of the suppliers of the afore-mentioned kits, Altronics, reasoned that the it could be be made even better by re-designing the PC board to a handier shape, adding a few extra (low-cost!) components to provide better input protection, moving the supply on-board and finally, housing the probe so it was much more like . . . a probe!