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Build 3-State Logic Probe

For less than 10 bucks you can build a piece of test gear that you'll find is one of the handiest in your tool kit

Original Design by Rick Walters

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)!

Click for larger image
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!

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