A handy tool for tri-state monitoring of logic circuits

When troubleshooting digital circuits, a logic probe provides a quick and easy way of determining the logic level at a particular node. Unlike voltmeters and oscilloscopes which are able to display a range of analogue voltage levels, the logic probe is essentially a digital instrument that provides a simple yet unambiguous “yes” or “no” indication of the node’s logic level.

Unlike many commercially available logic probes, the one described in this article is a precision instrument, in that it uses analogue techniques to sense the voltage levels, thereby providing an accurate indication of the true logic state. Like the author’s Volt Checker last month, this Logic Probe is a pocket-sized instrument housed in a small plastic case which is easy to construct and simple to use.