To check static or slow changing signals in devices using TTL voltage levels, a multimeter or scope can be too cumbersome. To know a signal is high, low or undefined can be good enough in most cases.
A long time ago I copied this from a Elektor (then Elektuur, probably from 1977-1980) diagram, making it as compact as possible with 0.1" vero board.
The original diagram is lost, but here is a reverse engineered diagram component list and board layout.
Last updated: 2014-08-10
