Digital Test and Measurement Starts with Logic Probe - Any point in a Digital Circuit can be High, Low, Float means TriState. The High Low in a 5V CMOS circuit is just 0V and 5V. In TTL and ECL it is much different
TTL Pulse Reading Logic Probe - This circuit can monitor TTL logic levels. TTL compatible CMOS logic families can also be monitored.
The Test Point could also be Pulsing or have analog values and waveforms, as in Mixed Signal Chips. Here are Logic Probe Circuits you can Build in your own Home Lab.
Simple Logic Probe - Here is a simple Logic Probe using a single chip. The circuits have been designed for the CD4001 CMOS quad NOR gate and CD4011 CMOS NAND gate
Andy Collinson - Logic Probe - This logic probe uses a single CMOS 4001 IC and can display high, low and pulsing outputs.
Pac-man Repair & Troubleshooting Guide - Logic Probe - A Logic Probe is one of the basic troubleshooting tools for common electronic logic circuits (TTL circuits, chips with the 74XX pre-fixes).
Multi-Instrument Digital Assistant – PIC16F84A
Multi-Instrument Digital Assistant - PIC16F84A - "It contains four instruments in one unit: logic probe, single channel logic state analyzer, frequency counter and serial code receiver."
Small Electronic Testing Tools
Then first thing in Electrical testing is the Continuity Tester, Then we use the Neon Mains Screwdriver, Then we need an Voltmeter or an Ammeter. In electronics we need a OhmMeter and LCR meter too.