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.