This circuit is designed to analyze the distortion of an audio signal. We found it in an article in the October 1988 Radio Electronics magazine that teaches how to use it.
This is the basic configuration of a monostable TTL with the 74121. The trip is done with an NC...
This fixed source configuration with 78xx series regulators has already been explored in several...
In the figure we have a TTL logic interfacing circuit as CMOS logic using a 6N139 optocoupler...