This circuit was found in a March 1988 Radio Electronics. The voltage scale is set at R3. 19 LEDs are used in this indicator circuit.
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...