This circuit was found in the October 1989 issue of Radio Electronics magazine. It drives an auxiliary relay with the call signal for a possible louder bell.
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...