The circuit shown was found in the February 1994 issue of Electronics Now magazine. The circuit records messages for a few seconds and plays them back as keys are selected.
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...